An apparatus for calculating an appropriate inverter output and an appropriate power storage capacity for a power storage system to be installed at a household, said power storage system to be installed at a household including an inverter configured to receive commercial AC power and convert the AC power to DC power, and a power storage device for storing electrical energy supplied with said commercial AC power by way of said inverter to supply the electrical energy to an electrical load, said apparatus for calculating an appropriate inverter output and an appropriate power storage capacity comprising: an interface for input of data obtained by measuring power usage during prescribed hours at the household where said power storage system is to be installed; and a display unit; said apparatus for calculating an appropriate inverter output and an appropriate power storage capacity being characterized in that it further comprises: an operation processing unit for finding annual average power usage at said household during said prescribed hours based on said measurement data, and determining the appropriate inverter output for the inverter of the power storage system to be installed at the household based on said average power usage at the household, and a storage unit for holding predetermined compensation factors corresponding to a plurality of inverter outputs, respectively, wherein said operation processing unit is configured so as to adopt said average power usage as the appropriate output for the inverter of the power storage system to be installed at the household, acquire from said storage unit a compensation factor corresponding to the determined appropriate inverter output, calculate an actual amount of use out of said power usage available from said power storage device by multiplying, by said acquired compensation factor, an amount of power used in one day during said prescribed hours obtained based on a result of measuring said power usage, determine the appropriate power storage capacity for said power storage device based on said actual amount of use, and cause a result of operation processing to be displayed on the display unit. an interface for input of data obtained by measuring power usage during prescribed hours at the household where said power storage system is to be installed; and a display unit; said apparatus for calculating an appropriate inverter output and an appropriate power storage capacity being characterized in that it further comprises: an operation processing unit for finding annual average power usage at said household during said prescribed hours based on said measurement data, and determining the appropriate inverter output for the inverter of the power storage system to be installed at the household based on said average power usage at the household, and a storage unit for holding predetermined compensation factors corresponding to a plurality of inverter outputs, respectively, wherein said operation processing unit is configured so as to adopt said average power usage as the appropriate output for the inverter of the power storage system to be installed at the household, acquire from said storage unit a compensation factor corresponding to the determined appropriate inverter output, calculate an actual amount of use out of said power usage available from said power storage device by multiplying, by said acquired compensation factor, an amount of power used in one day during said prescribed hours obtained based on a result of measuring said power usage, determine the appropriate power storage capacity for said power storage device based on said actual amount of use, and cause a result of operation processing to be displayed on the display unit.