There is no simple answer, and it will depend on a number of factors:
A - How much money you would like to invest?
The supply and installation of a system can cost approximately of £5,000 per kWp and you should allow approximately 8 square meters of surface area per kWp. An average system size would be 2kWp and the maximum size under the feed-in tariff is 4kWp.
B - How much of your electricity you want to generate?
For most households, systems between 1.5 & 3 kWp, that will generate between 1300 & 2500 kilo Watt hours (kWh) of electricity per annum are most commonly fitted, but remember, you also get paid for any excess electricity that you generate.
C - How many panels can be installed in an optimal location?
Your system will be designed to suit the location, space and budget available for the project. The cost of installation will also vary according to how easy or difficult it is to access the roof and the type of fixing that is required.