Electric showers above 7.5 kw require increased fuse rating and more imprtantly increased cable cross section i.e. you need
a) 40 Amp supply fuse (8.7kW)
b) 45 Amp supply fuse (9.8kW)
often people upgrade the shower without realising that they need to rewire the supply. You need a competent electrician to check the draw of current and that the cable is OK.
Graham (amateur shower fitter!), September 2004