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.