Pumped storage hydropower (PSH) is one of the most-common and well-established types of energy storage technologies and currently accounts for 95% of all utility-scale energy storage capacity in the United States. PSH facilities store and generate electricity by moving water between two reservoirs at different elevations. Vital to grid reliability, today, the U.S. pumped storage hydropower fleet includes about 22 gigawatts of electricity-generating capacity and 550 gigawatt-hours of energy storage with facilities in every region of the country.

A key player in creating a clean, flexible, and reliable energy grid, PSH provides energy storage and other grid services that can help to integrate additional renewable resources, such as wind and solar, with the power system. PSH is also the only currently commercialized technology for long-duration storage, which may become increasingly valuable as the power system evolves to include more variable renewables.

As demonstrated in the animation, water is pumped from the lower reservoir to the upper reservoir in times of high electricity supply, such as during the day, when electricity can be supplied by the sun’s charging of solar panels, and/or low demand. In times of reduced electricity supply and/or high demand, such as at night, when some electrical load remains but the sun is not shining and solar energy is inaccessible, water from the upper reservoir is released to the lower reservoir, generating electricity as it moves down through a turbine.


PSH plants operate much like conventional hydropower plants, except PSH has the ability to use the same water over and over again. To generate electricity when power from the plant is needed, water flows from the upper reservoir, because of gravity, through turbine(s) that rotate generator(s) to produce electricity. The water then flows into the lower reservoir where it remains until electricity demand lowers. When this occurs, the turbines spin backward to pump the water back into the upper reservoir so it can once again be used to generate electricity when needed.

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