The document offers for architects and mechanical and sanitary engineers a tool to calculate the energy demand of heating, domestic hot water and room ventilation systems in the design stage of a building. The results of the calculation may be used as proof of the total energy demand of a building according to the German Engergy Saving Regulation (EnEV). It specifies a general energy efficiency calculation method and it gives energy efficiency factors for common used systems of heat transmittion to the room, the heat distribution in the building and the heat generation. The building forms the limits for the calculation. The primary energy source will be taken into account with a primary energy factor.