Pumped hydro storage plants serve an important role on electric power systems: they improve system-wide efficiency and reliability by allowing system operators to time-shift power generated during periods of low demand for electricity for use during high-demand periods.

Pumped hydro storage operates by using electrically powered turbines to force water uphill at night to fill a reservoir. During times of the day when electricity demand is high, the water is released to flow downhill through turbines to generate electricity. Although more electricity is used to force the water uphill at night than is produced when it flows downhill during the day, shifting the availability of power from overnight generation to serve daytime load adds significant value.

via Pumped storage provides grid reliability even with net generation loss – Today in Energy – U.S. Energy Information Administration (EIA).