Fair point on the quibble. I was thinking more in terms of efficiency as energy produced versus energy to make the thing in the first place.

Sadly photovoltaic is only a worthwhile option in places with lots of guaranteed sunlight. The Mediterranean or southwest US might quality - the UK certainly doesn't. "Which?" magazine calculated a 20-year payback term on photovoltaic in the UK, assuming that it runs for 20 years without needing any servicing.

Solar heating is a much better bet though. You just need to be somewhere where installing it doesn't mess anyone else up. Detached houses in the country and most US houses (again mostly detached) are fair game. Houses in cities or on estates might be more of a problem.

Graham.