Derek Broughton wrote:

I don't know how you get your figures, but the average North American uses
some 40 THOUSAND US gallons of hot water yearly. The water heater usually
runs in excess of 120F, from a coldwater supply well below 70F. Raising
40,000gal * 50F = 320,000lb * 50F = 16,000,000 BTU, which if I am not
mistaken is going to cost around $25/million BTU to heat. So that comes
out to $400/year. A tad more than £20, and well worth replacing even with
a system that only generates half your hot water.

Where did you get your figures?
About 25 years ago, I was living in a drought-stricken area of the US.
Limits were put on water use - 50 US gallons per person per day. That was
for ALL water, not just the heated stuff. The only thing that suffered was
the garden - which wouldn't have been using heated water, anyway.