Five years ago, Staples made it a corporate priority to cut its carbon dioxide emissions. The executives at the office supply giant decided, however, that whatever methods they chose-improving efficiency, changing operations, or buying low-carbon energy-the effort also had to make financial sense. At the time, no one could argue that buying solar panels was a good investment, as payback periods for recouping capital were typically measured not in years but in decades.

But 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.

Staples has now installed about 10 megawatts of solar capacity at more than three dozen sites-the equivalent of 2,000 […]