Primary energy (PE) is an energy form found in nature that has not been subjected to any conversion or transformation process. It is energy contained in raw fuels, and other forms of energy received as input to a system. Primary energy can be non-renewable or renewable. Total Primary Energy Supply (TPES) is a term used to indicate the sum of production and imports subtracting exports and storage changes. The concept of primary energy is used in energy statistics in the compilation of energy balances, as well as in the field of energetics. In energetics, a primary energy source (PES) refers to the energy forms required by the energy sector to generate the supply of energy carriers used by human society. The use of primary energy as a measure ignores conversion efficiency. Thus forms of energy with poor conversion efficiency, particularly the thermal sources, coal, gas and nuclear are overstated, whereas energy sources such as hydroelectricity which are converted efficiently, while a small fraction of primary energy are significantly more important than their total raw energy supply may seem to imply. PE and TPES are better defined in the context of worldwide energy supply.