Homeowners and businesses have now installed one gigawatt of roof-top solar panels, according to a report released this week by the advocacy group Environment California.

A gigawatt – or a thousand megawatts – is enough energy for about 600,000 homes. Only five nations — let alone states — including Germany and Japan, have reached that level. “Even in a bad economy, the solar industry has been growing exponentially by 40 percent per year,” says Michelle Kinman of Environment California.

The goal comes five years after California’s Million Solar Roofs Initiative began, which mandates three gigawatts of rooftop solar by 2016.

The report credits the rapidly falling prices of solar panels for the growth, as well as the California Solar Initiative, a $2 billion solar rebate program. In 2007, the program provided a rebate of up to $2.50 per watt. As demand has grown, the program is designed to reduce the incentive. Today, it’s between 25-65 cents per watt.

“The real goal of the program is to create a sustainable solar industry in California that will continue to thrive without continued ratepayer incentives,” Scott Murtishaw of the Public Utilities Commission. “And I think that we’re achieving that. The installed cost of solar has fallen by roughly a quarter since the program began.”

The report says California is on track to meet the three-gigawatt goal in 2016. Adam Browning of Vote Solar says the state’s net metering cap could get in the way. Solar customers sign up for net metering contracts with their utility, so they’re credited for the electricity they generate. Right now, California utilities are only required to sign contracts for up to 5% of their overall load.
“There are clouds on the horizon and we’re going to have to lift the cap again,” says Browning.

Still, he says it’s a good story. “We’ve hit that transformation point with solar. It’s like cell phones. They’re nowhere until they’re everywhere. Once they hit a sweet spot, you see an explosion and I think that’s what’s happening now.”