British Gas has teamed up with Nissan to offer motorists an escape route from record pump prices by allowing them to power their cars free - after an outlay of nearly ?40,000, that is.  The energy company has launched a package that offers customers the chance to generate enough electricity at home to recharge their cars and sell some back to the national grid. The catch is that they must own an electric vehicle and have a roof large enough to accommodate an array of solar panels.  after buying Nissan's ?25,990 electric Leaf and 14 solar panels costing ?13,168, customers can expect to save about ?1,560 a year on running their car - based on the aa's figures for an average petrol car travelling 12,000 miles annually - as well as an estimated ?117 by using home-generated electricity for domestic appliances during the day.  That means the solar panels will pay for themselves in about eight years, although it may actually be longer, as most people will charge their car overnight, when the solar panels are inactive. and what happens if the sun isn't shining? That shouldn't be a problem, as British Gas claims the panels need only daylight, not direct sunlight, to work. Best of all, customers can take advantage of a government incentive called the feed-in tariff, which pays them generously to produce green electricity, whether they consume it or sell it back to the grid. The utility company claims this could amount to ?1,280 a year with its 14-panel solar array, potentially reducing the payback period to 4? years.  "a lot has been made of charging points at the roadside, but our research suggests that more than 80% of electriccar charging will be done at home," British Gas said.  Dominic Tobin