Pumped Hydro Storage Explained: How the World's Biggest Battery Works
Updated October 5, 2026
Pumped hydro storage stores electricity by pumping water uphill to a higher reservoir when power is cheap or plentiful. When power is needed, the water flows back down through turbines to generate electricity. It is the oldest and largest form of grid-scale energy storage in the world.
How pumped hydro works
A pumped storage plant has two reservoirs at different heights joined by tunnels or a penstock, with reversible pump-turbines in between.
- Charging: the machines run as pumps, using grid electricity to lift water to the upper reservoir.
- Storing: the water sits uphill as gravitational potential energy, for hours or days.
- Discharging: water is released downhill through the same machines, now running as turbines, and they generate electricity.
The energy stored depends on two things: how much water there is and the height difference between the reservoirs, called the head. Higher head means more energy per liter, which is why the best sites are steep.
Efficiency
Modern pumped hydro plants typically return about 70% to 80% of the electricity used to pump the water. U.S. Energy Information Administration data has put average pumped storage round-trip efficiency at roughly 79%, a little below utility-scale lithium-ion batteries. Losses come from pump and turbine efficiency, friction in the waterways, and evaporation.
Scale: still the giant of grid storage
Pumped hydro still makes up the large majority of installed energy storage worldwide. Global capacity is around 180 GW, according to the International Hydropower Association. The United States has roughly 22 GW, much of it built in the 1970s and 1980s to pair with nuclear and coal plants. The largest U.S. facility, Bath County in Virginia, can deliver around 3 GW.
Duration is its big advantage. Many plants can run at full output for 6 to 20 hours or more, far longer than the 2 to 4 hours typical of lithium-ion battery projects.
Types of pumped storage
- Open-loop: connected to a river or natural water body.
- Closed-loop: two purpose-built reservoirs not connected to a river. This has lower environmental impact and is the focus of most new projects.
- Variable-speed: newer machines can vary pumping power, which lets the plant help balance the grid even while charging.
Advantages and drawbacks
Advantages: very long asset life (often 50 to 100 years), long duration, low cost per kWh stored at large scale, proven technology, and large rotating machines that provide inertia.
Drawbacks: it needs specific geography, carries high upfront capital costs, takes years to permit, and often needs 5 to 10+ years from concept to operation. Environmental reviews can be complex, especially for open-loop designs.
Pumped hydro vs. lithium-ion batteries
The two mostly complement each other. Batteries are modular, quick to build and can sit almost anywhere, which suits short-duration needs and fast response. Pumped hydro wins on long duration and lifespan but loses on siting and speed of deployment. Many grid plans include both.
Bottom line
Pumped hydro is a century-old idea that remains the backbone of grid storage. As grids add more wind and solar and need many hours of shifting, closed-loop pumped storage is getting a second look, with Australia, China and the U.S. all developing new projects.
Frequently asked questions
How efficient is pumped hydro storage?
Typically 70–80% round-trip, meaning 70–80% of the electricity used for pumping is recovered when the water is released.
How long can pumped hydro store energy?
Water can be held for days or longer with only small evaporation losses. Most plants are designed to discharge at full output for about 6 to 20 hours.
Why isn't more pumped hydro being built?
Suitable sites are limited, upfront costs are high, and permitting and construction often take a decade. Closed-loop designs are reducing some of those barriers.