Does a solar system add to the sales value of a home? Solar energy advocates like to say it does, but that’s often based on anecdotal evidence.

The Berkeley Lawrence National Laboratory decided to test solar power home value, and the result is a report being issued Thursday that showed that, indeed, solar homes command higher prices.

The analysis showed solar homes in California were sold for a premium at between $3.90 per watt to $6.40 per watt, the Berkeley Lab report said. That spread amounted to an average of a $17,000 premium for a fairly new (roughly two years old) 3.1 KW system, which is the average system size in the data, the report said. The premium began to decrease as systems became older.

The lab researchers poured over data about roughly 72,000 homes sold from 2000 to mid 2009, including 2,000 solar powered home.

The average premium tracked closely with the average price of $5 per watt that California homeowners spent to install solar from 2001 to 2009 (the price took into account state and federal incentives), the report said.

California is the largest solar market in the country as a result of its incentive program that defrays a big chunk of the installation cost. The state is home to nearly 100,000 grid-tied solar systems, and more than 90 percent of them are located at homes. It also has a good data collection system to track installations and price movement, which Berkeley Lab has analyzed in several reports in recent years.