The environmental case for rooftop solar is almost unassailable: It's zero-emission, low-impact, locally generated power that doesn't kill birds or tortoises or eat up land. Installation provides jobs and keeps money within communities. As far as clean-energy solutions go, it's perhaps the least contentious. But there's a catch, and it's been a thorn in the solar industry's side for decades: the steep up-front cost of installing a new system. Returns on investment and savings down the line don't mean much when you lack the cash to get there. Sungevity is one of many players in the field to recognize this, but its forward-looking solar lease — little or no money down, free installation, and often a reduced energy bill from day one — doesn't just sidestep this roadblock, it obliterates it. The five-year-old Oakland company is now rapidly expanding in the United States and around the world, all on the back of one good idea serving another.