Primary energy

Primary energy is the amount of energy put into a supply system, as opposed to the “useful energy” that the supply system outputs to consumers.

For instance, the tons of coal fed to a coal plant are considered primary energy, whereas the electricity that leaves the plant is considered secondary energy. A coal plant with an efficiency of 40 percent consumes 2.5 times more primary energy (coal) than it produces in the form of electricity (secondary energy). For wind and solar, there is no difference between primary and secondary energy. See also: efficiency.