Renewable energy is energy sources that can be replenished naturally on a human timescale, such as sunlight, wind, rain, tides and geothermal heat. Renewable energy technologies are diverse and include solar photovoltaics, hydroelectricity, biofuels, wind power and geothermal power. These renewable energies are considered to be sustainable as they do not deplete natural resources or produce harmful by-products. Renewable energy sources offer a number of advantages over traditional fossil fuel-based sources of energy such as coal and oil. They are cleaner burning and produce less air pollution than fossil fuels; they can be used to generate electricity with no net emissions; they require less water for production; they are more cost-effective in the long run; and they provide a more reliable source of electricity than traditional sources.