BOSTON - Cape Wind has outlasted a decade of government review, a slew of court brawls, and fierce opposition from mariners, fishermen, Indian tribes, and Kennedys just to win the right to sell its wind-fueled electricity.

Now, it just needs customers.

In November, the nation's first offshore wind farm nailed down its first buyer when the Massachusetts Department of Public Utility approved a deal that sees Cape Wind selling half its power to National Grid, the state's largest electric utility.

But the other half of the Cape Wind project's electricity remains available with no obvious takers, raising the possibility the project will have to shrink, and that would make the power it generates even more expensive.

The top prospect for Cape Wind's unclaimed power is the state's second-largest electric utility, NStar. But NStar says it can find cheaper renewable power elsewhere.

"It's not that we're for or against Cape Wind at all," said NStar spokeswoman Caroline Allen. "We just want to make sure that we are . . . also being mindful of costs for our customers."

A matter of price

Price is always an issue with offshore wind, which costs more partly because it's expensive to build and maintain massive turbines at sea.

The 468-megawatt Cape Wind is expected to produce enough power for 200,000 homes during average wind conditions and is estimated to cost $2 billion to construct.

In its 15-year deal, National Grid agreed to pay 18.7 cents per kilowatt hour for Cape Wind power beginning in 2013, with a 3.5 percent annual increase. The starting price is twice what National Grid pays now for power from fossil fuels, and regulators say the contract will add 1.7 percent to its residential customers' bills.

Though its price is higher, Cape Wind has big selling points, including a large generating capacity that's far greater, for instance, than Maine's 132-megawatt Kibby Mountain wind farm, the largest wind farm in New England.

Also, Cape Wind's projected start date in late 2012 has it producing the green electricity needed to fill state renewable-power mandates years before any other U.S. offshore project.

Without other takers, Cape Wind would almost certainly have to build fewer than its proposed 130 turbines, said wind-energy analyst Mark Kaplan, an associate director at IHS Emerging Energy Research. Developers won't be able to finance a project with more turbines than needed to produce the power Cape Wind is contracted to sell, he said.

Costly losses

If Cape Wind sheds turbines, the price will go up per lost turbine, to a maximum 19.3 cents per kilowatt hour.

Kaplan also wondered whether Cape Wind's difficulties in finding enough customers has implications for offshore wind-power expansion. Developer Deepwater Wind recently announced plans for a 200-turbine, 1,000 megawatt project off Rhode Island.

"If Cape Wind is having difficulties identifying a second buyer, who's going to buy a gigawatt of offshore wind?" Kaplan asked.

Deepwater has said the project's large size will help lower the price of its power, which they estimate will be in the "mid-teens" in cents per kilowatt hour.

Sue Reid of the Conservation Law Foundation, a Cape Wind advocate, estimated that it has about a year to find new buyers or face the disappointing prospect of having to build a smaller project. But she was certain that buyers, perhaps including NStar, will want to be part of a new and badly needed clean-energy industry: "Really, are they going to choose to be on the wrong side of history here?"

Cape Wind has been assailed since it was proposed in 2001 as a threat to marine life and historic ocean views. The late Sen. Edward M. Kennedy called it a giveaway to a developer, and two Wampanoag tribes said its Nantucket Sound location would ruin its ancient religious rituals.

This year, opponents sued the Obama administration after it approved the project.

In their Nov. 22 decision, state utility regulators wrote that Cape Wind's power price is "expensive" but cited numerous benefits that make Cape Wind "cost effective," including about 160 permanent jobs and a stable price that acts as a hedge against volatile fossil fuel prices.

Without NStar, Cape Wind could conceivably cobble together buyers from around the Northeast.