David Hansen wrote:

John Rumm wrote this:-

Even if is saves you £100/year that would still be a 50 year payback.

Simple payback period is not the only reason for doing something.
What is the simple payback period on a new kitchen, or replacing
fluorescent kitchen lights with downlighters?

Neither of those are done for payback unless you count count keeping the wife
happy as payback. Will your wife be happier with solar water heating over nice
decoration might seem to be the right question to ask.

Graham