In an electric power storage system connected to a commercial power system, a controller includes a deterioration price holding unit that holds discharge costs in association with depths of discharge of a storage battery, a price information obtaining unit that obtains a price of electricity, and a depth-of-discharge switching unit that discharges the storage battery at a depth of discharge at which the discharge cost held in association with the depth of discharge does not exceed a difference in the price, so as to properly determine the depth of charge and discharge in consideration of life properties of the battery and the difference in the price in the dynamically changing price of electricity.