In this paper we show the advisability of usage of solar collectors for domestic hot water (DHW) preparation in buildings located in two countries: Poland and Spain. The analysis was conducted for a single-family house with horizontal roof. During the calculations we took into account factors depending on the building location such as national rules, climatic conditions, cost of system installation, and fuel and electricity prices. Based on the total design heat losses and demand for DHW, the demand for usable energy for heating the buildings and DHW preparation was calculated. A gas boiler was selected as the heat source of the building, and solar collectors (flat-plate and vacuum pipe) were chosen as the source of DHW preparation. Installation investment costs and annual operating costs have been stamped. Calculations show that for buildings located in Spain, heat losses were 36.5% lower and the demand for thermal power for DHW was 59.5% lower than for buildings located in Poland. Annual operating costs of the heating installation for both buildings were at similar levels, due to high fuel prices in Spain, while the operating costs of DHW installations were 28.2% higher for locations in Poland. The results show that the use of solar collectors in Poland is economically justified.