groSolar announced on Thursday that it acquired the residential solar division of  Borrego Solar Systems, creating the fourth-largest residential solar power installer in the United States.

The deal will help groSolar launch new programs for homeowners to access solar power and expand the company’s distribution network, groSolar CEO Jeff Wolfe said in a statement.

Financial terms of the deal were not disclosed. But the move gives groSolar a stronger presence in California, the largest solar market in the United States. Borrego had residential operations in the state’s most important solar markets: San Diego, Orange County, and the San Francisco Bay Area, in addition to offices in Massachusetts.

The company bought Chesapeake Solar of Maryland last year and Energy Outfitters of Oregon in 2006. The venture-backed solar installer has also signed a $175 million, four-year purchase agreement with Evergreen Solar and has an exclusive North American distribution deal with Motech Industries, which makes inverters.

But the solar industry is struggling as the broader financial slowdown has made businesses and homeowners less willing to invest in solar systems. And creditors, like banks, have slowed or stopped financing larger solar deals.

Still, the cost of solar systems has been falling in recent years and analysts predict they will continue that trend for the next several years. Installers should benefit since they can pass those savings on to their customers, analysts say.

El Cajon, California-based Borrego also announced that it would focus exclusively on its commercial and government business units. The company, which announced a $14 million round of venture funding earlier this month, said these units represented 75 percent of its new revenue in 2008.

White River Junction, Vermont-based groSolar has operations in Canada and 12 U.S. states, including New Jersey, New York, and California.