At 5:30 a.m. Monday, Barbara MacMullen went to the Block Island Power Co. to witness history.

“We shut down the diesel generators,” said MacMullen, chairwoman of the Block Island Power Co. Transition Team.

When the noisy, dirty machines the island had depended on for decades for its power were turned off, she and others gathered there heard birds singing and reveled in the latest milestone for this small island’s journey to renewable energy.

“It feels so great,” she said.

The shutdown of the diesel generators happened just as electricity from the nation’s first offshore wind farm began flowing to the island. The Block Island Wind Farm has been producing power since December but sending it all to the mainland via a 30-mile cable installed as part of the project. On Monday, with the completion of the on-island transmission system, the power once supplied by the diesel generators to this vacation spot began coming from the five turbines spinning 3 miles from shore, generating up to 30 megawatts of power hourly.

“This was an awesome day at the end of an 11-year journey,” said Nancy Dodge, former town manager and now chairwoman of the Block Island Power Co., which is partway through its transition from a privately owned electricity generator and distributor into a publicly owned distribution-only firm.

Jeffrey Grybowski, chief executive officer of Deepwater Wind, the company that built the wind farm, said Monday marked a milestone in the development of offshore wind in this country. His company has four larger offshore wind projects in the works, including one off the South Fork of Long Island and another off the coast of Maryland.

“It’s only fitting that Block Island has made history as the first town in the United States to be powered by offshore wind,” he said. “We’re confident that the example Block Island has set will inspire communities up and down the Eastern Seaboard to chart their own path toward a renewable future.”

MacMullen said at peak season in the summer, Block Island consumes about 5 megawatts of power, and offseason demand is about 1 megawatt. The excess power produced by the turbines is being sent through the cable to National Grid, the power distribution company for Rhode Island, resulting in lower and more stable electricity costs for islanders. Block Island residents and business owners for years dealt with frequent outages on days of peak demand, and high rates that would spike with the volatility of diesel fuel costs.

“This is a very, very important project for the folks on Block Island to have a reliable, consistent source of power,” said David Graves, director of communications for National Grid of Rhode Island. Through the cable, National Grid will be able to send power to the island on windless days when the turbines aren’t spinning.

The cable built and owned by National Grid, called the Sea2Shore Project, also includes a fiberoptic line that will supply the island for the first time with faster Internet service. The town will be in charge of building the fiberoptic network, MacMullen said.

Ratepayers will start reaping the benefits of lower utility costs in their June bill, she said, when they will pay 12.4 cents per kilowatt hour. Average rates since 2012 have been 25.6 cents per kilowatt hour, though they dropped in recent months to 16.8 cents with a fall in the price of diesel fuel.

“There won’t be any more variation in the rates that makes it hard to budget,” she said.

j.benson@theday.com