It's Not Easy Being Green

California's rooftop solar mandate takes effect in 2020, requiring all new homes be built with solar panels, upping the monthly mortgage payment an average $40, with average monthly energy savings of $80 per month.

The California Energy Commission (CEC) data highlight that despite the long-term savings, any additional expense in an already expensive market can be a barrier for new homebuyers. Danielle Hale, chief economist at Realtor.com, told CNBC that the cost may hit the "affordable" end of the market harder, "It's a very different perspective depending on if you're looking for affordable homes, or pricier homes," said Hale. "It's already difficult for builders to build, and I think this is just going to exacerbate that problem."

Although the law begins next year, a Realtor.com study found 8 of the 10 U.S. cities with the highest percentage of home solar panels are already in California, and more than six percent of homes in San Jose feature solar panels. The second highest percentage is in Salinas with 4.8 percent, and San Diego is fourth with about 3.3 percent "green" homes.

"We might see states or localities make those decisions," Hale said. "I think it's going to take a couple of years before people see how the ramifications play out and what it really does for building in California, before they decide if it's a good idea to follow suit."

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