Im trying to work out what sized boiler system is required for my house . I need to understand on average how many kws

Bath ?
Shower ?
Radiator ?

THe bigger factor is how many people reside in the house, which determine how much hot water is needed in volume... which determines how large a hot water tank you need. (assuming you aren't using a flash boiler that works on demand). You don't size them based on how many KW's of electric they use.