Large swaths of green pasture along Massachusetts highways are being transformed into solar power fields that state transportation officials say could save taxpayers $15 million over the next 20 years.

Ten sites — in Salisbury; Plymouth along Route 3; and Framingham, Natick, Stockbridge and West Stockbridge along the Mass Pike have been selected for the first phase of the project. And the Department of Transportation is canvassing another three sites along state highways for the second phase, with the goal of producing at least ?6 megawatts of solar power, said Michael Verseckes, a DOT spokesman.

“MassDOT’s development of solar (energy) facilities within the state highway layout is driven by the desire to create energy savings by producing electricity locally and economically, optimize the use of underutilized state land and reduce greenhouse gas emissions via renewable power generation technologies,” Verseckes said.

After a lengthy procurement process, the DOT selected Ameresco Inc., a Framingham renewable-energy company, last June to design, finance, construct, operate and maintain the solar panels.

A spokeswoman for the company, which already has installed scores of panels along the Mass Pike, referred all questions to the DOT, which said that Ameresco will recoup the cost of the project through a combination of state energy credits, federal tax incentives and the energy the DOT buys back from it at a discounted rate.

Over the life of the contract, the DOT, in turn, expects an estimated $15 million in savings and revenue from that discounted rate and the $17.50 per kilowatt that Ameresco pays to use the land.

Annually, solar panels at the 13 sites are expected to produce 7.8 million kilowatt hours of electricity — enough to power nearly 1,300 homes — and reduce carbon emissions by almost 7 million pounds, the DOT said.

M.J. Shiao, director of solar research at GTM Research in Boston, said projects such as the one the state has undertaken have become increasingly common as the cost of solar has come down.

“One of the largest barriers has been the perceived cost of solar,” Shiao said, “so as the cost decreases, that opens up new geographies where solar makes sense.”

Last year, one in three non-residential and non-utility solar power systems in the U.S. was installed for a government entity, a school or a not-for-profit, up from 28 percent in 2010, he said.

“There are often goals for these entities to use renewables or solar energy,” Shiao said. “Schools and governments are looking for a way to save on costs and redistribute those funds to other things they need.”