The main sources of renewable energy are biofuels, such as wood, hydropower, geothermal, solar, and wind. Unlike fossil fuels, renewable energy resources are being replenished continuously and will never run out. Some sources of renewable energy, such as solar power and wind power, are perpetual, meaning the wind will blow and the sun will shine no matter how much energy is used. Sources of renewable energy that rely on agriculture, such as woods, are renewable as long as they are not depleted and exploited too rapidly.