It stands to reason that if we can look up low airfares and bargain hotel prices on the Internet, why not comparison shop for solar panels, too?


That is exactly what Google had in mind when it unveiled Project Sunroof, which allows homeowners to calculate the solar energy potential of their roof. 


Launched in August 2015, it was initially made available in the San Francisco Bay Area and the California city of Fresno, as well as the Boston area. In December, Google expanded the program to include a total of 20 U.S. metropolitan areas in Arizona, California, Colorado, Connecticut, Massachusetts, Nevada, New Jersey, New York and North Carolina.


Carl Elkin, lead engineer for Project Sunroof, writes in a blog post that the tool is for all those homeowners who think that “my roof isn’t sunny enough for solar,” or “solar is just too expensive.” The tool uses high-resolution aerial mapping (the same used by Google Earth) to make its calculations.


After a homeowner enters his or her address, Project Sunroof crunches the numbers. It figures out how much sunlight hits the rooftop throughout the year, taking into account factors such as roof orientation, shade from trees and nearby buildings, and local weather patterns. Homeowners can also enter their typical electric bill to customize the results. The tool combines all this information to estimate the amount a homeowner could potentially save with solar panels. It can even connect homeowners with local solar providers.


According to Elkin, Project Sunroof not only tells homeowners whether their house is a good fit for solar panels, it can also determine whether paying for installation will pay off in the long run.


“In short, to see the effect sunlight can have on their wallet,” he said.