Basic concepts for evaluating rated power of generator sets powered by combustion engines are presented in this article. A relation between parameters of the engine and the generator are shown. Recommendations contained in the standards and technical literature are systematized. The scheme and methodology for evaluating rated power of the generator set, based on laboratory research, are suggested. Also, the main problems which occurred during research have been highlighted. The differences between the values of rated power assumed in theoretical consideration and obtained during laboratory research for the sample unit, have been commented upon.