Five years ago Mitchell Posin and Clarissa Allen had a vision: of sheep grazing under a windmill that powered their Chilmark farm. It was a vision of a working farm functioning with clean energy, from the grass the sheep ate to the compost tea they helped produce to the wind that spun the turbine.

On Monday morning that vision became reality when a 149-foot turbine was installed at the farm, the largest turbine to date on the Island.

Once it is fully operational, the windmill will produce 125,000 kilowatt hours per year.

Assembling turbine base. — Ivy Ashe

As the windmill went up in pieces on a gray, drizzly November day, Mr. Posin was right there in the middle of the action. “I hate the wind,” he said, referring to the salt spray that is a constant at the coastal farm perched above the south-facing shoreline. “Now we get to use it for something good.”

The power generated will be about double what is needed to power the entire farm. Mr. Posin said he is in negotiations to sell the excess electricity to the Home Port Restaurant in Menemsha.

The turbine cost $425,000 and was financed through a combination of loans from the Martha’s Vineyard Savings Bank and grants from the federal government. Mr. Posin said he hoped to pay it back over six years.

The farm is still waiting for a connection approval from NStar, which could take up to a month, Mr. Posin said.

Rick Bleakney of Baxter Crane raises tower. — Ivy Ashe

Construction was an all-day affair Monday and included members of the Allen-Posin family and a team from Great Rock Windpower of Oak Bluffs, who installed the tower. The turbine arrived on the 6 a.m. ferry; an hour later the pieces sat in rain-darkened skies on a pastoral hilltop overlooking South Road.

When the rain let up, Great Rock Windpower manager Gary Harcourt and his crew were ready to go.

“So it’s actually going to happen?” Mr. Posin said to Mr. Harcourt as the last crane hooks were attached.

“It’s really going to happen,” Mr. Harcourt replied.

The farm has been planning for the turbine since 2007.

“Does that make me crazy?” Mr. Posin asked Mr. Harcourt.

“It makes you patient, deliberate and thoughtful,” replied Mr. Harcourt.

“Like I said, crazy,” Mr. Posin said.

The turbine was approved in January after the Chilmark zoning board of appeals upheld a building permit for the Allen Farm and another for the Grey Barn, also on South Road. The permits were issued under a special agricultural exemption that allows working farms to skip the normal permitting process.

The Allen Farm has been in Clarissa Allen’s family for some 300 years and was once home to a smaller windmill.

Gary Harcourt, Rick Bleakney and Mitch Posin prepare a windmill segment. — Ivy Ashe

Mr. Posin said he invited the entire neighborhood to the installation, in the hope of drawing out supporters as well as opponents of wind energy.

“We’re hoping it’ll be here, people will see it and the tension of the ‘anti’ will subside,” he said.

The base for the turbine was dug in October; it is 18 feet deep with a diameter of 10 feet and contains 250,000 pounds of concrete. The tower is 120 feet tall with 30-foot blades. A taller turbine would have required approval from the Martha’s Vineyard Commission.

The turbine went up in sections over the course of the day — three pieces for the tower, a turbine and blades. The first section of tower was lifted into place at 9 a.m., and five minutes later, Mr. Posin and his son, Nathaniel Allen-Posin, were helping to secure the last bolts of the base. A total of seven people were on hand to hoist the turbine into place. The windmill — 42,000 pounds of machinery, fully installed — now sits atop the hill overlooking the south shore.

This was the 50th wind turbine the Great Rock team has installed, the 12th on the Vineyard. The company also installed a 120-foot wind turbine at Morning Glory Farm two years ago.

“These are the guys who teach other people how to build them. How lucky are we to have them on Martha’s Vineyard,” Mr. Posin said.

The tower was built in China, the turbine in British Columbia. Mr. Harcourt said he was at the factory the day the turbine left.

As his sheep huddled in a corner of a field, Mr. Posin said he hoped construction of the turbine would set a precedent for future farmers.

“If every farmer could do that it would help the world tremendously,” he said. “The beauty of this is we’re going to return energy to the land.”