As the energy-use for space heating continues to diminish due to better performances of the building envelope, the energy use for hot water production becomes increasingly relevant. Therefore, it becomes increasingly important to size domestic hot water installations correctly, e.g. to provide the expected comfort using the least possible energy. Different international standards and methods describe how size domestic hot water production installations, but the results for a same building vary considerably. This article compares the different methods on several buildings of different sizes. The causes of the differences in results are also commented.