Usually the process method of evaluating the energy effects of rationalization of an industrial energy plant is applied. But in this method the energy effects of the network relations between energy processes are neglected. The application of the mathematical model of energy management of an industrial plant in the system approach of the evaluation of rationalization effects provides possibilities to take into account all the interdependences between energy processes to obtain accurate results. The energy effects of the rationalization of industrial energy management are determined at the system boundary of an industrial plant. The final result of this calculation is a decrease of external supplies of energy carriers.