BRITaIN'S biggest energy companies will be handed a ?7billion windfall by government plans to boost "green" power generation, analysts have said.  Power companies which are already imposing sharp increases on household energy bills are set to benefit from measures to encourage the building of new nuclear power stations and windfarms, according to a report by Credit Suisse.  Chris Huhne, the Energy Secretary, will tomorrow set out plans to encourage low-carbon energy generation. The electricity market reform White Paper will offer financial incentives to low-carbon generators, guaranteeing them a fixed price for their power above the market price.  The incentives will be funded by households though their power bills.  according to the report, higher prices will bring in an extra ?24 billion for power companies between 2013 and 2020. Their costs are forecast to rise by ?17billion, giving a profit of ?7billion.  Companies' profits will be determined by the level at which the carbon "floor price" is set, something the bank said was subject to "unc