The financing technique that gave Europe an early lead in renewable energy is starting to cross the Atlantic. Put simply, the idea is to pay homeowners and businesses top dollar for producing green energy. In Germany, for example, a homeowner with a rooftop solar system may be paid four times more to produce electricity than the rate paid to a coal-fired power plant. This month Gainesville, Fla., became the first city in the United States to introduce higher payments for solar power.