Depends what power cabling you have to your shower. Mine burnt out because the installers did not put in the correct cabling (a few years back). I wanted to upgrade the shower (9.5 is the highest I think) and I had to have the cable upgraded to go with it. While the man was here he also put in a new power unit (fuse box). You really need to get in a reliable qualifed installer to advise you.
Are you sure you want an electric shower, col?
A shower that is fed from your hot water system (with built-in pump) is an alternative.
Or, if you have a combi boiler, a standard thermostatic valve shower.
I would agree with builder^^^. Mains fed via a pump is best. We have a Kholer Mira one which has a remote control fixed to the wall and works via Bluetooth. Just be careful where you position the pump as they can be noisey if fitted in the wrong place.