Hydroelectricity is Japan's main renewable energy source, with an installed capacity of about 50 GW (including pumped storage) and a production of 69.2 TWh of electricity in 2009, making Japan one of the biggest hydroelectricity producers in the world. Most of Japanese hydroelectric power plants are pumped-storage plants. Conventional hydropower plants account for about 20 GW out of the total installed capacity as of 2007.
Conventional hydropower potential of Japan is considered to be almost fully developed, with little opportunity for further capacity increase. In recent years, almost exclusively pumped storage plants were commissioned, significantly increasing the ratio of pumped storage capacity over conventional hydro. The large capacity of pumped storage hydropower was built to store energy from nuclear power plants, which until the Fukushima disaster constituted a large part of Japan electricity generation. As of 2015, Japan is the country with the highest capacity of pumped-storage hydroelectricity in the world, with 26 GW of power installed. After the 2011 nuclear power shutdowns, pumped-storage plants have been increasingly used to balance the variable generation of renewable energy sources such as solar, which have been growing rapidly in recent years.
As of September 2011, Japan had 1,198 small hydropower plants with a total capacity of 3,225 MW. The smaller plants accounted for 6.6% of Japan's total hydropower capacity. The remaining capacity was filled by large and medium hydropower stations, typically sited at large dams. Cost per kilowatt-hour for power from smaller plants was high at ¥15-100, hindering further development of the energy source.