In this work it was carried out the calculation of heat energy (the reference year consumption of thermal energy) which is used for heating residential buildings-family house in Slavonski Brod with the necessary input and output data for the calculation. The heat energy which is used for the building depends on the shape of the building, construction materials, thermal insulation, energy system characteristics and climatic conditions in which the building is situated. Natural gas is used as an energy source for heating in the above example, so for the purpose of calculation the consumption of natural gas (reference annual heat consumption) was taken the average consumption of natural gas from I / 2013 to XII / 2015. Since in Croatia heating family houses is carried out in addition to the use of natural gas and by applying other energy (electricity, fuel oil, liquefied petroleum gas, firewood, pellets ...), so in this paper a comparison of annual consumption for each energy source was made with respect to the reference annual consumption of thermal energy for heating, as well as the comparison of the annual cost for each energy source. With respect to ecological aspect it was made a comparison of annual CO2 emissions of each energy source in relation to the emission factor of each energy source.