Field of the Invention
The present invention relates to an energy storage
system including a solar cell, a commercial AC power source
and a storage battery in which the storage battery is
charged with the power from the solar cell and the power
from at least one of the solar cell, the commercial AC power
source and the storage battery is supplied to one or more
load devices. Background of the Invention
Background of the Invention
There has been known an energy storage system which is
fully charged with electric power from a solar cell and
discharges during an emergency such as a power failure.
Such an energy storage system discharges only in an
emergency. However, such a systems does not sufficiently
use the solar cell since the discharge from the storage
battery is carried out only in an emergency. Therefore,
there has been proposed an energy storage system which
allows power to be discharged up to a predetermined
threshold value in non-emergency situations to promote
effective use of electric power from the solar cell (see,
e.g., Japanese Patent Application Publication No. 2009-
159730) . The threshold value is set to a capacity which

makes it possible to supply electric power to be used upon a
power failure.
In terms of energy savings, it can be thought that
electric power which is used at home be obtained from solar
energy by charging a storage battery with electric power
generated by solar power generation in the daytime and
discharging the electric power from the storage battery at
night.
However, in the conventional energy storage system,
the electric power discharged from the storage battery
irrespective of whether it is day or night when the electric
power by the solar power generation is less than electric
power that is consumed by load devices. In this case, the
electric power required to the load device may not be
furnished by only the discharge from the storage battery at
night.
Summary of the Invention
In view of the above, the present invention provides
an energy storage system that can prevent a shortage of the
electric power of a storage battery at night.
In accordance with an aspect of the present invention,
there is provided an energy storage system including a
storage battery, wherein, when a charge level of the storage
battery is not greater than a reference charge level, a

discharge from the storage battery to a load device is
permitted or prohibited depending on a time zone.
The energy storage system may further include a solar
cell and a commercial AC power source, wherein the storage
battery is charged with electric power from the solar cell,
wherein electric power from at least one of the solar cell,
the commercial AC power source, and the storage battery is
supplied to the load device, wherein, in the daytime, when
an amount of electric power generated by the solar cell is
less than an amount of electric power consumed by the load
device and the charge level of the storage battery is higher
than the reference charge level, the discharge from the
storage battery to the load device is permitted, wherein, in
the daytime, when the amount of electric power generated by
the solar cell is less than the amount of electric power
consumed by the load device and the charge level of the
storage battery is not higher than the reference charge
level, the discharge from the storage battery to the load
device is not permitted, and wherein the discharge from the
storage battery which makes the charge level of the storage
battery lower than the reference level is permitted at night.
With such configurations, the storage battery is
charged with the power from the solar cell in the daytime,
and the discharge from the storage battery is permitted at
night when there is a shortage in power because the amount
of electric power generated by the solar cell does not

exceed the power consumed by the load device. Meanwhile,
when the charge level of the storage battery is less than a
first charge level, power that is supposed to be used at
night is ensured by prohibiting the discharge from the
storage battery. At night, the power stored in the daytime
is used by allowing the capacity of the storage battery to
drop below the first storage level. Accordingly, when the
electric power is supplied at night by the discharge from
the storage battery, it is possible to reduce the case where
the electric power by the discharge from the storage battery
cannot be supplied.
Further, a backup level may be set as a reference
value of the charge level of the storage battery, the backup
level being lower than the reference charge level and
corresponding to an amount of electric power that is used in
an emergency in which electric power is not supplied from
the solar cell nor the commercial AC power source to the
load device. In this case, supply of electric power from at
least one of the solar cell and the commercial AC power
source to the load device is performed so that the charge
level of the storage battery is equal to or higher than the
backup level in the daytime and at night. In the emergency,
the discharge from the storage battery which makes the
charge level lower than the backup level is permitted.
With such configurations, in the daytime and at night,
the charge level of the storage battery remains equal to or

greater than the second charge level. In an emergency in
which the supply of the electric power from the commercial
AC power source is interrupted, the discharge from the
storage battery is allowed so that the electric power can be
furnished to the load device even in the emergency.
Further, when, in the daytime, the amount of electric
power generated by the solar cell is less than the amount of
electric power consumed by the load device and the charge
level of the storage battery is not higher than the
reference charge level, determination process may be
performed based on past charge level change data in which a
time period during which the charge level reaches a fully
charged level is equivalent to a predetermined time period
or more, to determine whether or not the charge level
recovers to the reference charge level before night in a
state in which the discharge from the storage battery is
permitted, and when the charge level is determined to
recover by the determination process, the permission of the
discharge from the storage battery to the load device is
maintained.
With such configurations, in the daytime, when the
amount of electric power generated by the solar cell is less
than the amount of electric power consumed by the load
devices and the charge level of the storage battery has
reached a lower level from a higher level than the first
charge level, the following process is performed. That is,

by referring to, from among past charge level change data,
the past charge level change data in which a time period
during which the charge level reaches the fully charged
level is equivalent to a predetermined time period or more,
whether or not the charge level will recover up to the first
charge level is determined based thereon. Here, when the
charge level is determined to recover up to the first charge
level, the permission of the discharge from the storage
battery to the load device is maintained. Specifically, in
the daytime, when the charge level drops below the first
charge level, typical process does not permit the discharge
from the storage battery to ensure electric power to be used
at night. However, when the recovery of the charge level is
estimated by the determination, the discharge from the
storage battery to the load device is permitted. In this
way, the storage capacity of the storage battery can be
optimally used depending on the situation in which the
charge level of the storage battery is lowered, thereby
reducing the amount of electric power from the commercial AC
power source.
Further, in the determination process, based on
similarity between charge level change data indicating a
past change in the charge level of the storage battery, and
charge level change data on a date when the determination
process is performed, a past date having a charge level
change which approximates a charge level change on the date

of the determination process may be selected. Further, it
is determined whether or not a full charge level period
during which the charge level reaches a full charge level in
the charge level change data on the selected date is
equivalent to a predetermined time period or more, and when
the full charge level period is determined to be equivalent
to the predetermined time period or more, it is determined
that the charge level will recover and the permission of the
discharge from the storage battery to the load device is
maintained.
With such configurations, in the determination as to
whether or not the charge level will recover, as described
above, the past charge level change data, which approximates
the charge level change data on the date when the
determination processing is performed, is selected. And, it
is determined whether or not the full charge level period is
equivalent to the predetermined time period or more in the
charge level on the date of the selected charge level change
data. That is, since whether or not the charge level on the
date of the determination will recover later is determined
based on the past data on the date having data approximating
that on the date of the determination, the accuracy of the
determination can be improved.
Further, in the determination process, based on
similarity between power generation amount change data
indicating a past change in power generation amount by the

solar cell, and power generation amount change data on a
date when the determination process is performed, a past
date having a power generation amount change approximating a
power generation amount change on the date of the
determination process may be selected. And, it is
determined whether or not a full charge level period during
which the charge level reaches a full charge level in the
charge level change data on the selected date is equivalent
to a predetermined time period or more, and when the full
charge level period is determined to be equivalent to the
predetermined time period or more, it is determined that the
charge level will recover and the permission of the
discharge from the storage battery to the load device is
maintained.
With such configurations, in the determination as to
whether the charge level will recover, as described above,
the past power generation amount change data, which
approximates the power generation amount change data on the
date when the determination processing is performed, is
selected. And, it is determined whether or not the full
charge level period is equivalent to the predetermined time
period or more in the charge level on the date of the
selected power generation amount change data. That is,
since whether or not the charge level on the date of the
determination will recover later is determined based on the
past data on the date having data approximating that on the

date of the determination, the accuracy of the determination
can be improved.
Further, the reference charge level may be set to a
level that corresponds to an amount of electric power that
is consumed by the load device at night.
With such configurations, since the electric power
used at night can be furnished with the electric power
charged in the storage battery in the daytime, it is
possible to reduce the use of the electric power supplied
from the commercial AC power source.
Further, when time zones in which an electricity rate
of the commercial AC power source is determined include a
normal time zone in which the electricity rate is normal and
a low-price time zone in which the electricity rate is lower
than normal, the reference charge level may be set to a
level corresponding to an amount obtained by deducting an
amount of electric power that is used in the low-price time
zone from an amount of electric power that is consumed by
the load device at night.
Electricity rate of a commercial AC power source is
divided into normal price and low price electricity. That
is, the nighttime is divided into a normal time zone having
a normal electricity rate and a low-price time zone having a
lower electricity rate. The electricity charge for the
consumed electric power is produced by summing a product
obtained by multiplying the amount of electric power used in

the normal time zone by the normal electricity rate and a
product obtained by multiplying the amount of electric power
used in the low-price time zone by the lower electricity
rate. In addition, in consideration of the above, in the
present invention, the first charge level is set as a value
corresponding to the amount of electric power obtained by
deducting the amount of low-price electric power that is
used in the low-price time zone from the amount of electric
power that is used at night. That is, in the low-price time
zone, it is set such that there is a shortage in electric
power and the electric power from the commercial AC power
source is used in that time zone. Accordingly, as for the
electric power that is used at night, the electric power
that is supplied at the low rate is effectively used, and
thus the electricity costs can be reduced.
In accordance with the present invention, there is
provided an energy storage system capable of preventing a
shortage in the electric power of the storage battery.
Brief Description of the Drawings
The above and other objects and features of the
present invention will be more apparent from the following
description of exemplary embodiments taken in conjunction
with the accompanying drawings, in which:
Fig. 1 is a block diagram showing a configuration of a

power supply system including an energy storage system
according to an embodiment of the present invention;
Fig. 2 is a clock diagram showing a configuration of a
power control device in the energy storage system;
Fig. 3 is a flowchart showing a sequence of "power
control process," which is performed by the power control
device according to the same embodiment;
Fig. 4 is a flowchart showing a sequence of "daytime
power control process" which is performed by the power
control device;
Fig. 5 is a flowchart showing a sequence of "nighttime
power control process" which is performed by the power
control device;
Fig. 6 is a flowchart showing a sequence of "charge
level recovery estimation process" which is performed by the
power control device;
Fig. 7 is a timing chart showing an example of the
control state of the energy storage system;
Fig. 8 is a timing chart showing another example of
the control state of the energy storage system; and
Fig. 9 is a timing chart showing a further example of
the control state of the energy storage system.
Detailed Description of the Preferred Embodiments
Embodiments of the present invention will be described

in detail below with reference to the accompanying drawings
which form a part hereof. Same reference numerals will be
assigned to same or similar components throughout the
drawings, and redundant descriptions thereof will be omitted.
Referring to Figs. 1 to 9, an embodiment of the
present invention will be described. Furthermore, the
present embodiment illustrates the case where the
electricity supply management device of the present
invention is practiced as a part of an electricity supply
system.
As shown in Fig. 1, a house is provided with an
electricity supply system 1 that supplies an electric power
to various types of household devices (a lighting device, an
air conditioner, household electronic appliances, audio and
visual devices, and the like). The electricity supply
system 1 supplies to a variety of types of devices an
electric power from a household commercial AC power source
(AC power source) 2 and an electric power from a solar cell
3, using solar light.
The electricity supply system 1 supplies an electric
power not only to DC devices 5 but also to an AC device 6.
Each of the DC devices 5 is operated by a DC electric power,
and the AC device 6 is operated by an AC electric power from
the commercial AC power source 2. Although, in the
following descriptions of the embodiment, the house is given
as an example of a place where the electricity supply system

1 is installed, the place is not limited thereto.
Alternatively, the electricity supply system 1 . may be
installed and used in a multi-family house, an apartment, an
office, or a factory.
As a distribution board of the electricity supply
system 1, a controller 7 and a DC distribution board 8
(including a DC breaker) are provided in the electricity
supply system 1. Furthermore, in the electricity supply
system 1, a control unit 9 and a relay unit 10 are provided
as a device for controlling the operation of the DC devices
5 in the house.
An AC distribution board 11 for branching an AC power
is connected to the controller 7 through an AC power line 12.
The controller 7 is connected to the commercial AC power
source 2 via the AC distribution board 11, and is connected
to the solar cell 3 through a DC power line 13. The
controller 7 receives an AC power from the AC distribution
board 11, receives a DC power from the solar cell 3, and
converts the powers into a predetermined DC power as a power
for the devices. Furthermore, the controller 7 outputs the
resulting DC power to the DC distribution board 8 through a
DC power line 14 and to a storage battery 16 through a DC
power line 15. The controller 7 receives the AC power,
converts the DC power from the solar cell 3 or the storage
battery 16 into an AC power, and supplies the AC power to
the AC distribution board 11. The controller 7 exchanges

data with the DC distribution board 8 through a signal line
17.
The DC distribution board 8 is a kind of DC power
breaker. The DC distribution board 8 branches the DC power
inputted from the controller 7, and outputs the resulting DC
power to the control unit 9 through a DC power line 18, or
to the DC relay unit 10 through a power line 19.
Furthermore, the DC distribution board 8 exchanges data with
the control unit 9 through a signal line 20, or with the
relay unit 10 through a signal line 21.
The plural DC devices 5 are connected to the control
unit 9 through DC supply lines 22 each of which is capable
of carrying both DC power and data. A communications signal,
carrying data by using a high frequency carrier wave, is
superimposed on a DC voltage supplied as a power to the DC
device 5 through the DC supply line 22. That is, both power .
and data are carried to the DC device by means of the power
line carrier communications through the DC supply line 22
that has a pair of wires. The control unit 9 obtains the DC
power for the DC devices 5 through the DC power line 18, and
determines an operation control state of the DC devices 5
based on an operating instruction obtained from the DC
distribution board 8 through the signal line 20.
Furthermore, the control unit 9 outputs the DC power and the
operating instruction to the corresponding DC device 5
through the corresponding DC supply line 22, and controls

the operation of the DC device 5.
Switches (SW) 23 are connected to the control unit 9
through the DC supply line 22. The switches 23 are
manipulated when the operations of the DC devices 5 are
switched. Furthermore, a sensor 24 for detecting, for
example, radio waves transmitted from an infrared remote
control is connected to the control unit 9 through the DC
supply line 22. Accordingly, the DC devices 5 are
controlled by communications signals transmitted thereto
through the DC supply line 22 in response not only to an
operating instruction from the DC distribution board 8 but
also to the manipulation of the switches 23 or the detection
of the sensor 24.
Some of the DC devices 5 are connected to the relay
unit 10 through respective DC power lines 25. The relay
unit 10 obtains the DC power for the DC devices 5 through
the DC power line 19, and determines which one of the DC
devices 5 is to be operated based on an operating
instruction obtained from the DC distribution board 8
through the signal line 21. Furthermore, the relay unit 10
controls the operation of the determined DC device 5 by
selectively turning on and off the supply of power through
the DC power line 25 by a relay provided therein.
Furthermore, a plurality of switches 26 for manually
manipulating the DC devices 5 are connected to the relay
unit 10, and the DC devices 5 are controlled by selectively

turning on and off the supply of power thereto through the
DC power line 25 by the relay in response to the
manipulations of the switches 26.
A DC outlet 27 that is uprightly attachable to the
house, for example, in the form of a wall outlet or a bottom
outlet, is connected to the DC distribution board 8 through
the DC power line 28. By inserting the plug (not shown)
from the DC device into the DC outlet 27, a DC power can be
directly supplied to the DC device.
Furthermore, a power meter 2 9 capable of remotely
measuring the amount of used power from the commercial AC
power source 2 is connected to the AC distribution board 11.
The power meter 2 9 has not only the function of remotely
measuring the amount of commercial power used but also, for
example, the function of power line carrier communications
and/or wireless communications. The power meter 29
transmits the results of the measurement to an electric
power company or the like through the power line carrier
communications or wireless communications.
The electricity supply system 1 includes a network
system 30 that enables various types of household devices to
be controlled through network communications. The network
system 30 includes a home server 31 that functions as a
control unit of the network system 30. The home server 31
is connected to an external management server 32 through a
network N, such as the Internet, and also to a customer

premises equipment 34 through a signal line 33. Furthermore,
the home server 31 is operated by a DC power, obtained from
the DC distribution board 8 through a DC power line 35.
A control box 36 for managing the operational control
of various types of home devices by using network
communications is connected to the home server 31 through a
signal line 37. The control box 36 is connected to the
controller 7 and the DC distribution board 8 through the
signal line 17, and also directly controls the DC devices 5
through a DC supply line 38. A gas/water meter 39 capable
of remotely measuring, for example, the amount of gas or
water used is connected to the control box 36, and a
manipulation panel 40 of the network system 30 is also
connected to the control box 36. A monitoring device 41
including, for example, a door phone receiver, a sensor
and/or a camera is connected to the manipulation panel 40.
When operating instructions from the various types of
home devices are inputted to the home server 31 through the
network N, the home server 31 notifies the control box 36 of
the operating instructions, and operates the control box 36
to control the various types of the home devices to perform
operations based on the operating instructions. Furthermore,
the home server 31 may provide various types of information,
obtained from the gas/water meter 39, to the management
server 32 through the network N. When receiving from the
manipulation panel 40 a notification that the monitoring

device 41 has detected an abnormality, the home server 31
may also provide the notification to the management server
32 through the network N.
The energy storage system 100 includes the solar cell
3, the storage battery 16, the controller 7, and a power
control device 70. The energy storage system 100 controls
the storage battery 16 depending on the amount of power
generated by the solar cell 3 and the DC power usage amount
by the DC devices 5.
The solar cell 3 periodically measures the solar power
generation amount PWS, and outputs the measured solar power
generation amount PWS to the power control device 70 through
a signal line 51. Furthermore, the solar power generation
amount PWS varies depending on both the intensity of solar
light and the load connected to the solar cell 3. For
example, even when the solar cell 3 has a sufficiently large
capacity to generate a power, if the total amount of DC
powers used by the DC devices 5 connected to the solar cell
3 is smaller than the amount of power generated by the solar
cell 3, the solar cell 3 may generate the power in
proportion to the total power consumption amount of the DC
devices 5.
The storage battery 16 is charged and discharged in
response to a request from the power control device 70. The
storage battery 16 is managed by two levels, such as backup
level CLB and reserve charge level CLA (reference charge

level), which is a charge level higher than the backup level
CLB. The backup level CLB is set in such a way that an
electric power can be supplied for a predetermined period
when the supply of power is interrupted in case of emergency
such as a power failure in the nighttime or a fire. For
example, the backup level CLB is set to a charge level
corresponding to the amount of power that is used in case of
emergency. The storage battery 16 is controlled so that the
amount of electric power that is charged in the storage
battery does not become less than the backup level CLB
during normal other than the emergency.
The reserve charge level CLA is set to furnish
electric power that is consumed during the nighttime. For
example, the reserve charge level CLA is set to a charge
level corresponding to the amount of electric power per
night that is used during the nighttime. The storage
battery 16 periodically measures the charge level CL, and
outputs the measured charge level CL to the power control
device 7 0 through a signal line 52. The reserve charge
level CLA is set for each season. For example, the reserve
charge level CLA in spring and autumn is set to be lower
than in summer and winter. The set value can be changed
through an interface such as a touch panel or the like.
The controller 7 includes a DC/DC converter that
converts an electric power from the solar cell 3 into a low-
voltage DC power. By the DC/DC converter, the electric

power generated by the solar cell 3 is converted into a
power of a predetermined voltage. The controller 7 converts
an electric AC power from the commercial AC power source 2
into a DC power or converts the DC power from the solar cell
3 or the battery 16 into an electric AC power in response to
a request from the power control device 70. For example,
when the amount of DC power PWD used by the DC devices 5 is
larger than the solar power generation amount PWS by the
solar cell 3 and the DC power is insufficient, the AC power
is converted into a DC power by the controller 7, thereby
compensating for a deficit in the DC power.
Meanwhile, when a DC power usage amount PWD by the DC
devices 5 is smaller than the solar power generation amount
PWS by the solar cell 3 and an excess of power is generated
by the solar cell 3, the remaining DC power is converted
into an AC power by the controller 7. The controller 7
measures an AC-DC power amount obtained by converting the AC
power into a DC power and a DC-AC power amount obtained by
converting the DC power into an AC power, and outputs these
measured power amounts to the power control device 70
through the signal line 53.
The power control device 70 will now be described with
reference to Fig. 2.
As shown in Fig. 2, the power control device 70
includes an operation device 71; a communications unit 72
for performing information communications with an external

device including the solar cell 3, the control unit 7 and
the storage battery 16; a solar power generation amount
storage unit 73; a AC-DC power amount storage unit 74; a
battery charge level storage unit 75; a DC power usage
amount storage unit 76; and a battery reference value
storage unit 77.
The communications unit 72 receives information such
as the solar power generation amount PWS, the charge level
CL, the AC-DC power amount, and the DC-AC power amount
outputted from the solar cell 3, the storage battery 16 and
the control unit 7 through the signal lines 51 to 53. In
addition, the communications unit 72 outputs the information
to the computation device 71. Furthermore, the
communications unit 72 transmits operating instructions,
transmitted from the computation device 71, to the solar
cell 3, the storage battery 16 and the control unit 7.
The operation device 71 performs power control process,
daytime power control process, nighttime power control
process, and charge level recovery estimation process. The
operation device 71 creates power generation change data DTA,
total power generation amount per day DTB, and charge level
change data DTC based on the amount of solar power
generation amount PWS and the charge level CL of the storage
battery 16. The power generation change data DTA is data
including time points when the solar power generation amount
PWS and the solar power generation amount PWS at each time

point, and indicates variation in the solar power generation
amount PWS depending on the time point. The total power
generation amount per day DTB indicates the total solar
power generation amount by the solar cell 3 on that date.
The charge level change data DTC indicates variation in the
charge level CL depending on the time point.
The solar power generation amount storage unit 73
stores the power generation change data DTA and the total
power generation amount per day DTB as solar power
generation data DT. The solar power generation data DT is
maintained for several years. The solar power generation
data DT is used as reference data when estimating change in
the solar power generation amount PWS. The AC-DC power
amount storage unit 74 stores an AC-DC power amount and a
DC-AC electric power amount.
The battery charge level storage unit 75 stores the
charge level CL that is actually measured and the charge
level change data DTC. The DC power usage amount storage
unit 7 6 stores the DC power usage amount, i.e. the amount of
electric power that is consumed by the DC devices 5. The
battery reference value storage unit 77 stores the backup
level CLG and the reserve charge level CLA.
Referring to Fig. 3, a sequence of the power control
process will now be described. This process is repeatedly
performed at every operation cycle by the power control
device 70.

The control of the storage battery 16 is performed
differently during the daytime and the nighttime. That is,
in step S110, it is determined whether or not it is the time
when solar power generation is available. When the
determination is affirmative, that is, it is determined to
be the daytime in step S110, "daytime power control process"
is performed in step S120. Meanwhile, when the
determination is negative in step S110, "nighttime power
control processing" is performed in step S130. In addition,
the time zone during which power generation by the solar
cell 3 is available is set to range from the time of sunrise
to the time of sunset. That is, the length of the time zone
varies depending on the season.
Referring to Fig. 4, a sequence of the "daytime power
control processing" performed by the power control device 70
will now be described. This process is repeatedly performed
at every operation cycle by the power control device 70.
In step S210, electric power from the solar cell 3 is
assigned to the supply of power to the DC devices 5
preferentially. When electric power from the solar cell 3
is in surplus or is not enough to be supplied to the DC
devices 5, the following electric power control is performed.
In step S220, the solar power generation amount PWS by
the solar cell 3 is compared with the total DC power usage
amount PWD (power consumption) by the DC devices 5. Here,
when it is determined that the solar power generation amount

PWS is greater than the total DC power usage amount PWD,
that is, when the solar power generation amount PWS is in
surplus, it is determined, in step S230, whether or not the
charge level CL of the storage battery 16 has reached a full
charge level CLC.
When the charge level CL of the storage battery 16 has
not reached the full charge level CLC, in step S240, the
electric power from the solar cell 3 is assigned to the
supply of power to the DC devices 5 and the surplus electric
power is assigned to the charge of the storage battery 16.
Here, the solar power generation amount PWS is equivalent to
the sum of the total DC power usage amount PWD by the DC
devices 5 and the charged power amount PWE in the storage
battery 16. Meanwhile, when the charge level CL of the
storage battery 16 has reached the full charge level CLC, in
step S250, the electric power from the solar cell 3 is
assigned to the supply of power to the DC devices 5, and the
surplus electric power of the solar power generation amount
PWS is supplied to the AC device 6 after being converted
from DC to AC by the controller 7. At this time, the solar
power generation amount PWS becomes equivalent to the sum of
the total DC power usage amount PWD by the DC devices 5 and
the amount of DC-to-AC converted electric power.
In step S220, when it is determined that solar power
generation amount PWS does not exceed the DC power usage
amount PWD, in step S2 60, it is determined whether or not

the charge level CL of the storage battery 16 is greater
than the reserve charge level CLA. That is, it is
determined whether or not the shortage in the DC power usage
amount PWD can be supplemented by the power from the storage
battery 16.
When it is determined in step S2 60 that the charge
level CL of the storage battery 16 is greater than the
reserve charge level CLA, in step S270, an amount of
electric power corresponding to the shortage in the DC power
usage amount PWD is discharged from the storage battery 16
to be supplied to the DC devices 5. At this time, the sum
of the solar power generation amount PWS and the amount of
electric power PWF that is discharged from the storage
battery 16 becomes equivalent to the total DC power usage
amount PWD by the DC devices 5.
When it is determined in step S2 60 that the charge
level CL of the storage battery 16 does not exceed the
reserve charge level CLA, in step S280, it is determined
whether or not the charge level CL of the storage battery 16
is greater than the backup level CLB.
When the determination in step S280 is affirmative,
there is performed in step S290 a process to determine
whether or not the charge level CL will recover up to the
reserve charge level CLA (hereinafter, referred to as
"charge level recovery estimation process"). Next, when the
decrease in the charge level CL is determined to be

temporary by the charge level recovery estimation process in
step S300, it is estimated that the charge level CL will
recover up to the reserve charge level CLA, and the storage
battery 16 is discharged as in step S270. When it is
determined in step S300 that the decrease in the charge
level CL is determined not to be temporary, it is estimated
that the charge level CL will not recover, and in step S310,
an amount of electric power equal to the shortfall in the DC
power usage amount PWD is supplied to the DC devices 5 by
converting AC from the commercial AC power source 2 into DC.
At this time, the sum of the amount of AC-to-DC converted
electric power and the solar power generation PWS becomes
equivalent to the total DC power usage amount PWD by the DC
devices 5.
When it is determined in step S280 that the charge
level CL of the storage battery 16 does not exceed the
backup level CLB, in step S310, an amount of the electric
power that is equal to the shortfall in the DC power usage
amount PWD is supplied to the DC devices 5 by converting AC
from the commercial AC power source 2 into DC, and the
charge level CL is maintained at the backup level CLB.
Referring to Fig. 5, a sequence of the nighttime power
control process will now be described. This process is
repeatedly performed at every operation cycle by the power
control device 70. Since power generation by the solar cell
3 is not performed during the nighttime, electric power from

the storage battery 16 and the commercial AC power source 2
is supplied to the DC devices 5. When surplus electric
power is stored in the storage battery 16, discharge from
the storage battery 16 is performed preferentially.
Specifically, in step S320, it is determined whether
or not the charge level CL of the storage battery 16 is
greater than the backup level CLB. When the determination
in step S320 is affirmative, in step S330, the discharge
from the storage battery 16 is performed and electric power
is supplied to the DC devices 5. In contrast, when it is
determined to be negative in step S320, in step S340,
electric power is supplied to the DC devices 5 by converting
AC from the commercial AC power source 2 to DC.
Referring to Fig. 6, a sequence of "charge level
recovery estimation process" performed in the daytime power
control process will now be described. The charge level
recovery estimation process is performed in the daytime when
the solar power generation amount PSW is less than the DC
power usage amount PWD by the DC devices 5 and in the case
in which the charge level CL of the storage battery 16 does
not exceed the reserve charge level CLA and is greater than
the backup level CLB.
First, in step S410, it is determined whether or not
there was a time point on that date at which the charge
level CL exceeds the reserve charge level CLA before a time
point at which the charge level recovery estimation

processing is started. When the determination is
affirmative, it is estimated that the solar power generation
amount PWS, which was on an increasing trend, is on a
decreasing trend.
Meanwhile, when it is determined in step S410 that
there was no time point at which the charge level CL exceeds
the reserve charge level CLA, it is estimated to be a time
point when a small amount of solar power is generated
because of the sun not having risen or it being overcast
after the time point at which power generation was started.
Therefore, the charge level recovery estimation process is
stopped, and step S460 determines that the decrease in the
charge level CL is not temporary.
When the determination in step S410 is affirmative,
all of past charge level change data DTC on the same month
as that of the date when the daytime power control process
is performed are retrieved in step S420. In addition, when
there are no past data, reference data which has been stored
in advance is retrieved as reference data.
Afterwards, in step S430, the charge level change data
DTC on the corresponding processing date is compared with
the retrieved past charge level change data DTC, and data
similar to the charge level change data DTC on the
corresponding processing date is selected. The
determination as to whether or not the data is similar is
made based on whether or not the difference between the time

point at which the charge level CL on the corresponding
processing date first became higher than the reserve charge
level CLA (hereinafter, referred to as "reserve charge level
exceed time") and the time point at which the charge level
CL exceeds the reserve charge level in the past charge level
trend data DTC is within a predetermined tolerance range.
In addition, when a plurality of similar data is selected
from among the past charge level change data DTC, one
closest to the time point at which the charge level CL
became lower than the reserve charge level CLA is selected.
In step S440, it is determined whether or not there is
a time when the charge level CL has increased and reached
the full charge level CLC in the charge level change data
DTC that is selected as the similar data (a first
determination), and whether or not a time period until the
charge level CL reaches the full charge level CLC exceeds a
predetermined time period (a second determination). When
the first determination and the second determination are
affirmative, "the decrease in the charge level CL is
temporary" is outputted (step S450). That is, based on that
the time it takes to reach the full charge level CLC exceeds
the predetermined time period in a case similar to the
charge level change data DTC on the corresponding processing
date, it is estimated that the charge level CL on the
corresponding processing date also reaches the full charge
level CLC later or that the charge level CL is reversed on

the increasing trend. In contrast, when at least one of the
first determination and the second determination is negative,
output is made to the effect that the decrease in the charge
level CL is not temporary (step S460) .
Referring to Fig. 7, there will be described the
changes of a variety of parameters with respect to an
example of the control state of the energy storage system
100. Fig. 7 shows profiles when it is clear all day and
power generation is ideally performed by the solar cell.
At time point tl, the nighttime power control process
is performed. At this time, the DC power usage amount PWD
by the DC devices is furnished from the discharge from the
storage battery 16 since the discharge is available with the
charge level CL being greater than the backup level CLB.
At time point t2, the daytime power control process is
started. Power generation by the solar cell 3 is performed
from this time point, and power generated by the solar cell
3 is supplied to the DC devices 5. A shortage in electric
power occurs since the solar power generation amount PWS is
less than the DC power usage amount PWD by the DC devices 5.
This shortfall in the electric power is supplemented by the
commercial AC power source 2. In addition, in the daytime,
the discharge is not performed in principle when the charge
level CL does not exceed the reserve charge level CLA.
At time point t3, the solar power generation amount
PWS by the solar cell 3 exceeds the DC power usage amount

PWD by the DC devices 5, and the solar power generation
amount PWS is in surplus. The storage battery 16 is charged
with the surplus electric power. Then, the solar power
generation amount PWS increases with the rising of the sun,
and the charge level CL of the storage battery 16 also
increases.
At time point t4, the charge level CL of the storage
battery 16 reaches the full charge level CLC, no further
charge can be performed. The charge level CL remains in the
state of the full charge level CLC. At this time, while the
solar power generation amount PWS exceeds the DC power usage
amount PWD by the DC devices 5, the solar cell 3 is applied
with a load that is smaller than its capacity to generate
electric power. Therefore, the solar power generation
amount PWS becomes equivalent to the total DC power usage
amount PWD by the DC devices 5.
At time point t5, the solar power generation amount
PWS by the solar cell 3 becomes less than the DC power usage
amount PWD by the DC devices 5, and it becomes impossible to
cover the DC power usage amount PWD by the DC devices 5 with
the solar power generation amount PWS alone. At this time,
the discharge from the storage battery 16 is performed, and
the shortage in the amount of electric power is supplemented.
Afterwards, since the solar power generation amount PWS
decreases as the sun sets and the shortage in the electric
power increases, the charge level CL of the storage battery

16 also becomes lower.
At time point t6, the nighttime power control process
is started. Since the power generation by solar cell 3 is
not performed, power discharged from the storage battery 16
is supplied to the DC devices 5. Afterwards, the DC power
usage amount PWD by the DC devices 5 increases, and the
discharge amount from the storage battery 16 also increases.
At midnight, the DC power usage amount PWD by the DC devices
5 is lowered to standby power. In the example shown in Fig.
7, since a sufficient amount of electric power was charged
in the storage battery 16 in the daytime, the DC power usage
amount PWD by the DC devices 5 is furnished by the discharge
from the storage battery 16 in the nighttime.
Referring to Fig. 8, there will be described the
changes of the parameters with respect to another example of
the control state of the energy storage system 100. Fig. 8
shows profiles when power generation by the solar cell 3 is
temporarily decreased in response to a change in weather
conditions.
At time point tl, since the charge level CL is greater
than the backup level CLB, the DC power usage amount PWD by
the DC devices 5 is furnished by the discharge from the
storage battery 16. Afterwards, the charge level CL is
gradually lowered and approaches the backup level CLB.
At time point t2, the charge level CL is lowered to
the backup level CLB. At this time, discharge is not

permitted so that the charge level CL of the storage battery
16 is not lowered below the backup level CLB, and the charge
level CL is kept at the backup level CLB. At this time, the
DC power usage amount PWD by the DC devices 5 is supplied by
the electric power of the commercial AC power source 2.
At time point t3, the daytime power control process is
started. From this time point, power generation by the
solar cell 3 is performed, and the electric power generated
by the solar cell 3 is supplied to the DC devices. At this
time point, a shortage in the electric power occurs since
the solar power generation amount PWS is less than the DC
power usage amount PWD by the DC devices 5. In addition,
since the charge level CL has not reached the reserve charge
level CLA, the shortage in the electric power is furnished
from the commercial AC power source 2.
At time point t4, the solar power generation amount
PWS by the solar cell 3 exceeds the DC power usage amount
PWD by the DC devices 5, and becomes in surplus. The
storage battery 16 is charged with the surplus electric
power. Then, the solar power generation amount PWS
increases with the rising of the sun, and the charge level
CL of the storage battery 16 also increases. Afterwards,
the charge level CL exceeds the reserve charge level CLA.
At time point t5, the solar power generation amount
PWS by the solar cell 3 becomes less than the DC power usage
amount PWD by the DC devices 5. That is, the solar power

generation amount PWS decreases in response to a change in
weather conditions. At this time, the discharge from the
storage battery 16 is performed since it becomes impossible
to cover the DC power usage amount PWD by the DC devices 5
with the solar power generation amount PWS alone.
At time point t6, the charge level CL of the storage
battery 16 is decreased to the reserve charge level CLA. At
this time, "charge level recovery estimation process" is
performed to determine whether or not the decrease in the
charge level CL is temporary. This example shows a case in
which the charge level change data DTC of the processing
date is compared with the charge level change data DTC
selected from the data on the same month in the past, and
the charge level CL is determined to recover up to the
reserve charge level CLA. In this case, the discharge is
performed to a level that is lower than the reserve charge
level CLA. Afterwards, the decreasing trend of the solar
power generation amount PWS is reversed and becomes on an
increase trend.
At time point t7, the solar power generation amount
PWS exceeds the DC power usage amount PWD by the DC devices
5. At this time, the storage battery 16 is charged with the
surplus electric power of the solar power generation amount
PWS. Afterwards, the charge level CL gradually increases,
and exceeds the reserve charge level CLA again.
At time point t8, the solar power generation amount

PWS becomes less than the DC power usage amount PWD by the
DC devices 5, and it becomes impossible to cover the DC
power usage amount PWD by the DC devices 5 with the solar
power generation amount PWS alone. At this time, power is
discharged from the storage battery 16 to supplement the
shortage in the amount of electric power.
At time point t9, the nighttime power control process
is started. Electric power discharged from the storage
battery 16 is supplied to the DC devices 5. Afterwards, the
DC power usage amount PWD by the DC devices 5 increases, and
the discharge amount from the storage battery 16 also
increases so that the charge level CL is lowered.
Referring to Fig. 9, there will be described the
changes of the parameters with respect to a further example
of the control state of the energy storage system 100. Fig.
9 shows profiles when weather conditions are unstable and
thus the amount of electric power generated by the solar
cell 3 is small.
At time point tl, the charge level CL is on the backup
level CLB. Therefore, the DC power usage amount PWD by the
DC devices 5 at midnight is furnished by electric power from
the commercial AC power source 2.
At time point t2, the daytime power control process is
started. From this time point, power generation by the
solar 3 is performed, and the generated electric power is
supplied to the DC devices 5. Since the solar power

generation amount PWS is less than the DC power usage amount
PWD by the DC devices 5, there is a shortage in electric
power. This shortage in the electric power is supplemented
by the commercial AC power source 2.
At time point t3, the solar power generation amount
PWS by the solar cell 3 exceeds the DC power usage amount
PWD by the DC devices 5, and becomes in surplus. The
storage battery 16 is charged with the surplus electric
power. After that, the solar power generation amount PWS
increases with the rising of the sun, and the charge level
CL of the storage battery 16 increases also. Afterwards,
the charge level CL exceeds the reserve charge level CLA.
At time point t4, the solar power generation amount
PWS by the solar cell 3 becomes less than the DC power usage
amount PWD by the DC devices 5. That is, the solar power
generation amount PWS decreases in response to a change in
weather conditions. At this time, the discharge from the
storage battery 16 is performed since it becomes impossible
to cover the DC power usage amount PWD by the DC devices 5
with the solar power generation amount PWS alone.
At time point t5, the charge level CL of the storage
battery 16 is lowered to the reserve charge level CLA. At
this time, "charge level recovery estimation process" is
performed to determine whether or not the decrease in the
charge level CL is temporary. In this example, the charge
level change data DTC of the processing date is compared

with the charge level change data DTC selected from the data
on the same month in the past, and it is estimated that the
charge level CL does not recover up to the reserve charge
level CLA. Therefore, any discharge which makes the charge
level CL below the reserve charge level CLA is not permitted.
That is, at this time point, the discharge of the storage
battery 16 is interrupted, and a shortage in the electric
power is supplemented by the commercial AC power source 2.
At time point t6, the solar power generation amount
PWS by the solar cell 3 exceeds the DC power usage amount
PWD by the DC devices 5. That is, the solar power
generation amount PWS recovers and increases. The storage
battery 16 is charged with the surplus electric power of the
solar power generation amount PWS. Afterwards, the charge
level CL gradually increases.
At time point t7, the solar power generation amount
PWS by the solar cell 3 becomes less than the DC power usage
amount PWD by the DC devices 5, and it becomes impossible to
cover the DC power usage amount PWD by the DC devices 5 with
the solar power generation amount PWS alone. At this time,
the discharge from the storage battery 16 is performed to
supplement the shortage in the amount of electric power.
At time point t8, the nighttime power control process
is started. Electric power discharged from the storage
battery 16 is supplied to the DC devices 5. Afterwards, the
DC power usage amount PWD by the DC devices 5 increases, and

the discharge amount from the storage battery 16 also
increases to decrease the charge level CL.
At time point t9, the charge level CL is lowered to
the backup level CLB. In order to maintain the backup level
CLB, the discharge from the storage battery 16 is not
permitted. The shortage in the electric power is
supplemented by the commercial AC power source 2 .
The following effects can be obtained from the energy
storage system 100 in accordance with the present embodiment.
(1) In the present embodiment, in the daytime, the
discharge from the storage battery 16 to the DC devices 5 is
permitted when the solar power generation amount PWS is less
than the DC power usage amount PWD by the DC devices 5 and
"he charge level CL of the storage battery 16 is higher than
the reserve charge level CLA. In the daytime, when the
solar power generation amount PWS is less than the DC power
usage amount PWD by the DC devices 5 and the charge level CL
of the storage battery 16 is not greater than the reserve
charge level CLA, the discharge from the storage battery 16
to the DC devices 5 is not allowed.
At night, the discharge from the storage battery 16,
which makes the charge level CL of the storage battery 16
lower than the reserve charge level CLA, is permitted.
In this configuration, in the daytime, the storage
battery 16 is charged with the electric power from the solar
cell 3, and the discharge from the storage battery 16 is

permitted when power generated by the solar cell 3 does not
exceed the DC power usage amount PWD by the DC devices 5 to
cause a shortage in the electric power. Meanwhile, if the
charge level CL of the storage battery 16 is not greater
than the reserve charge level CLA, the discharge from the
storage battery 16 is not permitted so that electric power
to be used at night is ensured. At night, the capacity of
the storage battery 16 is permitted to drop below the
reserve charge level CLA, and the electric power stored in
the storage battery 16 during the daytime is used. In this
way, when the electric power is supplied by the storage
battery 16 in the nighttime, it is possible to suppress the
case where electric power cannot be supplemented by the
discharge from the storage battery 16.
(2) In the present embodiment, in the daytime and the
nighttime, the electric power is supplied to the DC devices
5 from at least one of the solar cell 3 and the commercial
AC power source 2 so that the charge level CL of the storage
battery 16 does not drop below the backup level CLB. In
addition, in an emergency in which the electric power is not
supplied from the solar cell 3 and the commercial AC power
source 2 to the DC devices 5, the discharge from the storage
battery 16, which makes the charge level CL lower than the
backup level CLB, is permitted.
According to this configuration, the charge level CL
of the storage battery 16 is maintained at a level equal to

or higher than the backup level CLB throughout the daytime
and the nighttime. Meanwhile, in an emergency in which the
electric power is not supplied from the commercial AC power
source 2 and the solar cell 3, the discharge from the
storage battery 16 is permitted. Therefore, the electric
power can be furnished to the DC devices 5 even in a power
supply emergency.
(3) In the present embodiment, in the daytime, when
the amount of the power generation of the solar cell 3 drops
below the DC power usage amount PWD by the DC devices 5 and
the charge level CL of the storage battery 16 is changed to
a lower level from a higher level than the reserve charge
level CLA, the following determination process is performed.
That is, based on the past charge level change data DTC, in
which the charge level CL is kept at the full charge level
CLC for a predetermined period or more, it is determined
whether or not the charge level CL will recover up to the
reserve charge level CLA before night even though the
permission to discharge from the storage battery 16 is
maintained. When the charge level CL is determined to
recover by this determination, the permission to supply
electric power from the storage battery 16 to the DC devices
5 is maintained.
According to this configuration, in the daytime, when
the amount of electric power generated by the solar cell 3
is less than the DC power usage amount PWD by the DC devices

5 and the charge level CL of the storage battery 16 is
changed to a lower level from a higher level than the
reserve charge level CLA, the following determination
process is performed. That is, the past charge level change
data DTC, in which the charge level CL is kept at the full
charge level CLC for the predetermined period or more, is
compared with the charge level change data DTC on the date
of performing the determination process. When the charge
level CL is determined to recover up to the reserve charge
level CLA, the permission to discharge from the storage
battery 16 to the DC devices 5 is maintained. That is, in
the daytime, when the charge level CL is lower than the
reserve charge level CLA, typical process does not permit
discharge from the storage battery 16 to ensure enough
electric power to be used in the nighttime. In contrast,
when the charge level CL is expected to recover even if the
discharge is continuously permitted, the discharge from the
storage battery 16 to the DC devices 5 is permitted. with
such process, the capacity stored in the storage battery 16
can be optimally used depending on the situation in which
the charge level CL of the storage battery 16 becomes lower,
and thus the use • of the electric power from the commercial
AC power source 2 can be reduced.
(4) In the charge level recovery estimation process,
based on the similarity between the charge level change data
DTC indicating the change of the charge level CL of the

storage battery 16 in the past and the charge level change
data DTC on the date when the determination is made, a past
date having a charge level change which approximates the
change of the charge level CL is selected. Further, it is
determined whether or not the full charge level period
during which the charge level CL reaches the full charge
level CLC in the charge level change data DTC on the
selected past date is equivalent to a predetermined period
or more. In the determination, when the full charge level
period is determined to be the predetermined period or more,
the charge level CL is determined to recover and the
discharge from the storage battery 16 to the load devices 5
is continuously permitted.
According to this configuration, in the determination
that the charge level CL will recover, as described above,
the past charge level change data DTC, which approximates
the charge level change data on the date when the
determination process is performed, is selected. In
addition, in the charge level CL on the date of the selected
charge level change data DTC, it is determined whether or
not the full charge level period is equivalent to the
predetermined time period or more. That is, since it is
determined whether or not the charge level on the date of
the determination will recover later based on the past data
on the date including the data approximate to that on the
date of the determination, the accuracy of the determination

can be improved.
(5) In the present embodiment, a charge level
corresponding to the amount of electric power per night that
is used in the nighttime is set as the reserve charge level
CLA. According to this configuration, the electric power
that is used in the nighttime can be furnished using the
amount of electric power that is charged in the storage
battery 16 in the daytime, and thus the use of the electric
power that is supplied from the commercial AC power source 2
can be reduced.
(Other embodiments)
The energy storage system 100 of the present invention
are not.limited to the aforementioned embodiment, but may be
modified as the following embodiments. In addition, the
following modifications are not limited to the
aforementioned embodiment, but may be implemented by
combining the modifications.
In the aforementioned embodiment, the backup level CLB
is set to the charge level CL that corresponds to the amount
of electric power in an emergency in which the electric
power is not supplied from the solar cell 3 and the
commercial AC power source 2. Instead of the above, the
backup level CLB may be set to a value that is greater than
the charge level CL that corresponds to the amount of
emergency electric power. In this case, it is possible to
supply more electric power in an emergency.

In the aforementioned embodiment, the reserve charge
level CLA is set to a level corresponding to the amount of
electric power consumed at night. Instead of the above,
nowever, the reserve charge level CLA may be set to a level
that is lower than the amount of electric power consumed at
night. With this configuration, part of the electric power
that is used at night is supplemented by the amount of
electric power charged in the storage battery 16 in the
daytime, so that the use of the electric power supplied from
the commercial AC power source 2 can be reduced. The above
is effective when the maximum capacity to be charged in the
storage battery 16 is less than the total amount of electric
power that is used at night.
In addition, the reserve charge level CLA may be set
as follows. Specifically, the reserve charge level CLA is
set to be equivalent to the amount of electric power
obtained by deducting the amount of electric power of the
commercial AC power source 2 that is used in the low-price
time zone from the amount of electric power to be used at
night. In this case, in the low-price time zone, the
electric power from the commercial AC power source 2 is used.
The reasons for the above are as follows.
The electricity rate of the commercial AC power source
2 is divided into normal rate and low rate depending on the
time zones. Specifically, one day is divided into a normal
time zone in which the electricity rate is normal, and a

low-price time zone in which the electricity rate is low.
In consideration of this, in the present invention, the
reserve charge level CLA is set to a value corresponding to
the amount of electric power obtained by deducting the
amount of low-price electric power of the commercial AC
power source 2 that is used in the low-price time zone from
the amount of electric power that is used at night. That is,
in the low-price time zone, the electric power is set to be
insufficient, and the electric power from the commercial AC
power source 2 is used in the low-price time zone.
Accordingly, as for the electric power used at night, the
electric power supplied at the low electricity rate is
effectively used, so that thus the electricity costs can be
reduced.
In the "charge level recovery estimation process" of
the foregoing embodiment, data similar to that on the date
of the determination process is selected and the
determination is made by comparing the charge level change
data DTC on the determination process date with past charge
level change data DTC. However, the following process may
be performed instead of the above.
Specifically, the determination as to whether or not
the charge level CL will recover is also based on the solar
power generation amount PWS. Thus, it is determined the
similarity between power generation amount change data DTA
indicating a past change in the solar power generation

amount PWS and power generation amount change data DTA on a
date when the determination process is performed, and a past
date having the change in the solar power generation amount
PWS approximating the power generation amount change data
DTA on the date of the determination is selected. It is
determined whether or not the full charge level period
during which the charge level reaches the full charge level
in the charge level change data on the selected date is
equivalent to the predetermined period or more. When the
full charge level period is determined to be equivalent to
the predetermined period or more, the charge level may be
determined to recover and the discharge from the storage
battery to the load device may be permitted. In the
similarity determination process, the similarity is
determined based on whether or not the time point required
for the solar power generation amount PWS to reach a
predetermined threshold on the determination process date
and the time point for the solar power generation amount PWS
to reach a predetermined threshold of in the selected past
power generation change data DTA are within a predetermined
tolerance range. When the time points of the both data are
within the predetermined tolerance range, both data are
determined to be similar. When the time points are not
within the predetermined tolerance range, the data are
determined not to be similar. In addition, in the
similarity determination process, instead of the above, the

similarity may be determined based on the curve data of both
change data.
According to this configuration, whether or not the
charge level CL on the date of the determination will
recover later is determined based on the past data on the
date having data similar to that on the date of the
determination, and thus the accuracy of the determination
can be improved.
Tn the "charge level recovery estimation process" of
the foregoing embodiment, the following recovery of the
charge level CL is estimated by comparing the change in the
charge level CL with the past data. This estimation is
performed on the assumption that the charge level CL is
associated with the solar power generation amount PWS.
However, the factors of decreasing the charge level CL
are regarded to include a decrease in the solar power
generation amount PWS and an increase in the DC power usage
amount PWD by the DC devices 5. Furthermore, in
consideration of this, in place of or in addition to the
foregoing "charge level recovery estimation process", the
following process may be performed.
Specifically, when the charge level recovery
estimation process is performed, it is determined whether or
not there was a time point when the charge level CL exceeds
the reserve charge level CLA on that date prior to the time
point when the charge level recovery estimation processing

was started, as in step S410. Here, when the determination
is affirmative, the past power consumption change data of
the DC devices 5, which was recorded over the same period as
the date of the estimation process, is retrieved. From
among the power consumption change data, there is selected a
past date having power consumption change data from a given
time (e.g. 6 A.M.) to the time when the estimation process
is started which is similar to that on the date. In
addition, in the charge level change data on the selected
date, it is determined whether or not there was a time when
the charge level CL increases to reach the full charge level
GLC (first determination) and whether or not the period
during which the charge level CL reaching the full charge
level CLC exceeds a predetermined time (second
determination). When both the first determination and the
second determination are affirmative, it is output that a
decrease in the charge level CL is temporary. When at least
one of the first determination and the second determination
is negative, it is output that the decrease in the charge
level CL is not temporary.
In the foregoing embodiment, in the step S410 of
"charge level recovery estimation process," it is determined
whether or not there was a time point when the charge level
CL exceeds the reserve charge level CLA before a time point
when the charge level recovery estimation process was
started. However, instead of the above, the following

determination process may be performed. That is, in the
same step, it may be determined whether or not the time when
the charge level recovery estimation process was started is
after a predetermined time. In this case, the predetermined
time is set such that, after the charge level CL is changed
to increase after the predetermined time, a sufficient time
is ensured until the charge level CL exceeds the reserve
charge level CLB. Since it is taken as a given that the
charge level CL will not recover up to the charge level CLB
when the charge level recovery estimation process is started
after 5 P.M., the predetermined time is set to a time point,
such as 0 P.M. In addition, the predetermined time may be
set depending on the season.
In the foregoing embodiment, in the step S220 of
"daytime power control process," the solar power generation
amount is compared with the total DC power usage amount by
the DC devices. In this case, the DC power usage amount by
the DC devices may be defined as the amount including the
amount of DC-to-AC converted electric power.
In the foregoing embodiment, the time zone, in which
power generation by the solar cell 3 is possible, is set as
a time zone from the time of sunrise to the time of sunset.
However, instead of the above, the corresponding time zone
may be set to a time zone from a predetermined time point
after several hours from sunrise to another predetermined
time point before several hours from sunset.

While the invention has been shown and described with
respect to the embodiments, it will be understood by those
skilled in the art that various changes and modification may
be made without departing from the scope of the invention as
defined in the following claims.

We Claim:
1. An energy storage system comprising a storage battery,
wherein:
when a charge level of the storage battery is not
greater than a reference charge level, a discharge from the
storage battery to a load device is permitted or prohibited
depending on a time zone.
2. The energy storage system of claim 1, further
comprising a solar cell and a commercial AC power source,
wherein the storage battery is charged with electric
power from the solar cell,
wherein electric power from at least one of the solar
cell, the commercial AC power source, and the storage
battery is supplied to the load device,
wherein, in the daytime, when an amount of electric
power generated by the solar cell is less than an amount of
electric power consumed by the load device and the charge
level of the storage battery is higher than the reference
charge level, the discharge from the storage battery to the
load device is permitted,
wherein, in the daytime, when the amount of electric
power generated by the solar cell is less than the amount of
electric power consumed by the load device and the charge
level of the storage battery is not higher than the

reference charge level, the discharge from the storage
battery to the load device is not permitted, and
wherein the discharge from the storage battery which
makes the charge level of the storage battery lower than the
reference level is permitted at night.
3. The energy storage system of claim 2, wherein a backup
level is set as a reference value of the charge level of the
storage battery, the backup level being lower than the
reference charge level and corresponding to an amount of
electric power that is used in an emergency in which
electric power is not supplied from the solar cell nor the
commercial AC power source to the load device,
wherein supply of electric power from at least one of
the solar cell and the commercial AC power source to the
load device is performed so that the charge level of the
storage battery is equal to or higher than the backup level
in the daytime and at night, and
wherein, in the emergency, the discharge from the
storage battery which makes the charge level lower than the
backup level is permitted.
4. The energy storage system of claim 2 or 3, wherein
when, in the daytime, the amount of electric power generated
by the solar cell is less than the amount of electric power
consumed by the load device and the charge level of the

storage battery is not greater the reference charge level,
determination process is performed based on past charge
level change data in which a time period during which the
charge level reaches a fully charged level is equivalent to
a predetermined time period or more, to determine whether or
not the charge level recovers to the reference charge level
before night in a state in which the discharge from the
storage battery is permitted, and when the charge level is
determined to recover by the determination process, the
permission of the discharge from the storage battery to the
load device is maintained.
5. The energy storage system of claim 4, wherein in the
determination process, based on similarity between charge
level change data indicating a past change in the charge
level of the storage battery, and charge level change data
on a date when the determination process is performed, a
past date having a charge level change which approximates a
charge level change on the date of the determination process
is selected; it is determined whether or not a full charge
level period during which the charge level reaches a full
charge level in the charge level change data on the selected
date is equivalent to a predetermined time period or more;
and when the full charge level period is determined to be
equivalent to the predetermined time period or more, it is
determined that the charge level will recover and the

permission of the discharge from the storage battery to the
load device is maintained.
6. The energy storage system of claim 4, wherein in the
determination process, based on similarity between power
generation amount change data indicating a past change in
power generation amount by the solar cell, and power
generation amount change data on a date when the
determination process is performed, a past date having a
power generation amount change approximating a power
generation amount change on the date of the determination
process is selected; it is determined whether or not a full
charge level period during which the charge level reaches a
full charge level in the charge level change data on the
selected date is equivalent to a predetermined time period
or more; and when the full charge level period is determined
to be equivalent to the predetermined time period or more,
it is determined that the charge level will recover and the
permission of the discharge from the storage battery to the
load device is maintained.
7. The energy storage system of any one of claims 2 to 6,
wherein the reference charge level is set to a level that
corresponds to an amount of electric power that is consumed
by the load device at night.

8. The energy storage system of any one of claims 2 to 7,
wherein, when time zones in which an electricity rate of the
commercial AC power source is determined include a normal
time zone in which the electricity rate is normal and a low-
price time zone in which the electricity rate is lower than
normal, the reference charge level is set to a level
corresponding to an amount obtained by deducting an amount
of electric power that is used in the low-price time zone
from an amount of electric power that is consumed by the
load device at night.

ABSTRACT

In an energy storage system, power from at least one
of a solar cell, a commercial AC power source, and a storage
battery is supplied to load devices. In the daytime, under
the condition that an amount of power generated by the solar
cell is less than an amount of power consumed by the load
device, the discharge from the storage battery to the load
device is permitted or prohibited if the charge level of the
storage battery is higher or is not higher than the
reference charge level. The discharge from the storage
battery which makes the charge level of the storage battery
lower than the reference level is permitted at night.