Since solar panels in Britain generate, on average, only 20 per cent of 
their potential maximum output (at a cost of £4,500 per kilowatt of 
installed capacity), he reported in the journal of the Institute of Civil 
Engineers that the average pay-back time of solar panels is more than 45 
years (although 70 years, he wrote, "is a more realistic figure"). Yet the 
average life of a photovoltaic cell is only 25 years.

In other words, if taxpayers were not so generous in providing £3,000 per 
kilowatt of capacity, no one in his right mind would dream of installing 
them.



http://www.telegraph.co.uk/news/main.jhtml?xml=/news/2006/11/19/nbook19.xml