For the past two years, Project Sunroof has walked people through all the information-gathering steps of installing solar panels: After you tell it where you live, its algorithms estimate how much solar energy falls on your roof, calculate how much solar panels would reduce your electricity bill, and deliver estimates from local installation firms like Solar City.

It can also walk you through similar steps if you’re interested in leasing or borrowing panels. “It highlights that, for many people, solar is often free. In many cases, including for my house, solar is better than free,” Elkin told me last week.

Now—in a nod to the powerful peer effects of solar power—it will also show you which of your neighbors have already installed panels. In its map view, Project Sunroof will show a red dot over any home or structure that appears to have rooftop solar.

“People want to know: ‘What if there’s some hidden gotcha in the contract?’—and usually there isn’t. ‘Does this work for other people like me? Is solar really viable in my neighborhood?’” Elkin says. “You can zoom around through your town and understand how common solar is in your neighborhood. And many people have found: Wow, there is a lot more solar in my neighborhood than I’d realized.”

Google created the data for this feature in-house, training a machine-learning algorithm on the common appearance of rooftop solar panels and then letting it loose on the cities and towns that Project Sunroof already covers. Right now, the company has analyzed installations on about 60 million buildings in the United States; it hopes to get to the remaining 40 million buildings in the next few years. The methodology doesn’t seem to be perfect yet—I noticed some rooftop solar installations in my own neighborhood that the algorithms missed—but it seems to identify most of them.

“I think the idea is a really great one,” says Gillingham, who has previously talked with Google about his research but did not know they were working on this feature.

His current research has found that people are even more likely to install solar panels if they can see their neighbors’ installations from the street—suggesting that day-to-day visibility, and not, say, word-of-mouth or local marketing efforts, is what nudges people to look into solar power. The new Project Sunroof feature “tells you that people nearby have installed solar panels even if you can’t see them from the road,” he says.

Gillingham did share some privacy concerns about the feature. State solar registries often hide address-level data, and they only share it with academic researchers confidentially. “However, if you go on Google Maps or Google Earth, you can pretty easily see the installation in pretty much all cases,” he says.