This paper addresses the different primary energy definitions, accounting methods, and their applications with a focus on electricity and heat generation from renewable energy. In addition to renewable energy sources, primary energy factors for electricity from waste, nuclear, and imported electricity are also discussed as these can be calculated in different ways.

This paper is the first part of a two paper series. In the second part of the study, the application of primary energy factors (PEF) in the three EU Directives - Energy Performance of Buildings Directive (EPBD), Energy Efficiency Directive (EED) and Renewable Energy Directive (RED) is discussed.