A startup called SunEdison came along and made an offer Staples couldn't refuse—employing a financial model that could give solar the edge it needs if it's to provide a significant portion of the world's energy. Under SunEdison's plan, Staples would get solar panels on its retail rooftops at no upfront cost and without any monthly equipment fee. Instead, it would agree to pay SunEdison a preset rate for the power the panels generate over a period of 20 years. "The bottom line is that we're able to purchase solar energy off our rooftops for less than electricity off the grid," says Mark Buckley, Staples's vice president for environmental affairs.