The main energy terms

The following terms generally describe different categories of energy:

Primary energy: typically, energy in its «raw» form. It hasn't yet converted and not recycled so that it can be better to use (for example, crude oil, sunlight, etc.)

Secondary energy: transformed and recycled energy in a form that can be used by an  ultimate customer (for example, light oil, electricity, etc.)

Useful energy: every form of energy, which can be used for the desired purposes. It is created by conversion of secondary energy, for example, in space heating, traffic, electric lighting, etc.

At each stage of the conversion process has energy losses, and lost thus the energy can no longer be used. The smaller the energy loss, the more effective the conversion process. Accordingly, the resulting loss of energy is the key factor in what amount of primary energy required for cooking a specific amount of energy.