An MIT spinoff company, Mapdwell, is determining  the solar potential of roofs for entire cities and providing a cost-benefit analysis for individual buildings. Their website shows a satellite map for a city, and you can click a building to see information about the financial benefit for installing panels on its roof. The company has mapped Boston, Cambridge, Wellfleet, New York, San Francisco, and a few cities in Chile, and it plans to have mapped all major metropolitan areas by year’s end. Results for Cambridge show a return of invested dollars within seven years.