Solar Impulse plane sets off for New York in last leg 6 July 2013 Last updated at 10:18 The Solar Impulse plane has taken off for the final leg of its bid to cross the US on solar power alone. The craft set off at 04:56 local time (08:56 GMT) on Saturday from Washington DC on a course for New York's John F Kennedy airport. Pilot Andre Borschberg anticipates a journey time of about 21 hours. The Across America transcontinental bid began in San Francisco, California, and included stopovers in Phoenix, Arizona, Dallas, Texas, and St Louis, Missouri. The Solar Impulse HB-SIA has the same wingspan as an Airbus A340 but at a weight of just 1.6 tonnes. By comparison, a fully laden A340 weighs about 370 tonnes. The plane's wing and stabiliser are covered with nearly 12,000 solar cells, which drive its four propellers and charge the plane's 400kg of lithium-ion batteries for night-time flying. The project has been billed as the first time that a solar-powered plane capable of flying day and night has attempted a journey across the US. Mr Borschberg and his fellow pilot of the single-seat craft have been alternating flying duties, with each leg of the trip designed to be under 24 hours in length. The prior leg was complicated by high winds and air traffic, so it was re-routed via Cincinnati at the last minute in order to stay under this limit. "It was difficult to find all these 'weather windows' in the States - especially this last one," Mr Borscherg told BBC News prior to the flight. We had so much to organise to make it feasible - to integrate an experimental airplane like Solar Impulse in the busiest air traffic in the world, flying from Washington to Kennedy (airport) was extremely hard. The Across America project is the last outing for the HB-SIA prototype craft. Mr Piccard's and Mr Borschberg's intention is to finish a larger, two-seat plane, the HB-SIB, and fly it around the world in the spring of 2015. That, Mr Borschberg said, would be "many times more complex than what we did here in the US." "Working in one country with one language makes it of course easier than working internationally on different continents with all the logistics," he said. The unpredictable side of the project requires a lot of preparation. The HB-SIA craft carried out the first inter-continental flight in 2012 and holds the world record for the longest manned solar-powered flight at 26 hours. During the Across America bid, it set the record for the greatest distance of a manned solar-powered flight. The project coincides with Piccard's and Borschberg's Clean Generation Initiative, an effort to encourage policy-makers and businesses to develop and adopt sustainable energy technologies. At a welcoming event when the craft reached Washington DC, US Secretary of Energy Ernest Moniz said: "If you look at the Department of Energy's research programmes and technology visions, it involves many of the same kind of technology advances that these gentlemen have put together in an integrated system to accomplish something that would not have been thought of a few years ago. I believe in 10 years we're going to see the fruits of all these technologies changing the world.