One of the main features of modern buildings is the improved isolation. This leads to a sinking power requirement combined with a more uneven time dependent behaviour. The economic efficiency of the widespread water heating systems is endangered by that. So this thesis compares the total costs of a gas-fired water heating system to the costs of an electric direct heating system. The calculation is based on the new VDI 2067. A new method of calculating the combined external temperature is introduced. As a result of this thesis it turned out that an electric direct heating system is in many cases the alternative with lower total costs.