I take it is a Truma Ultrastore (see photo) , if so then its 850 watt , older ones had one about 450W which are no longer available . The fitting is different .If you search ebay for Truma ultrastore heater element replacement it will return several suppliers . some sell the oriTruma part at a higher price and some sell equivalent ones. When I have bought these equivalent ones I found the element was actually closer to 1200W which is no big deal (the wiring and fuse will take the higher load) but the sealing ring supplied with the new element didn't seal properly ( thinner section) so I had to use the original one, Also when fitting the thermostat apply a little heat sink compound to the mating surface