Renewable energy is energy from a source that is replenished by natural processes on a sufficiently short timescale so the energy can be used by humans more or less indefinitely, provided the quantity taken per unit of time is not too great.
Types of renewable energy resources include:
Most renewable forms of energy, other than geothermal and tidal, are in fact stored solar energy. Water power and wind power represent very short-term solar storage, while biomass represents slightly longer-term storage.
Renewable energy resources may be used directly as energy sources, or used to create other forms of energy for use. Examples of direct use are windmills, solar ovens, and geothermal heat pumps. Examples of indirect use in creating other energy sources are electricity generation through wind generators or solar cells, or production of fuels such as ethanol from biomass.
Compare with nonrenewable energy.