A new federal study suggests that electricity for Oahu produced from offshore wind turbines can be economical for developers and consumers.

But not soon. And only if some fairly daunting challenges are met.

The National Renewable Energy Laboratory recently completed a cost and feasibility analysis for floating wind farm power in federal waters at least 3 nautical miles from the shoreline of Hawaii’s most populous island.

The analysis by the research facility at the U.S. Department of Energy produced many findings, including expectations that such a project would:

>> Produce electricity that costs less than what solar farms with battery storage deliver today.

>≥ Not likely be built before 2030.

>> Be most economical at a 600- megawatt scale, which could supply around 35% of Oahu’s power needs.

>> Cost about $2.1 billion.

Several major issues that could affect feasibility of offshore wind power development weren’t addressed, including port upgrades, onshore transmission infrastructure costs, potential financial incentives such as tax credits, environmental studies and public engagement.

Also, Oahu’s electrical utility, Hawaiian Electric, isn’t keen on having such a huge single power source.

Still, findings in the study are encouraging to some stakeholders.

“It’s doable,” said Noe Kalipi, vice president of development for Oregon-based Progression Energy LLC. “The ana­lysis clearly shows that offshore wind is technically feasible and can be price competitive.”

Kirsten Baumgart Turner, deputy energy officer for the Hawaii State Energy Office, called the cost estimates encouraging but noted that factors not addressed in the study are important to consider as part of further exploring offshore wind energy for the state.

“It is only a small piece of a very large puzzle,” she said in an email.

The Energy Office developed the scope of the study with the federal Bureau of Ocean Energy Management to help better understand the value proposition of offshore wind in Hawaii, with a focus on Oahu where electricity demand is highest.

The study builds on what has been a long-running initiative to tap offshore wind energy for Oahu where efforts have included task force meetings that began in 2012, regulatory work, public hearings and interest from developers.

Generally, the study said the cost for power from a wind farm off Oahu should be the same or slightly less than in California, largely due to Oahu’s higher wind speeds offsetting higher expenses.

Researchers factored higher costs in Hawaii for several things, including shipping, floating turbines, protecting turbines from hurricanes and using more numerous smaller-capacity cables Hawaiian Electric wants for resiliency purposes.

Floating turbines would be required because of deep water off Oahu, according to the report, which said such technology is nascent yet rapidly evolving and could be available as early as the middle of this decade.

The study’s estimated $2.1 billion development cost is for a 600-megawatt project built in 2032, given the time required for technology improvements, planning, permitting and construction.

If a 600-megawatt project were developed two years ago using existing technology, the long-term price for power would likely have been between 8 and 19 cents per kilowatt-hour depending on location, study authors estimated. This compares with 9 to 12 cents for existing solar-plus- storage projects.

In 2032 the estimated offshore wind power price is between 5 and 11 cents based on lower development costs.

Part of the expected lower future project and power costs result from more powerful turbines.

Average power capacity for offshore turbines last year was nearly 8 megawatts, and manufacturers have announced plans to produce up to 15-megawatt capacity turbines by 2024, the study said.

Researchers used 15-megawatt machines for their 2032 scenario and said this could be conservative given the industry’s doubling of capacity from 2015 to 2020.

A 15-megawatt turbine described in the study has a 722-foot fan diameter on a hub 541 feet above sea level, meaning the tip of a blade would top out at 886 feet in the air, or roughy as high as a 90-story building.

With bigger turbines, fewer would be needed — 40 for a 600-megawatt project.

The study said the footprint for such a wind farm would be roughly 77 square miles, which is close to half the size of Lanai.

Study authors looked at three areas off Oahu as potential sites.

One area spanning the island’s south shore and one northern spot off Kaena Point were previously identified by the Bureau of Ocean Energy Management, which leases federal waters for wind farms. Study authors also included an eastern site off Kailua that nearly reaches Molokai for cost assessment purposes.

The study said long-term project costs are lowest for the eastern area because winds are strongest and a substation connection point is close.

Long-term costs are highest for the northern site because of opposite conditions.

For the southern area, initial development costs would be lowest because of the proximity to ports, while long-term costs aren’t quite as good as the eastern area. However, the study said it’s unlikely that a project would be sited off the south shore given busy ship traffic from Honolulu Harbor and Pearl Harbor as well as a large coastal population.

For all three sites, the study said development and operating costs rise as the distance from shore increases.

Study authors said they selected a 600-megawatt size in collaboration with Hawaiian Electric to reflect the largest project that could reasonably be built off Oahu.

The company, however, said a wind farm this big would crowd out other power supplies and leave little flexibility to keep the lights on if such a large source suddenly went down.

“We don’t want 600 megawatts of any one single project on the system, even a cheap renewable,” company spokesman Jim Kelly said in a statement.

Developers including Progression Energy have expressed interest in building wind farms off Oahu as big as 400 megawatts. The study said smaller projects would likely result in higher energy costs.

Hawaiian Electric emphasized that is open to offshore wind farm energy that helps power Oahu, but, like study authors, noted that logistical requirements, environmental reviews and community acceptance present unique challenges.