Hydropower plays an important role as the backbone of America’s electrical grid. It is highly flexible and can rapidly respond to fluctuations in the demand for electricity with pumped storage. Often described as “giant batteries,” pumped storage hydropower (PSH) plants account for the bulk of utility-scale electrical energy storage in the United States and worldwide. According to the newly-released Hydropower Market Report, the existing 21.6 GW of PSH capacity make up 97 percent of U.S. utility-scale electricity storage.

With their ability to provide a wide range of ancillary services such as the ability to change output based on grid needs, PSH plants help ensure grid reliability. PSH uses pumps and turbines to draw energy from the power system and pump water into higher elevation reservoirs. This energy is returned to the power system as the pumped water is released into lower elevation reservoirs to generate electricity during periods of high electrical demand.

Pumped storage also supports the integration of variable renewables into the grid. More than 50 pumped storage hydropower projects, which function as large storage systems for other clean energy sources such as wind and solar, are in the various stages of planning and development, adding to the strength of our electric grid.

This map (above) from the Hydropower Market Report lays out the active PSH projects in the Federal Energy Regulatory Commission licensing process as of December 2014. These projects may add up to an additional 39 GW of total installed capacity, enough to power more than 17 million American homes. The map also displays a base layer showing the fraction of total installed generating capacity in each state that is attributed to wind or solar.

Because PSH is flexible and can support the integration of additional clean energy resources, pumped storage projects can help realize hydropower’s untapped resource potential in America. To learn more about the many facets of hydropower in addition to pumped storage, please visit the Water Power Program website.