Calculate it for yourself...
I'm amazed how many register readers can't operate a spreadsheet.
A 4kWp solar system (the biggest you can install without reducing your FIT rate or the power company complaining) costs:
- £15,000 if you go for the super fab latest panels
- £12,000 if you go for cheaper ones (but with the same guarantees)
- £9,000 if you install it yourself (admittedly unlikely)
I've ignored the discounts you can get through group buying of up to 20%.
With southern UK solar incident radiation at ground level (someone else posted the EU site for that) I generate roughly 4000 kWh per year (3940 as calculated but everyone I know with a PV system has achieved better than that). Working on the £15k system with no FIT and using all the generated power (i.e. ignoring the export income) and with inflation of 5% a year I will be at payback after 10 years and be better off by £40,000 in 25 years (although I'd expect the panels to continue generating for a while after that).
With the FIT I will be at payback after 5 years and £100,000 better off in 25 years (or £30,000 if you correct for the value of money by allowing for inflation - 340% over 25 years).
In the interest of being fastidiously fair, I'd probably have to replace the £2,000 inverter twice in 25 years (they tend to only last 10 years on average). On the other hand, electricity prices are going up faster than inflation because of all those renewable power installations (!?)
I really can't understand why new houses don't all have solar panels by default. Mine are paying a couple of months of my mortgages each year.
On a much more frivolous and badly researched note, if I used the generated power to charge a Tesla roadster and use it for business travel (no company car tax on electric cars by the way so I can buy the car out of pre-corporation tax profits) I would also get another £4,000 in mileage (10,000 miles at 40p a mile, tax free and I don't need to buy any petrol). That and the FIT would pay for the Tesla and the solar panels after only 11 years.