With the recent expansion of Project Sunroof, tens of millions of potential solar customers from across the U.S. can now Google their own rooftops to find out if their home is suitable for solar panels.

Google launched Project Sunroof last August in three cities -- San Francisco, Fresno and Boston. In January, the program expanded to 20 U.S. metropolitan markets in the most active solar states in the U.S., including California, Massachusetts, Arizona, New York, New Jersey, Nevada, Connecticut, Colorado and North Carolina.

Last month, Project Sunroof hit a new milestone by expanding to 42 states, with the ability to analyze roughly 43 million rooftops. According to Google, “thousands” of customers are visiting the Project Sunroof website each month, and the company is continuously working to expand its reach.

Project Sunroof is currently not available in Texas, Hawaii, Rhode Island, Idaho, South Dakota, Mississippi, Tennessee, Alaska and the District of Columbia. However, Google plans to reach all 50 states in the coming months.

“Google is in the business of providing universal access to information,” said Nicole Lombardo, head of business development and partnerships at Project Sunroof, in an interview. “So being able to take the imagery that we have and find new use cases for it that can help, in this case catalyze the renewable energy transition here in the U.S. ...is within our core.”

As a company, Google is also a large user of renewables and wants to “help others outside of the company choose cleaner power options,” she said.

Project Sunroof works by using high-resolution aerial imagery from Google Earth to help calculate a roof’s solar energy potential. Potential customers simply need to enter their address, and in roughly one second the program analyzes factors such as shade, roof orientation and local weather patterns to calculate how many hours of sunlight hit that roof in a typical year. All of this information is combined to create an estimate for how much the household could potentially save by going solar over the term of a 20-year lease.

Users can fine-tune the estimate by entering their electricity bill information. They can also view savings estimates for different financing options, including a loan and direct purchase. Potential customers can then click to see solar providers in their area.

Project Sunroof currently hosts a mix of solar companies, including major players like SunPower, local installers like Verengo Solar and new players like Pick My Solar. Interested customers can choose to share their contact information with selected providers on the Sunroof platform or contact them directly.

For customers, this amounts to a quick and easy way to learn more about solar and get an initial quote. For companies, Project Sunroof serves as a lead generator. But rather than receive a list of names, installers receive a list of customers that are actively looking at going solar and whose rooftops have already been prescreened.

“First and foremost, this is about how Google can catalyze the rooftop solar market,” said Lombardo, speaking at GTM's Solar Summit. “That was some of the first feedback we got from developers: volume, volume, volume -- we need more qualified homeowners.”

“The second piece of feedback was [the question of whether Google can] help us expedite the process of qualifying someone,” she added. “That’s where some of the new imagery that we have and being able to calculate whether or not they have enough roof space helps simplify a couple of steps.”

This service isn't free for solar companies. Providers have to pay to join the platform, and they have to bid to receive referrals. It’s up to users to actually send their information over to a provider, and the company pays only when Google shares its information.

Max Aram, founder and CEO at Pick My Solar, which operates a marketplace of rooftop solar installers, said he finds Project Sunroof to be valuable in two ways: “One is that when Google jumps into an industry it’s a validation for that whole industry,” he said. “The other thing is that the quality is good and the model is interesting. They’re not trying to buy and aggregate leads originated by other companies; they’re originating their own deals and controlling the search. So obviously, it’s going to be much easier for Google to provide these leads.”

As awareness around solar increases, Google could eventually eliminate the myriad of lead generation companies currently out on the market, said Aram. Google provides a key top-of-the-funnel service, because Google search is virtually universal. Once the leads are generated, Aram said he sees marketplace companies like Pick My Solar taking control of the sales process for a large segment of the market, similar to Kayak or Expedia for travel. Solar companies would then manage the installation.

A central challenge for Google is to continue to expand the pool of customers. Today, customers are primarily learning about the Project Sunroof through news articles and by using Google to search for "Project Sunroof" in order to find it.

“Because we’re still in an early stage, the majority of the market does not know this this tool exists,” said Lombardo. “So one of things we’re hoping to work on is making it more widely known.”

Google is also working to make the service more widely available. Project Sunroof is now available in 42 states, but those states do not have full Google Earth coverage. At the same time, Google is continuously trying to improve its imagery. There is a team of 10 employees, mostly engineers, working to refine the technology and offer more granular information on things like the boundary of a roof and height points like a chimney.

“We’re never done,” said Lombardo. “We are constantly helping to train the neural network behind this.”

In addition, Google is working on how to make the tool more valuable to solar companies. Sunrun and SolarCity, for instance, are not Project Sunroof partners. These companies have already made major investments in their own customer acquisition and engagement programs, and already spend a lot money on online marketing with Google and other companies. Paying more to access Google’s solar leads, particularly at the pace and scale at which national installers operate, is an expensive and potentially redundant proposition.

Lombardo acknowledged that the Sunroof project is evolving. Eventually Google plans to go beyond residential solar to support community and commercial solar projects. The company could also offer new services to solar companies, like tracking utility rate changes -- it just depends on technology barriers and demand.

“We’re definitely at a point where we’re looking for partnerships and ideas,” she said. “We’re here to help.”