Re: Roof tank?
The solar PV panels on my roof have significantly reduced my carbon footprint in the year since they were installed. some of the energy produced goes into charing my Hybrid Car. So for short journeys (< 20 miles round trip) I run totally on battery. No petrol consumed. I've not had to fill up since March 17th.
The rest is fed back into the grid for which I do get paid.
I estimate that the ROCI for the panels is around 3 years at current usage (fuel costs and ££ income from feedingthe grid).