"Renewable energy (sources)" or "RES" includes all sources of energy that are captured from on-going natural processes, such as solar power, wind power, water flow in streams (hydro power), biomass, biodiesel and geothermal heat flows. Most renewable forms of energy, other than geothermal and tidal power, come from the Sun. Some forms are stored solar energy such as Rainfall and wind power which are considered short-term solar-energy storage, whereas the energy in biomass is accumulated over a period of months, as in straw, or through many years as in wood. Capturing renewable energy by plants, animals and humans does not permanently deplete the resource. Fossil fuels, while theoretically renewable on a very long time-scale, are exploited at rates that may deplete these resources in the near future.

Renewable energy resources may be used directly, or used to create other more convenient forms of energy. Examples of direct use are solar ovens, geothermal heating, and watermill and windmills. Examples of indirect use are electricity generation through wind turbines or photovoltaic cells, or production of fuels such as ethanol from biomass (see alcohol as a fuel).

For aspects of renewable energy use in modern societies see Renewable energy development.

Other definitions of renewable energy sources

Renewable energy use should not harm the environment

The most common definition of renewable energy, used in legal documents and in the political discourse on sustainable energy, include only those sources that do not harm the environment during use. For example, large hydro power plants are excluded. (see:WWF Gold Standard)