Hydropower converts moving water into electric current. Depending on the installation, a river's flow can pass through turbines, a dam can store water to generate during peak hours, or a pumped-storage plant can return some of the electricity consumed to lift water. The world's leading renewable source of electricity generation, hydropower provides grid flexibility but depends on water resources and can permanently transform ecosystems and territories.
Key facts
- Principle: a turbine drives a generator using water flow and hydraulic head.
- Share of the mix: hydropower supplied around 14% of global electricity in 2024.
- Flexibility: reservoirs allow generation to be adjusted; pumped-storage plants store energy by pumping water.
- Limitations: droughts, seasonal variations, construction costs and ecological or social impacts.
What is hydropower?
Hydraulic energy comes from moving water. In a hydropower plant, river flow or water released from a reservoir turns a turbine. This drives a generator that converts mechanical energy into electricity. The amount generated depends particularly on available flow, hydraulic head and equipment efficiency.
Water can be held behind a dam or diverted from a watercourse before being directed to the plant through a canal or penstock. After passing through turbines, it is generally returned to the watercourse. Installations vary greatly in size: some supply a national grid, while others provide electricity to a valley or isolated community.
Main types of power plants
Run-of-river plants
They use a river's natural flow with little storage capacity. Generation therefore follows available water levels and can vary seasonally. They can provide regular generation when the river is abundant, but have less scope to adjust output than a large reservoir dam.
Reservoir plants
A dam creates a reservoir that holds back some incoming water. Operators choose when water passes through turbines within the limits of available volumes and basin management rules. These plants can rapidly increase output when electricity demand rises, particularly in the morning or evening.
Pondage plants have intermediate storage capacity suited to daily or weekly adjustments. Large reservoir plants store more water and can shift some generation between different periods of the year. This flexibility makes hydropower useful for balancing a grid where demand and solar or wind generation change hour by hour.
Pumped-storage hydropower plants
A pumped-storage plant connects two reservoirs at different elevations. When electricity is available and demand is lower, pumps move water from the lower reservoir to the upper one. When the grid needs electricity, water flows down through turbines.
A pumped-storage plant is a storage solution, not a net generation source: it returns less electricity than it consumed to lift the water. Its value lies in shifting energy over time and responding rapidly to grid variations. However, it requires a suitable site, substantial investment and water resource management.
Why hydropower matters in the electricity system
Hydropower plants are dispatchable to the extent that generation can be adjusted according to flow and reservoir capacity. Reservoir facilities can supply electricity at peak times, support grid frequency and complement variable sources. Hydropower can also provide grid services, such as a reserve that can be activated rapidly.
Globally, hydropower generated around 14% of electricity in 2024, making it the leading renewable generation technology. However, this role depends on hydrological conditions. In 2025, declines in hydropower output in Europe and Eurasia limited renewable growth in those regions, according to the International Energy Agency. A dry year can therefore reduce generation and force the system to use more other resources.
Dams and water management
A hydropower project operates within an entire catchment. Inflows depend on precipitation, soils, vegetation, evaporation and upstream uses. A reservoir can serve several purposes: generating electricity, regulating floods, supporting irrigation or ensuring water supply. These functions can complement one another but also compete during droughts.
Large dams alter flows and sediment transport, fragment aquatic habitats and can affect fish migration. Creating a reservoir can displace populations and transform farming or fishing activities. Some tropical reservoirs may also emit methane as submerged organic matter decomposes. Effects vary with the site, design and operation; they must be studied before the investment decision and monitored throughout the facility's lifetime.
Potential and challenges in Africa
Several African basins have substantial hydropower resources, but theoretical potential is not immediately available capacity. A project must be technically feasible, economically viable, grid-connected and acceptable in terms of social and environmental impacts. It must also withstand variations in water resources and fit into regional river management.
Large facilities can contribute to supplying several countries if interconnection lines, purchase agreements and sharing rules are established. Small plants can serve rural areas or local grids in certain settings. In both cases, maintenance, equipment availability, transparent decisions and benefit sharing are essential to sustained performance.
Frequently asked questions
Is hydropower always available?
No. Run-of-river plants depend directly on flow; reservoir dams can smooth variations, but their stocks remain limited by inflows, management rules and other water uses.
Does a pumped-storage plant generate renewable electricity?
It returns electricity by passing water through turbines after consuming electricity to pump it to the upper reservoir. It is therefore counted as a storage and flexibility resource, rather than a primary energy source.
Does hydropower emit no greenhouse gases?
A plant does not emit CO₂ through combustion during generation, but its life-cycle impact includes construction and, depending on the site, reservoir emissions. Impacts depend on each project's characteristics.