Residential sector consumes 70 % of the district heat. The domestic hot water system consumes 27 % of the total thermal energy consumption in these buildings in Riga. According to the mandate of the European Committee for Standardization (CEN) European standards for assessment of the energy performance of buildings have been developed. CEN standards give different methods and default values for calculations and procedures for energy performance assessment. Taking into account national or regional regulations and climatic conditions for all CEN standards application a national annex is required. CEN standards adapted to the status of Latvian standards (LVS) are not complemented with national annexes that are required for high grade use of standards at the national level. The target of this paper is assessment the suitability of CEN standards for the calculation of energy performance of the domestic hot water systems and search for the optimal solutions (methods and default values) for Latvian conditions. In this study there is performed assessment of the consumption of the heat energy in the apartment buildings for heating and hot water system, including: 1) assessment of the DHW volume and necessary energy amount; 2) assessment of the heat losses in the hot water distribution system during heating and non-heating seasons; 3) assessment of the auxiliary energy of the DHW system. The results of this paper give possible evaluation of economic feasibility and energy impact for improvements of domestic hot water systems. The method corresponding to CEN standards suitable for Latvian conditions is proposed.