Hydropower Explained: How It Works, Costs and Environmental Impact

Hydropower is the oldest source of renewable electricity and still the largest. The sun evaporates water, which falls as rain and snow on high ground; a hydro plant turns the potential energy of that water into electricity at over 90% efficiency, better than any other generator. It also offers what solar and wind cannot on their own: stored energy you can dispatch when needed. On the other side of the ledger, dams reshape rivers, displace communities and are more and more exposed to drought. This guide covers the physics, the main technologies, the costs and an honest account of the environmental impact.

Hydropower in the global mix

Hydropower generated about 4,435 TWh in 2025, roughly 14% of world electricity (Ember / Our World in Data). In the 2000s its share was 16–17%; other sources have since grown faster. Installed capacity is around 1,400 GW (IHA), plus roughly 190 GW of pumped storage.

Data: Ember / Our World in Data, latest year. China alone produces around 30% of the world’s hydropower. Browse countries on the country pages and plant locations on the power plants map.

The physics: head, flow and efficiency

The power you can get from falling water depends on two things: how far it falls (head, in metres) and how much of it falls each second (flow, in m³/s).

Net head is the gross vertical drop minus friction losses in the penstock (the pipe), which typically take 5–15% of gross head. Overall efficiency combines the turbine (85–95% for large units), the generator (95–98%) and the transformer.

Worked example: a small run-of-river plant

A mountain river supplies a design flow of 4 m³/s. An intake diverts it through a 1.8 km penstock to a powerhouse 85 m below. Friction costs 8%, leaving a net head of 78 m. Overall efficiency is 0.86.

• P = 9.81 × 4 × 78 × 0.86 ≈ 2,630 kW (2.6 MW)
• At a capacity factor of 45% (flows vary with the seasons): 2,630 × 8,760 × 0.45 ≈ 10.4 GWh/year, enough for roughly 3,000–4,000 European households.
• CO₂ avoided against the average EU grid: on the order of 2,500–3,000 t/year.

Run the numbers for your own site in the micro-hydro calculator.

Turbine types

Head and flow decide the turbine:

Types of hydropower plant

• Reservoir (storage) hydro. A dam stores water for weeks to years, so output can be scheduled to match demand. That makes it the most valuable flexible renewable. Examples: Hoover, Itaipú, Norway’s highland reservoirs.
• Run-of-river. Little or no storage, so output follows the natural flow. Lower environmental impact, but no flexibility.
• Pumped storage. Pumps water uphill when power is cheap and generates when it is expensive, with a round-trip efficiency of 70–85%. It holds over 90% of the world’s electricity storage capacity by energy. Size a scheme with the pumped hydro storage calculator.
• In-stream and small/micro hydro. Under 10 MW (small) or 100 kW (micro), and often off-grid.

Costs

Hydropower is capital-intensive, tied to specific sites and very long-lived. Turbines run 30–50 years before refurbishment; dams and tunnels last a century or more.

Old dams often supply the cheapest electricity on a grid. New megaprojects are another matter: an Oxford study of 245 large dams (Ansar et al., 2014) found average cost overruns of about 96% and schedule overruns of 44%. Test the economics with the LCOE calculator.

Hydropower as grid flexibility

As wind and solar expand, hydropower’s flexibility is worth more than the energy it produces. Reservoir plants can go from zero to full output in minutes, supply inertia and frequency control, and move energy across days or seasons. Through interconnectors to Germany, the UK, Denmark and the Netherlands, Norway’s hydro fleet works as a “battery” for Northern Europe. Adding pumps or extra turbines to existing reservoirs (“repowering”) often costs less than building new dams. See Energy Storage Technologies Compared.

The environmental and social ledger

Emissions. Most hydropower has life-cycle emissions of 4–30 g CO₂e/kWh. Reservoirs that flood vegetation in warm climates are different: as the organic matter rots, they can release large amounts of methane. The IPCC range runs from roughly 1 to over 2,000 g CO₂e/kWh. A synthesis by Deemer et al. (2016) estimated that reservoir surfaces emit around 1 Gt CO₂e per year worldwide, mostly as methane. Shallow tropical reservoirs with low power density (a large area per MW) are the worst.

Rivers and ecosystems. Dams fragment rivers, block fish migration, trap sediment (starving downstream deltas such as the Nile and Mekong) and change temperature and flow patterns. Only about a third of the world’s long rivers still flow freely (Grill et al., Nature, 2019). Fish ladders, minimum ecological flows and sediment flushing reduce the damage without removing it.

People. Large dams have displaced an estimated 40–80 million people worldwide (World Commission on Dams, 2000), more than 1.3 million of them for Three Gorges. Benefit-sharing and free, prior and informed consent are now standard on paper but applied unevenly.

Safety. Dam failures are rare but catastrophic. The 1975 Banqiao failure in China killed tens of thousands. The 2023 Derna disaster in Libya, where two dams built for purposes other than hydropower failed, killed thousands.

Climate risk: drought

Hydropower runs on rain and snow, and a warming climate makes both less predictable:

• 2021–2022: Brazil’s worst drought in 90 years forced expensive thermal generation.
• 2022: Sichuan, China, which gets around 80% of its power from hydro, rationed electricity during a record heatwave. Hydro output in Italy, Spain, Portugal and France fell sharply.
• 2023–2024: Global hydro generation dipped in 2023 because of droughts in China, Mexico and elsewhere. In 2024 low water at Lake Kariba caused severe blackouts in Zambia and Zimbabwe.

Wind and solar often peak in different seasons from hydro, so adding them lowers this risk.

Modernisation: the overlooked opportunity

According to the IEA, hydropower plants in North America and Europe are on average more than 40–50 years old, and many need major refurbishment within the next decade. Upgrading existing sites can add a lot of output with little extra environmental impact:

• New runners and generators. Swapping turbines designed in the 1950s–70s for modern, computer-optimised ones typically raises efficiency by 2–5 percentage points and capacity by 5–30%.
• Adding pumping. Converting a conventional reservoir plant to pumped storage, or installing a pump-turbine between two existing reservoirs, creates storage without a new dam. Several Alpine and Iberian projects do this.
• Variable-speed units. Doubly fed or full-converter machines let pumped-storage plants vary their pumping power continuously, so they can provide frequency control even while consuming power.
• Powering non-powered dams. Thousands of dams built for irrigation, navigation or flood control produce no electricity. The US Department of Energy estimates that fitting turbines to existing non-powered US dams could add around 12 GW of capacity.
• Digital operation. Better inflow forecasts and market-driven scheduling get more value from the same water.

The IEA warns that without modernisation, ageing plants will become less available just when grids need their flexibility most. Compare the economics with the LCOE calculator.

Where growth will come from

The IEA expects most new hydropower in Asia, Africa and Latin America, which still have large untapped technical potential. Africa uses only about 10% of its economic potential. In Europe and North America, growth will come mostly from modernisation, pumped storage and small additions to existing dams. China’s 60 GW Yarlung Tsangpo (Medog) project, under construction since 2025, will be the world’s largest.

Micro-hydro: power from a stream

For homes, farms and villages with running water, micro-hydro can deliver more energy per euro than any other renewable, because it generates day and night. A site with 20 m of head and 20 l/s (0.02 m³/s) gives roughly 9.81 × 0.02 × 20 × 0.6 ≈ 2.4 kW, or around 15,000–20,000 kWh a year, several times what a typical household uses. Check the minimum flow across the whole year, permits for water abstraction and fish protection, intake screens and how you will clear debris. Estimate your own site with the micro-hydro calculator.

Bottom line

Hydropower underpins many of the world’s cleanest grids: plants last a very long time, cost little to run and offer flexibility nothing else matches. Its drawbacks are limited sites, high upfront costs, ecological and social damage and growing exposure to drought. On a grid built around wind and solar, existing hydro and pumped storage gain value as flexible partners. Compare hydro-rich grids such as Norway and Brazil on the world energy map.

Figures in this guide are typical ranges drawn from the sources above; your own numbers depend on location, prices and equipment. Use the linked calculators for site-specific estimates.