The rule with instantaneous water heaters is always to choose the highest power rating possible for the site conditions. The higher the power rating, the greater the volume of water that can be heated.
For example, the DBX electronic instantaneous water heater is available in the following power ratings: 18, 21, 24 or 27 kW 400 V 3~. The 27 kW rating provides the highest level of convenience without consuming more useful energy. However, the site supply must be dimensioned for the DBX 27. That requires a 3/PE 400 V~ supply and a 6 mm2 conductor cross-section. The electronic controls ensure that only the power actually needed is drawn.
If you are not sure of the site power supply conditions then you should choose a Multiple Power System instantaneous water heater. The electrician can then set the maximum power input (18, 21, 25 or 27 kW) when installing the unit.