^^^^^ I have wondered on this point as we looked at a couple of houses to buy which had got panels installed. I asked one estate agent for details as we liked the house but i just got a blank look and the i'll ask the owners. never heard any more and we found somewhere we liked more.
Also what happen if you have to remove the panels to repair the roof, do you have to pay the company for loss of income whilst the panels are not generating electricity? I know this is not very likely but i always like to have every eventuality covered.