Sunshine has been turning grapes into raisins for thousands of years. Now, it’s turning them into wine.

The county’s biggest photovoltaic installation is nearing completion at booze giant Constellation Wines’ Gonzales winery. Once online, the 1-megawatt system will produce more than half the winery’s energy, reducing 1.6 million pounds of carbon dioxide emissions per year – the equivalent of taking 2,000 cars off the road.

Constellation spokeswoman Nora Feeley describes the solar installation as social corporate responsibility for the nation’s richest winemaker, which pulls in about $2 billion annually: “It’s+ an ongoing global commitment.”

But it’s mostly a matter of economics. The solar system is expected to trim the winery’s costs while giving it green marketing points – and making money for the power managers who came up with the idea.

Kelly McMahon, sales director for Auburn-based Pacific Power Management, says he first noticed the massive winery warehouse during a drive down Highway 101. “Big, flat roof,” he says. “And it looks like it uses a lot of electricity.”

McMahon convinced the winery to sign onto what is known as a “power purchase agreement” in which PPM designs, installs and owns the solar system at no cost to the winery. Constellation purchases the electricity, with an option to buy the system after 20 years.

That’s a particularly good deal for PPM because the winery doesn’t use much energy in the summer, when PG&E rates are highest. “If we produce more than we’re using, the meter will spin backward and we’ll get credit at the full retail rate,” McMahon says. “Those grapes are still out in the field!”

Since October, PPM has been installing almost 6,400 185-watt panels on 170,000 square feet of the winery’s warehouse roof – a system with a market value of about $8.5 million, according to McMahon. He expects the installation to be complete by the end of March and online a few weeks later.