The combined heat and power system offers the possibility of reducing noticeably the CO2 emission with regard to the separate generation of electric energy and heating power. Equations for the calculation of the CO2 emission reduction are derived the assumptions, made in the process elucidated, and the occurring parameter discussed. Within this process it is remarkable that for the calculation of the CO2 emission of the combined hear and power system there is no division of the fuel use necessary. The emission reduction is above all dependent on the fact whether for the combined heat and power system lower-carbon fuels are used than for the separate generation of electricity and hearing power.