Abstract: The thermal output of a combined heat and power generation system is regulated by varying the speed of rotation while maintaining the position of the control means of the thermal engine in its maximum efficiency range. For that purpose, the thermal engine in its full load position or efficiency optimum is brought to a lower or higher speed of rotation exclusively by respectively reducing or increasing the electric current fed into the mains, so that the efficiency of the thermal engine is not substantially affected. The electric current may be maintained at the desired frequency by frequency converters.