Recovery of waste heat from the electrical generator into the water preheating circuit of the Steam Thermal Power Plants (STPP) is a method frequently used to increase electricity production without additional consumption of primary energy. However, the manufacturers of turbine-generator sets do not explicitly indicate the effect of the method on their performances. Using numerical modeling of steam cycle, the paper presents the method's effects on the STPP performances. It also shows a method, simple, fast, and enough accurate, for estimating these effects, that could be applied when it is not possible to recalculate the whole cycle.