humadoon:
How can we install it? it seems bit difficult ..
Er... connect to cold water supply, connect to its own earth with grounding rod if necessary, if using a 10kw then you'll need some 16mm cable I think (check) and perhaps you'll need to upgrade the wiring to the external supply to 25mm if not already at that level or above (not sure about UK standards). Install a Safe-T-cut with the right amps (remember physics ?) 10,000/220=45 amps so you'll need a 50 amp Safe-T-cut. Perhaps check your board has a good overall safety.
Of course, get some help if you cannot do it yourself. easy enough for an amateur if it is just a replacement for an existing unit but not so easy if it is a new install.