A power storage service system configured to control a power system using an assembled battery system (1, 1a) including a plurality of battery cells, the plurality of battery cells including first and second battery cells, the power storage service system comprising: a residual capacity measurer (210, 210a) configured to detect at least a residual capacity of each of the first and second battery cells, the residual capacity representing a residual quantity of dischargeable electric quantity in each of the first and second battery cells; a residual capacity equalizer (120, 120a) configured to decrease the residual capacity of the first battery cell by releasing an equalization current from the first battery cell; characterized by a residual capacity equalization current calculator configured to calculate a current value of the equalization current from the first battery cell; a self-discharge current difference calculator (223, 223a) configured to calculate at least a self-discharge current difference of the second battery cell from a criterion value by using a difference between the residual capacities of the first battery cell and the second battery cell detected by the residual capacity measurer (210, 210a) at a first point of time, a difference between the residual capacities of the first battery cell and the second battery cell detected by the residual capacity measurer (210, 210a) at a second point of time, and an integrated value of the equalization adjustment quantity for equalizing the residual capacities of the first and second battery cells between the first point of time and the second point of time; and a residual time calculator (320, 320a) configured to calculate a residual time of the assembled battery system (1, 1a) by using the residual capacity of each of the first and second battery cells detected by the residual capacity measurer (210, 210a), the self-discharge current difference of the second battery cell calculated by the self-discharge current difference calculator (223, 223a), and the current value of the equalization current calculated by the residual capacity equalization current calculator, wherein the residual time is a time it takes for an effective SOC range of the assembled battery system to become zero, the effective SOC range being obtained from a difference between an SOC limit range and a difference between the residual capacities of the first and second battery cells, the SOC limit range being a range of SOC in which each of the first and second battery cells is allowed to be charged or discharged. a residual capacity measurer (210, 210a) configured to detect at least a residual capacity of each of the first and second battery cells, the residual capacity representing a residual quantity of dischargeable electric quantity in each of the first and second battery cells; a residual capacity equalizer (120, 120a) configured to decrease the residual capacity of the first battery cell by releasing an equalization current from the first battery cell; characterized by a residual capacity equalization current calculator configured to calculate a current value of the equalization current from the first battery cell; a self-discharge current difference calculator (223, 223a) configured to calculate at least a self-discharge current difference of the second battery cell from a criterion value by using a difference between the residual capacities of the first battery cell and the second battery cell detected by the residual capacity measurer (210, 210a) at a first point of time, a difference between the residual capacities of the first battery cell and the second battery cell detected by the residual capacity measurer (210, 210a) at a second point of time, and an integrated value of the equalization adjustment quantity for equalizing the residual capacities of the first and second battery cells between the first point of time and the second point of time; and a residual time calculator (320, 320a) configured to calculate a residual time of the assembled battery system (1, 1a) by using the residual capacity of each of the first and second battery cells detected by the residual capacity measurer (210, 210a), the self-discharge current difference of the second battery cell calculated by the self-discharge current difference calculator (223, 223a), and the current value of the equalization current calculated by the residual capacity equalization current calculator, wherein the residual time is a time it takes for an effective SOC range of the assembled battery system to become zero, the effective SOC range being obtained from a difference between an SOC limit range and a difference between the residual capacities of the first and second battery cells, the SOC limit range being a range of SOC in which each of the first and second battery cells is allowed to be charged or discharged.