Project Sunroof was thought up by Google engineer Carl Elkin, who worked on it in his 20% time (Google allows employees to pursue side projects in addition to their regular job). The pilot project is starting small, and only works in three cities for now: Boston, San Francisco, and Fresno ("where one of the engineer's mom is from").
If I were to drop $91 a month to lease solar panels for my San Francisco home, I could save $14,000 over the next 20 years, according to the tool. Annually, I'd save $700, or about one month's rent for a walk-in closet in San Francisco.
According to local weather data, I get 1,856 hours of sunlight every year. That takes into account seasonal changes, the angle of my roof, and potential sun-blockers like a neighbor's house or trees. Using the Google Maps satellite image of my roof, Project Sunroof determined I have 961 square feet of space that can be covered in solar panels.
Google (GOOG) cleverly charges solar providers to list their information below these estimates. If you fill out a contact form at the bottom of the page, the providers you've selected can reach out and try and sell you their goods.
Related: Alphabet's coolest tech bets
CNNMoney (San Francisco) August 17, 2015: 5:10 PM ET