Calculation of electrical loads is the primary task in design of each power supply system, and the reliability of the values obtained from such calculations directly affects the technical and economic indicators of the entire enterprise. This article discusses all stages of development of methods and approaches for determining the calculated values of electrical loads from experimental representation in the 1920s to modern methods. The approved method for calculation of loads is considered in detail and tested for reliability of the reference data and the actual values of the utilization coefficient obtained as a result of the studies carried out for the object of mining and processing production. We have confirmed the influence of the measures to improve energy efficiency and, as a consequence, the need to adjust the existing method for calculation of electrical loads.