Power storage systems include storage batteries and pumped-storage power systems. The Center for Low Carbon Society Strategy (LCS) already calculate the future manufacturing costs of the storage battery systems and storage battery capacity required for low carbon power supply systems in various scenarios for 2050. Here, we will present the calculated results of the pumped-storage power capacity required for sustainable future energy use and its cost. In Japan, the present amount of storage capacity of existing pumped-storage power plants is around 130GWh, however, the capacity is not sufficient for the future and also the capacity utilization factor is as low as 3%. According to our studies, it is possible to create 20,000 pumped-storage power plants with the storage capacity of 50MWh per plant (10MW, 5hours/cycle), and the total amount of storage capacity 1,000GWh can be expected (the capacity of installed pumped-storage power system: 200GW, and the storage capacity of 300TWh/y in case of the capacity utilization rate 17%). This result showed that there is the potential of the new pumped-storage power more than the storage battery capacity (360-510GWh) estimated to be necessary for the future low-carbon power supply system (80-85% reduction from the current CO₂ emissions originating from power supply in 2050). The power generation cost was calculated as 23yen/kWh.This is 1.4 times the cost of the storage battery. There is room for cost reduction due to increase in operation and water storage capacity in the future.