A power calculation apparatus calculating amounts of power input to and output from a plurality of vehicles each including a battery, the power calculation apparatus comprising: a power-amount acquisition unit configured to acquire a power amount information regarding amounts of power input to and output from the plurality of vehicles, the amounts of power being detected by a plurality of measuring instruments provided in the plurality of vehicles or interposed between a power grid and the plurality of vehicles; a group setting unit configured to classify the plurality of measuring instruments based on an error information regarding errors possessed by the plurality of measuring instruments to set a plurality of measuring instrument groups each including equal to or more than a predetermined number of measuring instruments, the errors being measured in advance; and a power-amount calculation unit configured to calculate a total power input and output in each of the measuring instrument groups set by the group setting unit, based on the power amount information regarding amounts of power detected by equal to or more than the predetermined number of measuring instruments belonging to each of the measuring instrument groups, wherein the group setting unit sets the plurality of measuring instrument groups such that a mean value of errors of amounts of power detected by equal to or more than the predetermined number of measuring instruments belonging to a same measuring instrument groups is a predetermined value or less, and characterized in that the power-amount acquisition unit acquires a first power amount information regarding the amounts of power detected by the plurality of measuring instruments provided in the plurality of vehicles or a second power information regarding the amounts of power detected by the plurality of measuring instruments interposed between the power grid and the plurality of vehicles according to a setting for determining whether the first power amount information or the second power amount information is acquired when an instruction regarding the setting is input. a power-amount acquisition unit configured to acquire a power amount information regarding amounts of power input to and output from the plurality of vehicles, the amounts of power being detected by a plurality of measuring instruments provided in the plurality of vehicles or interposed between a power grid and the plurality of vehicles; a group setting unit configured to classify the plurality of measuring instruments based on an error information regarding errors possessed by the plurality of measuring instruments to set a plurality of measuring instrument groups each including equal to or more than a predetermined number of measuring instruments, the errors being measured in advance; and a power-amount calculation unit configured to calculate a total power input and output in each of the measuring instrument groups set by the group setting unit, based on the power amount information regarding amounts of power detected by equal to or more than the predetermined number of measuring instruments belonging to each of the measuring instrument groups, wherein the group setting unit sets the plurality of measuring instrument groups such that a mean value of errors of amounts of power detected by equal to or more than the predetermined number of measuring instruments belonging to a same measuring instrument groups is a predetermined value or less, and characterized in that the power-amount acquisition unit acquires a first power amount information regarding the amounts of power detected by the plurality of measuring instruments provided in the plurality of vehicles or a second power information regarding the amounts of power detected by the plurality of measuring instruments interposed between the power grid and the plurality of vehicles according to a setting for determining whether the first power amount information or the second power amount information is acquired when an instruction regarding the setting is input.