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Short answer
Pumped storage (pumped storage hydropower, PSH) stores electricity by moving water between two reservoirs at different elevations: water released from the upper reservoir runs through turbines to generate power, and when demand is low the turbines run in reverse to pump the water back up. It is a way of storing energy for the grid, not a source of new energy.
Definition and scope
The US Department of Energy (DOE) Water Power Technologies Office describes PSH as a type of hydroelectric energy storage with “a configuration of two water reservoirs at different elevations.” The same water is used “over and over again,” which is why DOE compares a PSH plant to a large battery, even though no battery chemistry is involved.
How it is used in practice: the pumped storage cycle
- Generating: water is released from the upper reservoir and flows down through turbines to produce electricity. It collects in the lower reservoir.
- Waiting: the water stays in the lower reservoir until electricity demand falls.
- Pumping: when demand is lower, the turbines “spin backward to pump the water back into the upper reservoir,” so it can generate electricity again when needed.
DOE describes the pumping step as using power, so the system moves energy in time rather than creating it.
Types: open-loop vs closed-loop pumped storage
DOE distinguishes two configurations by whether the reservoirs connect to an outside water body.
| Feature | Open-loop PSH | Closed-loop PSH |
|---|---|---|
| Connection to natural water | “An ongoing hydrologic connection to a natural body of water” | Reservoirs “are not connected to an outside body of water” |
| Equipment (DOE) | Turbine/pump and generator/motor move water and create electricity | Turbine/pump and generator/motor move water and create electricity |
Both rows of equipment are identical in DOE’s description: the difference is the water source and its surroundings, not the generating principle.
Where it matters for sector decisions: grid storage and energy supply
DOE calls PSH “the most dominant form of energy storage on the electric grid today” and says it plays an important role in bringing more renewable resources onto the grid. Figures DOE publishes:
- PSH accounts for 88% of all utility-scale energy storage in the United States, per the 2024 edition of the Hydropower Market Report.
- The country has 43 PSH plants.
- The US fleet has about 22 gigawatts (GW) of generating capacity and 550 gigawatt-hours (GWh) of energy storage.
- PSH is described as the dominant commercialized technology for long-duration storage.
The first known uses were in Italy and Switzerland in the 1890s; the United States first used PSH in 1930.
Capacity (GW) is not storage (GWh)
GW is how much power the fleet can deliver at a moment; GWh is how much energy it can hold. Dividing DOE’s two fleet figures (550 GWh by 22 GW) gives about 25 hours of full-power generation on average. That is our arithmetic on DOE’s rounded numbers, an illustration of why long-duration storage is measured in both units, not a DOE statement about any plant.
What this means for labs, clinics and supply planning
Editorial interpretation, not a DOE statement: pumped storage is a grid-scale asset operated by utilities, so a laboratory, clinic or supplier does not buy or operate it. It matters indirectly. DOE says PSH helps bring more renewable resources onto the grid, and facilities that depend on continuous power for cold storage, instruments or sterilisation draw on that grid. When reading an energy-supply claim, check whether it states power (GW) or stored energy (GWh) and which report and year it comes from.
Common misunderstandings
Is pumped storage a power source?
No. It returns energy that was first used for pumping; it shifts supply in time.
Is pumped storage a battery?
Only by analogy. DOE compares it to a large battery in function, but it stores energy as water elevation, not chemically.
Why do published shares of US storage differ?
Shares are tied to the year and report. DOE currently cites 88% from the 2024 Hydropower Market Report; older figures, such as 93% or 96%, still circulate in other sources. Cite the share together with its report and year.
FAQ
What is pumped storage in one sentence?
A hydroelectric system with two reservoirs at different elevations that generates power by releasing water downhill and stores energy by pumping it back up.
What is the difference between open-loop and closed-loop?
Open-loop systems keep a hydrologic connection to a natural body of water; closed-loop reservoirs are not connected to an outside water body.
Related energy topics
Other CASRAI entries on energy in research settings: cold-storage energy (research), open infrastructure energy efficiency and sustainable HPC.
Sources
- US Department of Energy, Water Power Technologies Office, Pumped Storage Hydropower, read 2026-11-08.
- US Department of Energy, Water Power Technologies Office, How Pumped Storage Hydropower Works, read 2026-11-08.
