In simple terms
Hydropower is electricity made from moving water. A river or a reservoir sends water through a turbine, the turbine spins a generator, and the generator produces current.
It is the oldest large-scale renewable source of electricity and still the largest.
How it works
Two things set the output: how much water flows, and how far it falls. The drop is called head, and more of either means more power.
Water runs down a pipe called a penstock, pushes against the blades of a turbine and spins them. The turbine drives a generator, and the current goes out to the grid.
There are three main designs. Run-of-river plants use the current as it arrives. Storage plants hold water behind a dam and release it when it is wanted. Pumped storage pumps water uphill when power is cheap and lets it back down at peak demand, using more electricity than it makes, so it works as a battery rather than a source.
National shares vary enormously. Hydropower supplied 17 percent of global electricity in 2020, but only about 5.6 percent of US utility-scale generation in 2025. It meets most of the demand in 28 emerging and developing economies with 800 million people between them.
Why it matters
Hydropower provides almost half the world low-carbon electricity, more than all other renewables combined and 55 percent more than nuclear.
Output can be raised or cut within minutes, so it steadies grids that increasingly depend on wind and solar, which cannot be scheduled.
It also depends on rainfall. Drought, shrinking glaciers and changed river flows hit generation directly, and a single flood can destroy years of built capacity.
Where you’ll see it
- Dams and reservoirs on large rivers.
- Small run-of-river schemes serving mountain villages.
- Pumped storage sites pairing two reservoirs at different heights.
- National electricity statistics and the fuel mix printed on energy bills.
Example
A mountain river drops 200 meters over a few kilometers. A run-of-river plant diverts part of the flow through a penstock to a turbine at the bottom, generating power without flooding a valley behind a dam.
Often confused with
Pumped storage uses the same machinery but is not a source of new energy. It consumes more electricity pumping water uphill than it returns, and exists to move power from cheap hours to expensive ones.
Key facts
- Hydropower supplied 17 percent of global electricity generation in 2020, the third largest source after coal and natural gas.1
- It provides almost half of the world low-carbon electricity, 55 percent more than nuclear and more than all other renewables combined.1
- It meets the majority of electricity demand in 28 emerging and developing economies with a combined population of 800 million.1
- Output depends on the volume of water flowing and the elevation drop, known as head.2
- In 2025 hydropower supplied about 5.6 percent of US utility-scale electricity generation and 23.1 percent of US renewable electricity.2
Related concepts
In the news
Quick checkWhat is the difference between a run-of-river plant and a pumped storage plant?Show answer
A run-of-river plant generates from the river flow as it arrives. Pumped storage pumps water uphill using cheap power and releases it at peak demand, so it stores energy rather than producing it.
Sources
- International Energy Agency. Hydropower Special Market Report: Executive summary. Undated (accessed 18 September 2026)
- US Energy Information Administration. Hydropower explained. Undated (accessed 18 September 2026)
Editorially reviewed by Specialty Digest Editorial TeamLast reviewed September 21, 2026Researched and drafted with AI assistanceReport an issue