"Peter Parry" plehead extrodanaire wrote in message
...

I don't believe it.

I'm sure you don't. Elementary understanding of physics and maths
has always been rather beyond your meager abilities.

If you spent £500 20 years ago and you haven't recouped
the cost? Do some sums.

I did, and it was monitored for much of its life. It saved an
average of GBP 18 per year. If the money had been put in a savings
account it would have earned far more than that.

£18 a year? It must be facing north, or poorly and inefficiently
installed - must be if you had anything to do with it.