This European Standard is part of a set of standards covering the methods for calculation of system energy requirements and system efficiencies of heating systems in buildings. In particular this standard is one of a number of standards dealing with domestic hot water systems. The scope of this specific part is to standardise the methods for calculation of: thermal losses from the domestic hot water generation system; recoverable thermal losses for space heating from the domestic hot water generation system; auxiliary energy of the domestic hot water generation systems. These values are input data for calculation of the overall energy use according to prEN 15603 and prEN 15316-1. This European Standard specifies the: inputs; calculation methods; outputs.