Rooftop solar panels are proving profitable when homeowners sell, according to a recent study from the Lawrence Berkley National Laboratory.

Researchers evaluated 1,894 sales of California homes that featured rooftop solar arrays between 2000 and 2009 and compared them with 70,425 sales of homes without solar arrays.

On average, homebuyers paid an extra $24,705 for homes with rooftop solar arrays, according to the study titled “Exploring California PV Home Premiums.”

Of course, the premiums varied depending on the homes and the size and age of the solar arrays. Bigger and newer rooftop solar arrays demanded a bigger home price premium that typically more than covered the seller’s investment in the renewable energy system.

But the value assigned to rooftop solar drops dramatically with time, according to the study. For every year the system has been installed, it depreciated an average of 9 percent.

That surprised Berkley researchers because systems are only expected to lose 1 percent of their efficiency each year.

“They might be perceived as older technology, even if they’re still producing electricity at the expected rate,” report author Ben Hoen told SF Gate.

Solar panel efficiencies have been climbing in recent years as the cost of installing solar systems has declined.

The cost to install a typical solar array was $10.73 per watt in 2009 and has fallen to $5.71 in 2014.

The Wall Street Journal talked with some high-end homebuyers and sellers following the release of the Berkley study this week about the impact solar arrays have on real estate values.

Palm Springs Realtor Benjamin Leaskou told the Journal that solar PV systems help to sell high-end homes.

“When you have multiple fridges, pool lighting, landscape lighting and HVAC zone, you can run into $1,000 to $2,000 a month electric bills,” Leaskou told the Journal. “But if you put in a large enough kilowatt system, it can offset that by half if not 75 percent.”

That could explain the premium Berkley researchers found for larger systems.

The study was a deeper exploration of a trend researchers discovered in a report released in 2011.

While the research didn’t include leased solar arrays, which account for nearly 80 percent of California’s residential solar installations, a future study is planned to evaluate the resale value of leased solar installations.

“The take-away here is the market is showing that PV is valued by homebuyers,” Hoen told SF Gate.