Payback? Many many years.
All the 'calculated' claims are in best scenarios (which you will no-where near meet)
Companies that install them free pocket the Government ''feed-in tariff' for as much as 25 years. Better to install on a bank loan if you ask me, so long as you receive the feed in tariff.
Your roof need to be more than about 20 sq M. and 45 deg. approx, and face approx. south.
payback is index linked - so assuming inflation at 3% (currently 4.5%) over the next 25 years, total gain should be in the region of £57,132.06. Total gain includes the savings on electric bills and selling 50% of generated power back to the company at .03p per Kw.