Any form of energy which is replaced by nature, with or without human assistance. In some contexts, energy may only qualify as renewable if it is also continually available. Common forms of renewable energy include wind, solar, geothermal and tidal energy, as well as energy from combustible products such as wood and alcohol. (Alcohol is usually produced from grain or grain byproducts.)

Most sources of energy classified as nonrenewable actually fit the textbook definition of renewability. Coal, oil, gas and nuclear energy, all of which are considered nonrenewable energy sources, can all be replaced by nature, but there's a catch…Earth's next batch of these energy sources won't be available for another few million years.

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