Energy sources that are either inexhaustible (solar, wind) or replenished over a short period of time (hydro, biomass, geothermal). Most renewable energy ultimately comes from the sun - indirectly in the case of wind, water, and biomass; directly in the case of solar (PV) generation. Natural gas and coal, for example, are not renewables because their use consumes gas and coal reserves at a much quicker rate than they can be replenished.