Primary energy designates energy from sources that involve only extraction or capture, with or without separation from contiguous material, cleaning or grading, before the energy embodied in that source can be converted into heat or mechanical work. Primary energy is not derived from any other form of energy. By convention, sources of energy that occur naturally such as coal, natural gas, fuel wood are termed primary energy.

Secondary energy designates energy from all sources of energy that results from transformation of primary sources.

På svenska skiljer man mellan "primära energikällor" och "energibärare".