Until now buildings are most seen as creating a demand for energy. However, if we want to develop an energy system being independent of fossil fuels in the future, this will require new higher standards for energy efficiency and a radical introduction of new and renewable energy technologies, all together implying that buildings in the future might act as prosumers that is both demanding and producing energy. In this report we look at the overall consequences for the energy system of introducing new technologies as photovoltaics and heat pumps in combination with strong energy conservation measures. A number of energy system scenarios are prepared based on technical simulations for single-family houses carried out by the University of Aalborg.