Southern California Edison said Thursday it signed the largest wind energy contract ever for a U.S. utility.

The agreement with Alta Windpower Development, a subsidiary of Australia-based Allco Financial Group Inc., covers at least 1,500 megawatts of electricity and more than doubles the utility’s wind energy portfolio. Financial terms were not disclosed.

The new turbines would generate twice the power of the biggest U.S. wind farm, the Horse Hallow Wind Farm in Texas, said Christine Real de Azua of the American Wind Energy Association (AWEA). One megawatt is enough to power for about 650 average homes.

The deal covers new projects slated for California’s Tehachapi area, including more than 50 square miles of wind parks.

The State of California has set a goal that 20 percent of the state’s electricity will be from renewable sources — in addition to hydro power — by 2010. At the end of 2005, 17 percent of the electricity delivered by of So Cal Ed, the state’s second-largest electric utility, was generated by using renewable sources such as wind power.

Shares of the utility’s parent, Rosemead-based Edison International, were down 16 cents to 45.71 in midday trading Friday.