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.