A power storage system includes a power control unit including an inverter, and a battery for storing electrical energy supplied with commercial AC power by way of the inverter to supply the electrical energy to an electrical load. An apparatus for selecting specifications of a power storage system includes a data input interface unit for input of data obtained by measuring power usage during prescribed hours at a location of power consumption where the power storage system is to be installed, and an operation processing unit for finding annual average power usage during prescribed hours based on the measured data, and determining an output of the inverter based on the average power usage.