Hydroelectricity, or hydroelectric power, is a form of renewable energy that uses the power of moving water to generate electricity. Hydropower supplies one-sixth of the worlds electricity, almost 4500 TWh in 2020, which is more than all other renewable sources combined and also more than nuclear power. Hydroelectric power plants are usually located on or near a water source, and the energy available from the moving water depends on both the volume of the water flow and the change in elevation, also known as the head, from one point to another. Most hydroelectric power comes from the potential energy of dammed water driving a water turbine and generator. The power extracted from the water depends on the volume and on the difference in height between the source and the waters outflow. A large pipe, called the penstock, delivers water from the reservoir to the turbine. Once a hydroelectric complex is constructed, it produces no direct waste, and almost always emits considerably less greenhouse gas than fossil fuel-powered energy plants.

Hydroelectricity has several advantages and positive aspects, including:

• Reuse: This resource comes from rainwater, and water used in the process can be reused.
• Duration: Hydroelectric facilities have a long useful life.
• Sustainable: Hydroelectric energy helps to reduce greenhouse gas emissions.
• Flexibility: Hydroelectric resources make it possible to cover peak demand times by making flexible use of the water in reservoirs.
• Control: Hydroelectric plants can store electricity by pumping water to an upstream storage reservoir.

Hydroelectricity accounted for about 6.2% of total U.S. utility-scale electricity generation and 28.7% of total utility-scale renewable electricity generation in 2022.