Calculator here
http://www.tlc-direct.co.uk/Technica...ltageDrop.html
10 kw showers are OK giving adequate shower but I would have thought
7.5 kw just too small to be any good. If you are putting in a cable why
not go next size up - 10kw needs 10mm cable, go for 16mm and be covered
for a bigger shower.
I find electric showers very handy - cheap to buy and fit (depending on
cable and pipe route) reliable, quick response and always available,
but if you want the best then go for mains pressure via combi boiler ,
but then combis come with a whole load of problems of their own.
cheers

Jacob