I had a 3.9KW 20 panel system fitted last November. The estimated generated KW was 3218 per annum. I passed that two months early. I was reluctant to have them fitted as the government was talking about changing the system of payments, so I wrote to Chris Huhne and had a reply confirming that there would be no retrospective changes to the payments described above, ie if you join on a tariff, it is guaranteed (as much as anything ever is) for 25 years, index linked. I estimate I am getting at least 12.5% return on capital at today's price of electricity, which of course will stay at this level, I don't think. My electricity meter is only reading about 10KW more than it was in March, and I have used the immersion heater to heat water since March, and have not used the gas boiler since April. My array points to 168 degrees at 27' to the horizontal. Personally, I think its a no-brainer, as it is highly unlikely you will ever get a better return on capital, and which can only increase as prices rise. PM me if you want to discuss it in more detail. I agree with those advising against the so-called 'free' offers.