This standard is part of the method for calculation of system energy requirements and system efficiencies. The scope of this specific part is to standardise the methods for calculation of the heat losses from the domestic hot water generation system and it defines the: inputs; outputs; calculation method. This standard covers the domestic hot water requirements in all buildings. The general approach to calculate energy consumptions and losses of domestic hot water systems is as follows: calculation of domestic hot water requirements of a dwelling, a zone or a building ( WQ ); calculation of heat losses due to the distribution or circulation of domestic hot water supplied ( dWQ , ); calculation of heat losses in hot water storage units ( sWQ , ) and heat losses due to the production or generation ( gWQ , ). In order to be coherent with calculation methods for space heating systems, emission losses representing taps and control should be taken into account.