That will take time. When the first offshore farm was built, in Denmark in 1991, developers were not thinking that it would suddenly become a mainstream form of energy, said Michael Hannibal, chief executive of the offshore division at Siemens Wind Energy, which supplied the turbines for that first plant. It took about a decade of testing and planning — and putting in place a set of programs and generous subsidies — for the market to begin taking off in Europe.

Part of what has driven the higher rate of adoption — especially in Britain, Denmark and Germany — is that Europe lacks as many cheap, clean (or cleaner) alternatives that can replace coal, diesel and nuclear plants. Electric rates are generally higher, natural gas is more expensive and open land for wind and solar fields is harder to find than in the United States, making an expansion to the seas more economically viable.

In the United States, the industry has been hampered by local opposition, fluctuating subsidies and a lack of specialized suppliers and expertise.

Offshore wind projects take a long time to plan and construct because they are more complicated and demand larger equipment that must function in the hostile environment of the high seas, said George Favaloro, a managing director in PricewaterhouseCoopers sustainable business solutions practice. The obstacles are surmountable, he said, but that would take an expectation that the industry will take off.

“We’re having a hard time making all that happen, frankly, because we’re rich in alternative sources of energy and rich in alternative sources of renewable energy,” he said. “It’s sort of a specialized set of circumstances now where this would be an attractive investment.”

That was what Block Island offered. Like many islands, its power prices are high because it is not connected to the mainland grid and depends on diesel for its electricity, and the state undertook an extensive planning process before designating the energy zone. With strong support from three governors over the seven years that Deepwater Wind, the developer, has worked on the project, it is about to come to fruition, said Jeffrey Grybowski, the chief executive.