Hydropower is electricity generated by moving water. It works by channeling water through turbines that spin a generator, converting the kinetic energy of flowing or falling water into electrical power. In 2024, hydropower produced roughly 4,500 terawatt-hours of electricity worldwide, about 14% of the global total, making it the single largest source of renewable power generation.
How Water Becomes Electricity
The basic mechanics are straightforward. Water flows through a pipe called a penstock and pushes against blades inside a turbine. The spinning turbine drives a generator, which produces electricity. The more water flowing and the greater the height it falls from, the more energy is available to capture.
What makes this process remarkable is its efficiency. Hydroelectric plants convert about 90% of the water’s potential energy into electricity, according to the U.S. Bureau of Reclamation. For comparison, solar panels typically convert 15% to 22% of sunlight into power, and natural gas plants operate at roughly 40% to 60% efficiency. No other major electricity source comes close to hydropower’s conversion rate.
Three Types of Hydropower Facilities
Impoundment (Dam and Reservoir)
This is the type most people picture: a large dam holding back a reservoir of river water. Operators release water from the reservoir through turbines to generate electricity on demand. Because the reservoir stores water, these facilities can ramp production up or down depending on how much electricity the grid needs at any given moment. The world’s largest hydropower plant, China’s Three Gorges Dam on the Yangtze River, is an impoundment facility with a capacity of 22.5 gigawatts. Brazil’s Itaipu Dam, on the border with Paraguay, follows at 14 gigawatts.
Diversion (Run-of-River)
A diversion facility channels a portion of a river’s natural flow through a canal or penstock, using the river’s own downhill slope to push water through turbines. These plants may not need a dam at all, relying instead on the river’s existing current and elevation drop. They tend to be smaller and less disruptive to the surrounding landscape, but they can’t store water, so their output depends on seasonal river conditions.
Pumped Storage
Pumped storage works like a giant rechargeable battery. Two reservoirs sit at different elevations. When electricity demand is low (and power is cheap), the facility pumps water uphill to the higher reservoir. When demand spikes, that water is released back down through turbines to generate electricity. This setup doesn’t create new energy; it stores energy generated by other sources like solar, wind, or nuclear for use at peak times. Pumped storage is the most dominant form of large-scale energy storage on the electric grid today and plays a growing role in integrating other renewables, which produce power intermittently.
Where Hydropower Stands Globally
Hydropower has been the world’s leading renewable electricity source for decades. Solar panels overtook it in total installed capacity in 2023, but hydropower still generates more actual electricity, holding a 47% share of global renewable power production. That gap exists because dams can run around the clock when water is available, while solar panels only produce during daylight hours.
In terms of cost, hydropower is competitive with other renewables. The U.S. Energy Information Administration estimates a capacity-weighted average cost of about $26 per megawatt-hour for new hydroelectric projects entering service in 2030. That’s cheaper than solar, offshore wind, and geothermal on the same weighted basis, though onshore wind comes in lower at roughly $19 per megawatt-hour. The raw construction cost of a dam is high, but what tilts the economics in hydropower’s favor is lifespan: a typical hydroelectric facility operates for 65 to 85 years with relatively low maintenance costs. The turbines and generators run for long stretches without needing replacement, which spreads the upfront investment over many decades of production.
Environmental Tradeoffs
Hydropower produces no direct carbon emissions during operation, but it’s not without environmental impact. Dams fundamentally alter the rivers they sit on, changing water temperature, chemistry, flow patterns, and sediment loads. These changes ripple through the surrounding ecosystem, affecting native plants and animals both in the water and along the banks.
Fish are especially vulnerable. Dams can block migration routes that species like salmon depend on for spawning. Fish that do pass through turbines face a 5% to 10% mortality rate with current technology, though newer turbine designs aim to bring that below 2%. Many modern dams include fish ladders or bypass channels to help, but these solutions don’t fully eliminate the problem.
Reservoirs also produce greenhouse gases. Submerged vegetation and organic matter decompose underwater, releasing both carbon dioxide and methane. The exact amount varies widely depending on the reservoir’s size, climate, depth, and the type of land that was flooded. A shallow tropical reservoir surrounded by dense vegetation will produce significantly more methane than a deep reservoir in a cooler climate. This means the carbon footprint of hydropower isn’t zero; it’s just far smaller than fossil fuels in most cases.
Why Hydropower Still Matters
Hydropower fills a role that solar and wind can’t easily replicate on their own: reliable, controllable power. Grid operators can increase or decrease a dam’s output within minutes by adjusting water flow, making hydropower valuable for balancing the fluctuations that come with weather-dependent renewables. When the sun sets or the wind dies down, hydropower (especially pumped storage) can pick up the slack almost instantly.
That flexibility, combined with a 90% efficiency rate, decades-long operational lifespan, and competitive costs, explains why hydropower remains central to electricity systems in over 150 countries. It’s the oldest large-scale renewable technology, and as grids integrate more solar and wind, its role as a stabilizer and storage medium is becoming more important rather than less.