Renewable energy resources occur naturally and can be tapped again and again, unlike fossil fuels such as coal, oil and natural gas, which take millions of years to form and can only be used once. Renewable energy resources include firewood, hydro, wind, solar, geothermal and tidal power.

The main renewable energy used worldwide is "biomass", mainly firewood or organic waste that is the principal source of energy for more than 2 billion people in developing nations.

Biomass, which includes biofuels as a tiny fraction, accounted for about 10 percent of the world's primary energy use in 2004. Second was hydropower with a 2 percent share. Other major renewable energy resources such as wind, solar, geothermal and tidal power made up just 1 percent. In contrast, fossil fuels supply about 80 percent of world energy.