9kW, as in, a 40A current in 220V? If you consider 127V in some regions, it's a 70A current, so wires at least 3mm thick for 40A, or 5mm for 70A. These seem much scarier (and prone to fire risks if not respected) than the shower itself. Without taking into account the likelihood of a breaker box tripping at such high loads.

It seems that the standard rated power of Brazilian showers is 5.5 kW. But then again, Irish weather seems harsher than Brazilian weather.