Basis energy terms
Generally, these are the terms that characterize the different categories of energy1:
Primary energy: basically energy in its raw form. It has not yet been converted or processed in order to be able to use it better (i.e. crude oil, sunlight, etc.)
Secondary energy: converted and processed energy in a form that can be used by the end consumer (i.e. extra light heating oil, electricity, etc.)
Useful energy: every form of energy that affords the consumer a desired use. It is created by converting secondary energy into room heating, movement, electrical light, etc., for example.
There are losses of energy at each stage of the conversion process, whereby this lost energy can no longer be used. The less loss of energy there is, the more efficient the conversion process works. Correspondingly, resulting losses of energy are a decisive factor in how much primary energy is required for the preparation of a particular amount of useful energy.
 Vgl. Wesselak V. & Schabbach T. Regenerative Energietechnik Springer Berlin / Heidelberg 2009, S. 4