"Alec" wrote in
:

What shower rating gives optimal balance between efficiency and
running cost? My current shower is an 8.5kW unit and while output is
moderate it performs reasonably well. For my new house I am thinking
of fitting bigger rated unit, 9.5 or 10.5kW.

Alec

Bigger is better; they all (I theenk) can be run at less than max power.

However even a 10.5 has problems raising bitter cold winter water to a nice
temperature at a decent flow rate- it's the price you pay for this sort of
shower.

The power rating makes no difference, it's just that higher power can get
more water hotter, but it's how much flow and how hot you choose that
dictates cost, but a smaller shower will only get cold water up to a decent
temp at a dribble.

You will always wish you have more power in winter

Mike R