wmc there are many variables to consider when calculating the pay back period.
The cost you pay for the install has a major influence on the pay back period. It goes without saying the cheaper you get the system the shorter the payback.
Next I suggest would be the orientation of your roof, if your roof faces due south your likely to get more bang for your buck.
Then can you afford battery storage? I would advise anyone to get battery storage before solar. Most U.K. homes use circa 10kwh of electricity per day, if you can afford a battery of at least 10kwh you would be changing your electricity cost from 25p per KWh to 7 or 8.5 p per kWh.
I am just about to install Fogstar batteries. 1 No 16.1 kWh battery will cost you £2,600 ish, including VAT. If you can get an electrician to install it for a reasonable price it will pay for itself in three/four years depending on your electricity use.
Kezos time period are about the norm so if you assume seven years you won’t be far off.
The only person who got all his work done by Friday was Robinson Crusoe.
Anything i post over three lines long please assume it is an article lol.