How many homes does an offshore wind farm actually power?

An offshore wind press release often boasts two key figures: the size of the project in terms of maximum power output (megawatts) and the energy it’ll supply to the region (homes powered). 

South Fork Wind, the country’s first commercial-scale wind farm, completed last year, is a 132-megawatt project that provides enough energy to power approximately 70,000 homes, according to the developer, Orsted.  

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And per Vineyard Wind, its project will generate “800 megawatts of electricity and power over 400,000 homes.”

The pairing of those two figures — homes and megawatts — has led to confusion and strong skepticism from some readers, according to several emails sent to The Light. Understandably so, since the duo is missing a third metric, called capacity factor (which we’ll explain below). 

Using federal data on energy sales — figures not widely reported until now — The Light has done the math to estimate how many homes these two wind farms are powering.

By our calculations, South Fork Wind powered 70,000 Long Island homes this winter, but on average (across the year), fell short of that number. As for Vineyard Wind, a project that is still in progress, we calculated enough energy was produced in April, June and July to power about 33,000 homes. 

But before getting into those calculations, one must first understand how turbines operate offshore. 

Understanding power output  

A turbine’s output is affected by wind speeds and downtime for maintenance; it varies on a given day, week, and month; and it is linked to an important concept: capacity factor.

Capacity factor, a metric used by all energy projects — natural gas and coal plants included — is the measure over one year of a facility’s average energy generation, compared with the maximum amount it can generate (known as “nameplate” capacity). 

If a 100-megawatt project has a capacity factor of 50%, it produces 50 megawatts on average. 

Knowing the relationship between the nameplate capacity (the output if turbines ran 100% of the time at optimal wind speeds) and capacity factor (which reflects actual output) helps with understanding and disentangling language from developers.

Gordon Stewart, a professor of engineering at Roger Williams University, said wind developers’ framing of figures can be confusing.

“Four hundred thousand homes is accurate, I think,” said Stewart, speaking approximately about Vineyard Wind’s language. (He did his own calculation and arrived close to that figure, assuming a 50% capacity factor.) 

A 2020 construction and operations plan from Vineyard Wind listed a capacity factor “in excess of 45%.” The company did not respond to multiple inquiries on whether that is the current number.

But suggesting it’ll generate 806 megawatts when finished is “not that accurate,” Stewart continued. That’s because 806 megawatts is the output with a 100% capacity factor.

In other words, the homes-powered estimate is linked to actual output, which developers don’t include in their press releases. For Vineyard Wind, a 50% capacity factor means that about 400 megawatts of actual output would power its estimate of 400,000 Massachusetts homes on average over the course of a year. 

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Nuclear power plants have the highest capacity factor of any energy source: 90% or more. Last year natural gas in the U.S. had a capacity factor ranging from 20% to 59%, depending on the type of plant, while coal-powered plants sat at about 42%, according to federal data. The capacity factor for offshore wind can range from the 40s to 50s, and is typically higher than that of land-based wind farms, which had a capacity factor of about 34% last year. 

Unlike a coal plant, offshore wind farms don’t operate with a discrete on/off switch, explained Stewart, who specializes in offshore wind engineering. Wind turbines operate with a “cut in” and “cut out” system that responds to wind conditions.

“The way it works with turbines is you let the rotor spin without extracting power, freely, in low winds,” he said. The turbine’s generator system will sense when the wind speed reaches the “cut in” threshold, and will start extracting power.  

Variable wind speeds mean that turbines can run at full power, lower power, or no power over a given week. 

Determining homes powered by Vineyard Wind

At Vineyard Wind, the partially constructed wind farm about 15 miles south of Martha’s Vineyard, one turbine can put out between zero and 13 megawatts (its nameplate capacity), depending on wind conditions. 

The rated wind speed for the project’s GE Vernova Haliade-X blade model is 11 meters per second, which equates to about 25 miles per hour. The turbine will not produce the full 13 megawatts unless the wind is 25 miles per hour or greater. If the wind is weaker, say 15 miles per hour, it will put out fewer megawatts. If the winds are too weak, it won’t produce power. If the winds are too strong, the turbine will shut down to prevent damage. 

According to data published by the Federal Energy Regulatory Commission, Vineyard Wind sold about 57,268,000 kilowatt-hours of energy in April, May and June. (The federal agency releases reports on a quarterly schedule.) 

Per a FERC official, the energy-sold figures (labeled in the quarterly reports as “standardized quantity”) “generally represent energy outputs to the grid by each seller.”

The wind farm was operating four turbines in mid-May, according to the state’s energy office, and 17 turbines in late July, according to Vineyard Wind’s parent company, Iberdrola. 

The project sold about 12,281,000 kilowatt-hours of energy in the first quarter of the year, from January through March. It was much lower, likely because fewer turbines were installed and operating at the time. 

Without responses from Vineyard Wind (the company declined comment) — but with the limited information from FERC — we attempted to calculate homes powered. 

According to the U.S. Energy Information Administration, the average monthly household usage in Massachusetts in 2024 was 580 kilowatt-hours, lower than the national monthly average in 2022 of 899 kilowatt-hours. 

Using the Massachusetts-specific usage, we estimated that Vineyard Wind powered about 33,000 homes across April, May and June. 

These figures require a big caveat. First, homes powered is a calculation that is best done over the course of a year, as wind speeds vary seasonally. Vineyard Wind hasn’t been operating for a full year. 

Second, Vineyard Wind remains under construction, with more than half of its turbines not yet operational, not fully installed and in need of commissioning. While a project is under construction, operational turbines may go offline for maintenance, which will reduce output. 

And third, we used the average annual household electricity consumption for a three-month period. It’s possible the seasonal average was higher or lower compared with that of the whole year (for example, New England’s usage generally peaks in the summer). 

What could Vineyard Wind produce once it’s finished? Using a capacity factor of 45%, which could be applied when the project is fully built and operational for at least one year, The Light calculated that Vineyard Wind’s 400,000 homes-powered approximation is accurate, given the state’s average household electricity consumption in 2024.

We could not check this math with people who would have access to more detailed figures. Vineyard Wind did not answer The Light’s questions, and ISO New England, the region’s grid operator, said it does not release the electrical output of specific projects, citing “confidentiality rules” that bar it from releasing “generator-specific production information.” 

Aggregated energy output for all wind power in New England is publicly available, but it does not distinguish between land-based and offshore wind generation.

Determining homes powered by South Fork Wind

In the first half of this year, South Fork Wind attained a capacity factor of 53%, according to Orsted spokesperson Meaghan Wims. In other words, the 132-megawatt project put out about 70 megawatts, on average, over the past six months. 

Wims said South Fork Wind has “consistently delivered reliable energy to 70,000 homes across Long Island” since the project achieved “full operation, in late 2024.” According to press releases, all 12 turbines were powered up by March 2024. 

Because South Fork Wind started operations last year, The Light had over a year of FERC data to approximate homes powered by the project. 

Quarterly, the power sold rose from spring 2024 to winter 2025. This is to be expected, as winds are stronger in the winter. From January to March of this year, we calculated that South Fork Wind produced enough power for 73,000 homes. 

(Again, the above figure requires a caveat, as we used the average annual household electricity consumption for a three-month period, but the seasonal average could have been higher or lower.)

According to PSEG, which manages the grid for Long Island, New York, the average monthly electricity usage by a household last year was 715 kilowatt-hours. 

Over a year, that is 8,580 kilowatt-hours used by a single household. Between April 2024 and March 2025, South Fork Wind sold approximately 456,583,000 kilowatt-hours of energy, according to FERC reports. Divide that number by 8,580 kilowatt-hours and one gets approximately 53,000 Long Island homes powered across the year, lower than the 70,000 figure cited by South Fork Wind. 

We shared our math and asked Orsted about this. In response, Wims, the spokesperson, cited an EIA report for 2020 that listed electricity usage lower than PSEG’s 2024 estimates. 

Using that 2020 EIA estimate, we calculated about 66,000 homes powered.  

Wims said in an email that the project’s estimate was calculated using the statewide average of electricity consumption.

“These estimates are calculated when projects are proposed and are meant as a benchmark to help people understand the relative amount of energy that will be produced over a project’s multi-decade operating life,” Wims wrote.

“Note that South Fork Wind reached full-production status in late 2024, so it’s the months thereafter that best approximate the wind farm at its full production,” she said. 

Last year, LIPA, the area utility company that oversees PSEG, said South Fork Wind was steadily producing about 70 megawatts (which tracks with an approximate 50% capacity factor), and that one megawatt is enough to power about 400 homes, Newsday reported. LIPA told the outlet the average output of the project will vary between 40 megawatts in the summer and 80 megawatts in the winter.  

Per LIPA’s estimations, that would put homes powered far lower than South Fork Wind’s figure. 

The Light inquired about this, and a LIPA spokesperson confirmed The Light’s calculations of homes powered, stating that over the past 11 months, South Fork Wind had produced enough energy for about 70,000 homes based on the average residential usage for all of New York State. 

However, “if we adjust the calculation to reflect the higher average consumption on Long Island — about 9,000 [kilowatt-hours] per home annually — the output is equivalent to powering roughly 53,000 homes,” the spokesperson said. 

South Fork Wind has used varying language on its site — stating approximately “70,000 Long Island homes” and “70,000 New York homes” powered.

A note on our methodology

The Light consulted with two engineering professors, Eric Hines of Tufts University and Gordon Stewart of Roger Williams University, to review the soundness of our approach and our calculations. The Light also shared its math with Vineyard Wind and South Fork Wind for responses. We contacted federal and state agencies, as well as the respective utility companies, to obtain recent and region-specific estimates on average monthly or annual household electricity consumption. As stated in our story, our calculations are estimations. The homes powered at any given time will vary due to grid demand, maintenance work on turbines, and seasonal changes in wind speeds. Further, calculations may vary depending on the denominator, which in this case is the average annual or monthly household electricity consumption (a figure that varies over time). 

Editor’s note: The New Bedford Light’s newsroom is scrupulously independent. Only the editors decide what to cover and what to publish. Donors, founders, and board members have no influence over editorial content.

Email Anastasia E. Lennon at alennon@newbedfordlight.org.

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