The consumption of various forms of primary and secondary energy is one of the main sources of greenhouse gas emissions to the atmosphere. Also, the increase in the prices of energy resources is an important factor affecting the economic profitability of running a business organization. Legal requirements in the European Union also affect the need to implement appropriate solutions aimed at increasing energy efficiency, which translates into the need of implementing Energy Management Systems, based the ISO 50001 standard, in many enterprises.. In the case study presented in the article, which is based on a company from the energy industry in Poland, the most important Energy Performance Indexes and the impact of the quality of their information on the results obtained were reviewed. In the analyzed example, the main process used only 28% of the total energy consumption in the organization. Insufficient attention to auxiliary processes led to an undercut of Energy Performance by nearly 11% in the first year of operation. It is partic-ularly important to properly collect data on auxiliary processes, which are very often omitted or treated in general in companies, and as shown may constitute a significant share in the total amount of energy consumed.