California has hit a major renewable energy milestone: 1 gigawatt — or 1,000 megawatts — of solar power has been installed on rooftops throughout the state, according to a report to be released Wednesday by Environment California, a statewide advocacy group.

One gigawatt is roughly the size of two coal-fired power plants and is enough energy to power 750,000 homes. Five countries have hit the 1 gigawatt installation mark to date: Germany, Spain, Japan, Italy and the Czech Republic. California has installed more solar power than France, China and Belgium.

The cumulative tally for California includes solar panels installed on existing homes and commercial buildings as well as new-home construction. It includes solar connected to the electric grid by large utilities like PG&E as well as solar within municipal utilities in cities like Palo Alto and Santa Clara.

The report credits the California Solar Initiative, the state’s aggressive program to encourage homeowners, businesses, local governments and nonprofit organizations to install solar panels on their roofs, with the milestone. About 600 megawatts has been installed through the California Solar Initiative.

“California can become the Saudi Arabia of the sun if it continues to get behind big, successful solar programs,” said Michelle Kinman of Environment California and co-author of “Building a Brighter Future: California’s Progress Toward a Million Solar Roofs.”

Despite the state’s weak economy, solar installations saw tremendous growth in 2010, thanks to falling prices and the popularity of no-money-down leases.

“Of the total 1,000 megawatts of rooftop solar installed statewide, a record 205 megawatts was installed in 2011 alone,” Michael Peevey, president of the California Public Utilities Commission, said in a statement.

The report will be released Wednesday at the Oakland offices of Sungevity, a startup that serves the residential market in eight states. Sungevity has more than 3,000 customers nationwide and sold 2.2 megawatts of solar in October.

PG&E has 55,000 residential and commercial solar customers within its Northern California territory, more than any other utility in the country. And San Jose is PG&E’s leading solar city, with 25.9 megawatts installed. Large school districts like San Jose Unified and the East Side Union High School District have installed solar panels, as have leading companies like eBay (EBAY) and scores of homeowners. Bakersfield, Fresno, Santa Rosa and Livermore are also solar hot spots.

Much of the solar growth has been fueled by state incentives. The California Solar Initiative provided $3 billion in incentives for solar energy projects, with the amount of the incentives decreasing over time as the solar market matures. Rebates for residential systems are given upfront — a one-time payment based on system size. The rebate level for PG&E residential customers has fallen quickly in recent years and stands at 25 cents per watt. So if you’re putting in a 5 kw system, that results in a $1,250 rebate.

Contact Dana Hull at 408-920-2706. Follow her at Twitter.com/danahull.