The Uk feed in tariff in the Uk. With a rate of 21.0p per kw a typical 2.22 kWp system can earn you an income of around £650 a year.
The feed in rate consists of two elements
Used Rate; 21.0p Export Rate: 3.0p
The unit rate of 21.0p will be paid regardless of whether the electricity is used by the homeowner or an extra 3p per kW is exported to the national grid. The export tariff is paid in addition to the unit rate.

The payment will be made every year for the next 25 years guaranteed. So bearing in mind that typical solar PV system will last around 35 years, you would be looking at a payment of £10,000 for a typical installation, you would be looking at a rate of return of 10%, and this is TAX FREE.

After 30 years compound interest would turn the £10,000 in a savings account to £27,568.

You can earn a 7%-10% tax free return, an income that will rise in line with inflation. At the same time, you get to do more than your fair share in reducing the UK's carbon emmissions.