A new Zillow analysis shows that homes with solar energy systems sold for 4.1% more, on average, than others nationwide in the past year.

The sale premium varies by market: e.g., 5.4% in New York, 4.4% in San Francisco and 3.6% in Los Angeles. Zillow based its report on homes sold between March 1, 2018, and Feb. 28, 2019, that included any variants of the following words in the listing description: solar panel, solar collector, solar cell, solar battery, solar energy, solar power, solar electricity, photovoltaic, PV panel, and PV system.

Zillow: U.S. Homes With Solar Sell For 4.1% More

In honor of Earth Day, Zillow is helping inform homeowners and buyers about their own solar potential by putting a “Sun Number” on more than 84 million homes nationwide. The Sun Number measures the roof of each home and calculates the pitch, orientation and size of each roof plane. It determines the amount of sun that hits every square meter of the home’s roof, taking into account factors such as trees or taller buildings that could block sunlight. Finally, it adds local factors such as the cost of electricity and solar and the local weather conditions. The end result is a number between zero and 100; the higher the score, the more suitable for solar that house is.

Though certain markets’ solar energy potential seems obvious – e.g., good in Phoenix and not as good in Seattle – the Sun Number shows far more nuance, says Zillow. Zillow analyzed more than 500 metro areas, and although the top 10 are largely communities in the Southwest, there are some surprises: San Jose, Calif., (90) tied with Phoenix and Yuma, Ariz., for eighth. San Francisco checks in well ahead of sunny San Diego and Tampa, Fla., at a more than respectable 87, especially given its reputation for fog. Nationally, the median Sun Number is 78.

“Energy conservation isn’t only good for the environment – it can also translate into big savings on electricity bills, as well as help to reduce the strain on the electrical grid,” says Zillow’s senior economist, Sarah Mikhitarian. “The Sun Number provides a starting point for potential energy savings, but speaking with a local expert can help homeowners decide whether it pencils out. Homes with solar energy systems often sell for more than comparable homes without solar power. This premium is largely reflective of the future energy cost-savings associated with system.”