Had a couple of showers to fit over the past two weeks, also quoted for a complete new install. The tricky part is deciding what advise to give the customer if it is worth paying extra for a 10.5Kw. As it happens I got to run and play with both an 8.5 and a 10.5 within a few days of each other. My conclusion was that if you like a stonking hot shower and you have the water pressure available then a 10.5Kw might be what you need. Otherwise I concluded a 9.5 would do for most people. The 8.5 (7.8Kw at 230V) was OK too, I would call it adequate and economical on water and energy.
The 10.5Kw when running really hot produced a lot of steam/moisture in the air so bear in mind what ventilation is available as well.
The 10.5Kw when running really hot produced a lot of steam/moisture in the air so bear in mind what ventilation is available as well.