Ok let's give him benefit of doubt (for 30 sec's

and help say no to nuclear!
0.2kw system in Brighton
(I'll help him out location wise)
optimally positioned
(this time out of the shade)
and pitched Optimally
(probably more by luck than judgement)
we will also give him the benefit of efficiency because he will of course have chosen the inverter and panel for best fit for power production.
We'll be generous and say 950kwh/Kwp so:
0.2x950 = 190kwh/Yr
Fortunately for us we don't have the feed in tariff to worry about!
So a simple calculation and I'll be generous again and say they haven't been paying attention to the Electric bills and they are paying 16p/kwh so:
190x0.16p=£30.40/Yr
I'll ignore the Rpi issues and fuel price increases because frankly I can't be bothered so on a straight Roi that's
£795/30.40=26.15yr's to get your money back.
Don't forget tho you'll have to replace the inverter every 18months
(and thats not included in the costs)
as it was only supposed to power a laptop from a car battery!!!