Energy supply is the total quantity of usable energy available to people for doing work. We use forms of energy to operate machinery; to heat and cool our homes and offices; to cook; to provide light; and to transport people and goods. Heat energy is the most commonly used form of energy.
Energy may be obtained directly from an energy source, as when people burn wood to produce heat. Energy also may be obtained indirectly. For example, at a power plant, oil is burned to create steam. The steam turns a generator, thus producing electric power.
Energy that is produced by businesses and governments and sold to the public is called commercial energy. About 85 percent of all commercial energy comes from petroleum, coal, and natural gas. These sources are called fossil fuels because they developed from the fossilized remains of prehistoric plants.
Earth contains only a limited supply of fossil fuels. But the amount of fossil fuel burned by people to produce energy has nearly doubled every 20 years since 1900. Someday, the supply will run out. Scientists and engineers are working to develop other sources of energy to replace the shrinking reserves of fossil fuels.
