My mate does solar panels, however, these are purely for heating the water - not PV panels. A "mates rates" installation is about £3,500 and most customers estimate it reduces their electricity bill by about £500 per annum, so the payback time isn't too bad. In most instances, the solar panels heat all the water from about March-Novemberand; even in the winter the solar panels still heat the water by 20 degrees or so, so less mains electricity is needed to fully heat it and your bolier is doing less work, so should last longer.