BOULDER - Boulder-based companies Main Street Power Co. Inc. and Lighthouse Solar Inc. on Wednesday announced the recent completion of a combined 7.5 megawatts-worth of solar projects across the country.

Main Street Power partnered with MS Solar Solutions Corp., a subsidiary of Morgan Stanley, to finance a 2.5 megawatt project on a brownfield site in Stow, Massachusetts. The 12-acre site was otherwise unusable for development or farming because of contaminated groundwater.

Main Street Power will own and operate the 8,769-panel solar array, which is expected to generate enough power to offset the use of 300 typical American homes. A local utility, Hudson Light and Power Department signed a 25-year power purchase agreement to buy all of the array's output.

The array is the second in recent months opened by Main Street Power on a brownfield site. Because such arrays can require a lot of land, Main Street Power project manager Rob Cooper said the move toward using brownfield sites is a growing trend in the solar industry rather than using greenfield sites that could be cleared for other developments.

"It's a nice way to do solar on otherwise unusable pieces of land," Cooper said.

Cooper said the Stow site poses no risk to those who will be maintaining the array in coming years, and he said Massachusetts officials will continue to monitor the groundwater there.

While Main Street Power's announcement Wednesday covered one site, Lighthouse announced that it completed installation of projects totaling 5 megawatts in the fourth quarter of 2013 in Arizona, California, Colorado, New York and Texas.

Lighthouse spokeswoman Stephanie Chen said Colorado projects accounted for a good chunk of the 5-megawatt total but couldn't disclose how much. The Colorado projects included those at the Twenty Ninth Street mall, Flatirons Crossing mall, the University of Colorado-Boulder's East Campus and at Western Disposal Services Inc. in Boulder.

Other clients for the Lighthouse projects in the fourth quarter included Panasonic and the University of Texas.