Taking advantage of offshore wind power appears to be of special significance for the climate protection plans announced by the German Federal Government. For this reason, a comprehensive system analysis of the possible CO2 reduction including the consideration of all relevant processes has to be performed. This goal can be achieved by linking a life-cycle assessment model of offshore wind utilisation with a stochastic model of the German electricity market. Such an extended life-cycle assessment shows that the CO2 emissions from the construction and operation of wind farms are low compared with the substitution effects of fossil fuels. Additionally, in the German electricity system, offshore wind energy is the main substitute for medium-load power plants. CO2 emissions from the modified operation and the expansion of conventional power plants reduce the CO2 savings, but the substitution effect outweighs these emissions by one order of magnitude. The assumptions of the model, shown here to be above all CO2 certificate prices, have a considerable influence on the figures shown due to a significant effect on the future energy mix.