I want to remove my hot and cold water cylinders (very noisy and inconveniently sited) and heat the water by electric tankless on demand heater/s. There is only a wash hand basin, kitchen sink and bath  with shower to supply and only one user. I live in NE England. What capacity of heater/s will be required? For instance ,do you think one 12KW multipoint heater would be enough or two in parallel or, indeed, a heater for each outlet? Or is my idea half-baked?  Not keen on gas ,oil or anything else.