Pumped hydroelectric energy storage, more commonly referred to simply as “pumped storage,” converts electricity into potential energy by pumping water from a lower reservoir to an upper reservoir during off-peak periods. When power is needed, stored water is released to flow downhill through a turbine. Because pumped storage plants combine massive rotating equipment (hydro generators) with an ability to operate flexibly and respond quickly, they can offer a variety of grid-support (ancillary) services to a utility in addition to bulk power generation.

Pumped storage has been utilized for the past hundred years. As of 2021 there were 160,000 megawatts (MW) of pumped storage generating capacity installed around the world, including more than 60 plants each with a capacity of 1,000 MW or greater. 42 pumped storage plants are operating in the US today.

1.     Pumped storage hydro projects have some of the lowest cost per kilowatt-hour (kWh) of storage capacity.

2.    Pumped storage facilities have lifetimes exceeding 75 years, and likely closer to 100 years, and their storage capacity does not degrade. No other energy storage technology comes close to offering this kind of lifespan.

3.     With economical storage durations of eight hours or more, pumped storage can more easily qualify as firm (dependable) capacity, particularly as penetrations of renewable energy resources increase.

4.     Pumped storage facilities have long been providing the same kind of fast-response frequency regulation service that batteries, flywheels, and other emerging storage technologies are designed for.

5.     Pumped storage facilities have among the lowest levels of “embodied energy” per life cycle. In other words, compared with other energy storage technologies, much less energy is used in their construction compared with the amount of energy flowing in and out of them over their lifespans.