The biggest thing stopping the sun is money. Installing a rooftop array of solar panels large enough to produce all of the energy required by a building is the equivalent of prepaying its electricity bill for the next seven to 10 years—and that’s after federal and state incentives. A new innovation in financing, however, has opened up an additional possibility for homeowners who want to reduce their carbon footprint and lower their electric bills: get the panels for free, then pay for the power as you go.