Free solar panels are offered in return for a long term lease of your roof space. The installer will provide the panels and maintain them and you benefit from the electricity they generate, which on an average property is likely to equate to around £5 per week. In return the installer will benefit from the Government FIT payments. You should seek advice prior to agreeing to such a contract. These agreements may include compensation payments which must be made in the event of roofing works if the system is not working. Also consider what happens in the event of the property being sold. It is quite possible the new owner may have to honour the existing contract, which may prove unpopular with any possible purchasers.
A standard 3k Solar PV (Photovoltaic) system generates around 2400 kwh of electricity per year. If we estimate the best local electrcity suppliers tariff is 14p per kwh, that would make the annual benefit worth £336, equivalent to £6.46 per week.