Hydropower

Hydropower turns the potential energy of falling water into electricity. It is the oldest renewable source of power and still the largest. In 2025 it generated about 4,400 TWh, roughly 14% of world electricity, more than nuclear, wind or solar on their own. Global installed capacity, pumped storage included, is around 1,400 GW according to the International Hydropower Association. Hydro also offers something wind and solar can’t provide by themselves: large-scale, long-duration storage and dispatchable output that can follow demand within minutes. That flexibility makes it the backbone of grids in Norway, Brazil and Canada, and a more and more valuable partner for variable renewables elsewhere.

How hydropower works

Water held at height has potential energy. As it flows down a pipe called a penstock, pressure builds at the bottom. The water then passes through a turbine, whose runner turns that pressure and velocity into rotation. A synchronous generator on the turbine shaft produces alternating current at grid frequency, and a transformer steps up the voltage for transmission.

Output depends on just two site variables: the flow rate and the hydraulic head, which is the vertical distance the water falls.

Power scales linearly with both flow and head. A mountain site with a small stream and a 600 m drop can therefore match a lowland river with a big flow and a 6 m drop, although the two need completely different turbines. The micro-hydro calculator applies this formula to a small stream.

Turbine types

• Francis turbines are reaction turbines: water enters radially through adjustable guide vanes and leaves axially. They cover the widest range, from roughly 10 m to 700 m of head, and drive most large dams, including the 700 MW units at Three Gorges and Itaipu. Peak efficiency is above 94%.
• Kaplan turbines are propeller-type reaction turbines with adjustable blades, built for low heads (about 2–40 m) and large flows. You find them in run-of-river plants on big rivers. Bulb turbines, a Kaplan variant with the generator sealed inside the water passage, handle very low heads, including tidal barrages.
• Pelton turbines are impulse turbines. Fast jets from nozzles hit spoon-shaped buckets on a wheel. They suit high heads, from about 200 m to over 1,800 m, with moderate flows, as in alpine and Norwegian schemes.
• Crossflow and Turgo turbines fill niches in small hydro, where low cost and durability count for more than peak efficiency.

Types and main variants

Reservoir (storage) hydropower

A dam blocks a river to form a reservoir that stores water for days, seasons or even years. Operators release it when electricity is worth most, so the plant works like a huge battery charged by rain and snowmelt. Reservoirs also supply irrigation, flood control and drinking water, and those uses can clash with power generation. The biggest plants are all reservoir schemes: China’s Three Gorges (22.5 GW, typically around 100 TWh a year), Baihetan (16 GW), Itaipu on the Brazil–Paraguay border (14 GW, which set annual records above 100 TWh), Xiluodu (13.9 GW), Belo Monte in Brazil (11.2 GW) and Guri in Venezuela (around 10 GW).

Run-of-river

Run-of-river plants divert part of a river’s natural flow through a channel or tunnel to the turbines, or sit behind a low weir with little storage (often a few hours of pondage at most). Output follows the river, peaking with spring snowmelt or the rainy season. Flooding and resettlement are much less of an issue, but fish passage and minimum environmental flows still have to be dealt with. Many large river plants on the Danube, Rhine and Columbia run largely as run-of-river.

Pumped storage

Pumped hydro storage moves water between an upper and a lower reservoir. It pumps uphill when electricity is cheap or plentiful and generates when power is short. Modern reversible pump-turbines reach a round-trip efficiency of about 70–85%. Global installed pumped storage is roughly 180–190 GW (IHA, 2024) and still holds over 90% of the world’s stored electricity in energy terms; China is building by far the most new capacity. The largest plant, Fengning in Hebei, reached 3.6 GW. “Closed-loop” or off-river schemes keep away from rivers altogether, and the Australian National University has identified hundreds of thousands of possible off-river sites worldwide. You can size a scheme with the pumped hydro storage calculator or compare it with batteries in battery storage.

Small, mini and micro hydro

Definitions vary. Small hydro usually means below 10 MW (some countries use 30 or 50 MW), mini below 1 MW and micro below 100 kW. Micro hydro powers remote villages in Nepal, Peru and East Africa and is often the cheapest off-grid electricity wherever a steady stream has a usable drop.

Efficiency and capacity factor

No other generation technology converts energy as efficiently. Well-maintained large plants reach 90–95% from water to wire, against 35–60% for thermal power plants and around 20–23% for solar modules. The capacity factor is another matter: it is limited by how much water there is, and by the choice to run for peak demand instead of constant output.

IRENA’s cost database puts the global weighted-average capacity factor of new hydropower at roughly 45–55% in recent years, with wide spread. Norwegian and Swiss peaking plants run fewer hours by design, while equatorial rivers with steady flow run more. Output also swings from year to year with rainfall. Drought cut hydro output sharply in Europe in 2022, in China’s Sichuan and Yunnan in 2022–2023, and in Brazil in 2021, a growing climate risk for systems that lean on hydro.

Costs

Hydropower is capital-intensive and every site is different, but once built it produces cheap power for decades. IRENA’s Renewable Power Generation Costs in 2023 (2024) gives a global weighted-average LCOE of about USD 0.057/kWh for hydropower commissioned in 2023. Total installed costs averaged roughly USD 2,800/kW, up from earlier in the decade as sites became more remote and complex. Individual projects run from below USD 0.03/kWh for large plants in China, Brazil and Africa to well above USD 0.15/kWh for difficult small or remote sites. Lazard’s LCOE+ doesn’t cover new hydro in detail. NREL’s ATB ranks new US hydropower on undeveloped stream reaches among the pricier renewables, because the best US sites were dammed decades ago; upgrading existing dams and adding turbines to dams that don’t generate is often far cheaper.

Two things skew simple LCOE comparisons. First, civil works can last a century, so plants written off over 30–40 years keep generating at very low marginal cost; much of the existing fleet in Europe and North America predates 1980. Second, cost overruns are common. A study of 245 large dams by Ansar et al. (2014) found average overruns of about 96% in real terms, with schedules slipping 44% on average. Pumped storage is better measured by LCOS, and it usually competes with batteries for durations of eight hours or more.

Land, water and materials

Large reservoirs flood a lot of land. Land use per unit of energy differs by orders of magnitude: well under 1 m² per MWh/yr for high-head alpine schemes, tens of square metres for shallow lowland reservoirs such as Balbina in Brazil, which flooded about 2,400 km² for 250 MW. Three Gorges submerged around 630 km² and forced the resettlement of about 1.3 million people. The World Commission on Dams (2000) estimated that 40–80 million people worldwide had been displaced by dams, many without fair compensation.

Hydropower doesn’t consume water the way thermal cooling does, but reservoirs lose water to evaporation, and in hot, dry climates that can mean large volumes per MWh. Dams break rivers into pieces, trap sediment that would otherwise rebuild deltas and farmland downstream, and change water temperature and oxygen levels. The materials are mostly concrete, rock fill and steel. Spread over a very long service life, that gives very low material use per lifetime MWh.

Lifecycle CO2 emissions

No renewable has a wider emissions range. The IPCC AR5 WGIII Annex III (2014) reports a median of about 24 gCO2e/kWh, with a range from roughly 1 to 2,200 gCO2e/kWh. High-head alpine and Nordic plants sit at the low end. The extreme top comes from a few shallow tropical reservoirs where flooded biomass and organic inflows rot without oxygen. They release methane from the reservoir surface, through turbines and spillways (“degassing”), and further downstream. UNECE’s 2021 assessment gives roughly 6 to 150 gCO2e/kWh for representative plants, leaving out the most extreme reservoirs.

Methane, with a global warming potential around 80 times that of CO2 over 20 years, drives the high cases. Reservoir emissions peak in the first decade after flooding and depend on climate, depth, area per MW and organic loading. The G-res tool from the IHA and UNESCO is now widely used to estimate net reservoir emissions before construction. Most temperate and boreal hydro, and run-of-river plants in general, have lifecycle emissions similar to wind and nuclear.

Safety and environmental impacts

Dam safety. Averaged per TWh, hydropower kills few people, but the risk has a heavy tail. In 1975 the extreme rainfall of Typhoon Nina brought down the Banqiao and Shimantan dams in Henan province, China, which then caused dozens of downstream dams to fail. Death toll estimates range from about 26,000 to over 170,000 when famine and disease are included. Other major failures include Vajont, Italy (1963, about 2,000 deaths, when a landslide sent a wave over the dam) and Machchu-II, India (1979). Modern dam engineering, spillways designed for extreme floods, monitoring and emergency plans have made catastrophic failure rare. Ageing dams and heavier rainfall from climate change are a growing worry.

Ecology. Dams block migratory fish such as salmon, sturgeon and eels. Fish ladders, bypass channels and fish-friendly turbines reduce the harm but rarely remove it. Changed flow patterns affect wetlands and floodplains, and “hydropeaking” (sudden flow changes to follow demand) hurts river life downstream. Dam removals in Europe and the US, such as on the Elwha River in Washington, show how fast fish populations can come back.

Social impacts. Displacement, lost farmland and cultural sites, and disputes over shared rivers (the Nile with the Grand Ethiopian Renaissance Dam, the Mekong, the Indus) make large hydro politically contentious. The IHA’s Hydropower Sustainability Standard sets criteria for consultation, compensation and environmental flows.

Hydro-dominated power systems

Several countries get most of their electricity from hydropower. Norway generates close to 90% from hydro and uses its huge reservoirs to balance Nordic and continental markets through interconnectors. Brazil has historically produced around 60% of its electricity from hydro, from Itaipu, Belo Monte and the Tucuruí complex. Canada gets roughly 60% from hydro, led by Quebec, British Columbia and Manitoba, and exports large amounts to the United States. Paraguay, Ethiopia, Nepal and the Democratic Republic of Congo are over 90% hydro. Individual plants are on the power plants map.

Outlook

The IEA’s Hydropower Special Market Report (2021) called hydro the “forgotten giant” of clean electricity and expected growth to slow outside China as the best sites are taken and social and environmental opposition hardens. Most new conventional capacity is going up in China, India, Africa and South-East Asia. Pumped storage is growing faster, driven by the need to balance wind and solar; China alone has well over 100 GW under construction or approved. Large gains with little new impact are available from modernising and uprating ageing fleets in Europe and North America, adding generators to existing dams, floating solar on reservoirs, and running hydro and solar together. Climate change cuts both ways: flows rise in some high-latitude basins and fall in the Mediterranean, southern Africa and parts of South America. The technology comparison page sets hydropower against other sources, and marine energy covers its ocean-based cousin.