Pumped-storage projects differ from conventional hydroelectric projects. They normally pump water from a lower reservoir to an upper reservoir when demand for electricity is low. Water is stored in an upper reservoir for release to generate power during periods of peak demand. For example, in the summer water is released during the day for generating power to satisfy the high demand for electricity for air conditioning. At night, when demand decreases, the water is pumped back to the upper reservoir for use the next day.

These projects are uniquely suited for generating power when demand for electricity is high and for supplying reserve capacity to complement the output of large fossil-fueled and nuclear steam-electric plants. Start-up of this type of project is almost immediate, thus serving peak demand for power better than fossil-fueled plants that require significantly more start-up time. Like conventional projects, they use falling water to generate power, but they use reversible turbines to pump the water back to the upper reservoir. This type of project is particularly effective at sites having high heads (large differences in elevation between the upper and lower reservoir).

State Project	Name	Capacity	Head	Owner
California	Helms	1,050 MW	1,630 feet	PG&E Company
Virginia	Bath County			VEPCO

The 1,050-megawatt Helms pumped-storage project, operated by Pacific Gas and Electric Company in Fresno County, California, has a head of 1,630 feet - the highest in the United States. The largest operating, privately owned pumped storage-project is jointly owned by Virginia Electric and Power Company and Allegheny Generating Company in Bath County, Virginia. In addition, the largest non-federal, publicly owned pumped-storage project is part of the California Aqueduct Project and has a capacity of 1,275 megawatts. It is operated by the City of Los Angeles. The largest federally owned pumped storage project is the Tennessee Valley Authority's 1,530-megawatt Raccoon Mountain project on the Tennessee River in Tennessee.