For a comprehensive assessment of energy systems as to their potential for CO2 reduction, the balancing may not be confined to the CO2 emissions through the fuel use in operation, but has to ascertain the release of climatic-influencing tracer gases on the entire journey through the Life of the system. In the following contribution such a balancing method will be developed and will be applied to fossil, nuclear and regenerative energy systems for the electricity generation and heat supply. Furthermore, a longterm scenario of the transition to a CO2-reduced energy supply for the Federal Republic of Germany is introduced, and in the case of its application, the expected CO2 emissions are determined, where by for the first time the energy consumption and its resultant emissions are taken into account.