Apartment buildings are one of the largest district heat consumers in Latvia. The residential sector consumes 70 % of the district heat. The domestic hot water (DHW) system consumes 27 % of the total heat energy consumption in these buildings in Riga. To achieve targets of European energy efficiency policy the Energy Performance of Buildings Directive establish a certification of the energy performance of buildings. According to the mandate of the European Commission, the European Committee for Standardization (CEN) have developed European standards for assessment of the energy performance of buildings. European standards give different methods and default values and procedures for the energy performance assessment. Taking into account national or regional regulations and climatic conditions for all European standards application a national annex is required. European standards adapted to the status of Latvian standards (LVS) not complemented with national annexes that are required for wholesome use of standards at the national level. The target of the Doctoral Thesis is assessment the suitability of European standards for the calculation of energy performance of the DHW systems and finds the optimal solutions (methods and default values) for Latvian conditions. The Doctoral Thesis presents the analysis of energy consumption for DHW system in apartment buildings including hot water energy need (tapping requirements), distribution losses in the circulation loop and recoverable losses (heat gains). The aggregate data contains information on 39 buildings with total heated area of apartments 158 thousand m2 in Riga and 57 buildings with total heated area of apartments 91 thousand m2 in Bauska. The Buildings have been constructed by standard type designs from nineteen sixties to nineteen eighties. The results of the study provide an evaluation of the representative DHW consumption and energy performance indicators in apartment buildings. The study shows measurement based data and the results suitable for the development of a method for calculation of energy performance of DHW systems adjusted to Latvian conditions and the adoption of EPBD standards at national level. The results of Doctoral Thesis provide the base for economic and energetic evaluation of possible improvements of DHW systems of buildings.