The towns of Edgartown and Tisbury this week signed agreements with the Cape and Vineyard Energy Cooperative estimated to save them a collective $9.4 million in electricity costs over the next 20 years.

And the energy will not only be cheap, but clean. The cooperative will build large solar arrays at three different locations in Edgartown, with a combined production capacity of 4.5 megawatts, and one in Tisbury with a capacity of about 1.25 megawatts.

It is expected the solar arrays will begin commercial production of power by late next year, providing for the towns’ municipal electricity needs and feeding excess power into the grid.

The three Edgartown sites are at town-owned property at Katama Farm off Mattakesset Way, at Pennywise Path behind the Morgan Woods affordable housing development and at Nunnepog, a town well off Edgewood Drive. The town is leasing the land at no cost.

Tisbury’s array will go up on the town-owned former landfill site near the Park-and-Ride.

The two Island towns are part of a larger move toward solar generation through the cooperative, which was established in late 2007, and now has 19 members — 16 towns, two counties and the Cape Light Compact — including Tisbury, West Tisbury, Edgartown and Dukes County.

Last fall the energy cooperative issued a request for proposals on behalf of seven members, including the two Island towns, as well as Barnstable, Brewster, Chatham, Eastham and Harwich for solar arrays on a total of 10 sites.

Cape Light Compact administrator and energy cooperative clerk Margaret Downey said they received eight proposals, and negotiated with three companies before selecting one for the projects.

That company is American Capital Energy Inc., a six year-old concern based in Chelmsford, which has installed or contracted some 50 megawatts of solar power across the country, and claims about 20 per cent of the utility-scale projects in America. Its largest completed project in the state is a 1.8-megawatt array in Pittsfield.

The project comes at no cost to the towns and the energy group will front all of the expenses. The incentive for the company, and the reason the power can be sold so cheaply to the towns, at 8.5 cents per kilowatt hour, are state and federal subsidies and tax breaks for alternative energy.

Edgartown is expected to save $269,000 in energy costs in the first year of operation alone, and the savings will increase over time, to a projected $557,000 in the 20th year. Total savings over the 20 years are forecast to be at least $8.1 million.

Tisbury’s smaller project will have just over one megawatt of generating capacity, enough to produce 1.25 million kilowatt hours of electricity each year.

It is projected to generate savings of $65,636 per year in the first year, or about $1.3 million over the 20-year contract period.

But the savings for both towns could be greater if, as appears likely, the cost of power from other sources rises substantially.

Construction is expected to begin in the next six months and be completed by November 2012. The majority of construction will take place during the off-season.

Representatives of the company, who attended both meetings, said construction itself would be relatively quick, taking between three and six months. The greatest logistical issue was simply transporting the concrete foundations or ballast, on which the arrays will be mounted.

Permitting and interconnection with the grid would take longer.

They also said most of the work on the projects could be carried out by local subcontractors, which could provide a small boost to the struggling Island construction industry.

Voters approved the lease agreements at the towns’ respective town meetings in April.

In Tisbury, the proposal went through relatively easily. In Edgartown, however, there was some opposition from people concerned that the Katama site would take up valuable farmland.

There was some question about whether the project should be reviewed by the Martha’s Vineyard Commission; MVC executive director Mark London said he did not know whether the project would trigger a referral.

Edgartown selectman and board chairman Arthur Smadbeck said it does not. Time is of the essence, he added, due to strict federal and state funding timelines. Any delay in permitting could kill the project.

“There’s always a possibility that through mishandling of something like this that you could still foul things up, so it’s going to be very important not to do that,” Mr. Smadbeck said yesterday. “It’s not regional, it’s a town project . . . we absolutely don’t need the Martha’s Vineyard Commission input on any aspect of this project.”

Mr. Smadbeck said he hopes Edgartown will point the way for other towns.

“I think that first and foremost it’s about the economic advantage, it’s a great economic deal,” Mr. Smadbeck said. “But beyond that is the fact that through something we can do at a town level we’re making an impact on an issue that is global. I’m very proud that Edgartown is right in the forefront of this kind of project.”