Pumped hydro facilities use off-peak electricity to pump water from a lower reservoir into one at a higher elevation. When the water stored in the upper reservoir is released, it is passed through hydraulic turbines to generate electricity.
The off-peak electrical energy used to pump the water up hill can be stored indefinitely as gravitational energy in the upper reservoir. Thus, two reservoirs in combination can be used to store electrical energy for a long period of time, and in large quantities.
Utilities generally prefer to operate large coal and nuclear power stations at full power all the time (referred to as "baseload generation"), so in the middle of the night, these plants often produce more power than is needed. Pumped hydro energy storage can be used to smooth out the demand for baseload generation from such plants, storing off-peak electricity during the night and releasing it during times of peak demand, usually in the afternoon and early evening. It can also be used to provide emergency power injection to the grid when a plant goes offline unexpectedly.