**Abstract**

According to the European energy policy, the energy use of technical systems in buildings is given at the level of primary energy. This calculation requires knowledge of the primary energy conversion factors according to their source; however, there is currently no single European-wide recognized method for their determination. The aim of this study is to present and compare three methods for determining primary energy factors, namely the method of partial substitution, the physical energy method, and calculation according to EN 15603 standard. For the case study, the electricity factor for Slovenia was calculated according to the aforementioned methods. The results of this study showed that the methods differ in the evaluation of individual primary sources, which has a significant impact on the PEF value. We found that with the partial substitution method, we do not get representative results about the PEF. The method of physical energy defines the efficiency of production from renewable energy sources as 100%. The question arises if we can truly assume that the use of PE is equal to the actual production of electricity. In the third method, defined in the EN 15603 standard, which provides two PEFs, a certain measure of criticality of the assumed factors for the different sources of energy is used.

**Keywords:** primary energy, primary energy factor, electricity mix, renewable energy sources
