Ther's little point having panels on a north facing roof as others have said. Obviously in the N. hemisphere a south facing aspect is best but you still can have them on east & west facing roofs - and if you have both you will need 2 inverters which will put the cost up a bit. Alternatively, they can be put in a 'standalone' situation say in your garden etc.
Accredited MCS companies will go thro' an approved Govt. calculation to demonstrate what you can expect to generate in your specific case.
In addition to MBP's link, here is one that got me into the subject :-
Feed-in Tariff scheme
and on the left hand side of this page are sub links to different aspects, including solar panels.
With respect to installers, BE VERY CAREFUL! We ultimately had 4 quotations but the first was a real rip-off. There is an industry code-of-practice, (I think that's in the Energy Saving Trust link) & our first broke virtually all the rules AND wanted 25% deposit of his outrageous quote, i.e. nearly £5,000, before he would leave. Needless to say we sent him packing - I should have reported him but unfortunately didn't. There is/was a recent Which report on Solar Panel selling techniques.
Probably best if you explore local installers but they should be accredited MCS. Ours were and were Chartered Surveyors.
With respect to Ron's earlier post, I'm afraid you don't usually get anything for free in life. In his link case, the panels and installation would cost you nothing but that company would 'own' the system and claim the Feed-in-tariff.
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