The focus is still on the Northeast. That region has dense cities with strong electrical demand, high power prices, opposition to new power plants on land and some of the world’s stiffest ocean breezes off the coast. And the water remains relatively shallow many miles from shore, so wind farms could be installed far enough away that most of them would not be visible from the beaches.

With Northeastern states committing to the idea, the big question is: How much would it cost to get thousands of offshore turbines up and running?

When the first offshore projects were built two decades ago, European nations had to promise the developers extremely high prices for the electricity generated by their turbines, sometimes three or four times the wholesale power price, to get a new industry going.

Since then, offshore wind turbines have become a big business in Europe, worth billions, and the companies installing them have been able to create economies of scale. Recently, European nations have scrapped their old subsidy methods and have used competitive bidding to drive down the cost of the projects.

In some ways, the United States benefited by waiting for the industry to mature, as it can now take advantage of those falling costs. Installation is still pricier here than in Europe, and may be for a while, because few American companies have invested in the boats and other gear necessary to do the work.

The Block Island turbines were built overseas by a division of General Electric and were installed by a ship from Norway, brought over at a cost of millions of dollars, with help from an American vessel.