The number of turbines on the Kentish Flats wind farm is being increased from 30 to 45, with the operation being co-ordinated from Ramsgate.
Swedish energy company Vattenfall said six turbines had so far been installed on its site, off Herne Bay.
All 15 will be switched on later this year, and it is expected they will be fully operational and exporting power to the national grid by early 2016.
Matthew Green, project director, said the original 30 turbines could produce enough power for 40,000 homes.
The additional 15, which are slightly larger, will be enough for another 35,000 homes.
"They have larger rotors so we're able to make use of lower wind speeds to get more power," Mr Green said.
Vattenfall also owns and operates the Thanet offshore wind farm.

Shiyi Ying, 25, was given permission to take to the stage towards the end of the ceremony at Southampton University to pop the question to Yu-Yen Ou, 24.
He said: "Deep inside I know you are the only one I can share the rest of my life with," before she accepted.
Vice-Chancellor Professor Don Nutbeam said it was "certainly a first" in his 25 years attending graduations.
Both students were receiving their masters degrees. Mr Ying, from Beijing, had studied for an MSc International Management and Ms Ou, from Taipei, had studied MSc International Financial Markets.
After Ms Ou said yes the couple hugged - prompting applause from fellow graduands, guests and staff gathered inside the theatre.
Prof Nutbeam said: "I'd like to offer the couple huge congratulations and every best wish for the future."

It looked at the number of accidents recorded at each junction, then weighted the results according to the severity of injuries sustained and how many cyclists were active in the area.
The Arkleston Road/A761 junction in Paisley headed the list.
The charity, which promotes walking and cycling, said the results highlight the importance of better infrastructure.
The findings will be presented at the Scottish Transport Applications and Research conference this week.
Locations with dangerous junctions included Wallyford, Dundee, Tranent, Inverness and Stirling.
Roundabouts were featured in eight of the dangerous junctions, with seven others being T-junctions or staggered junctions.
Sustrans Scotland director John Lauder said: "Safety is often cited as the main reason why people don't cycle for more of the journeys they make every day.
"This research highlights the importance of having high-quality cycling infrastructure in place at junctions, so that collisions can be prevented.
"We know that better cycle infrastructure increases the feeling of safety and ultimately the number of people on bikes."
He added: "Put quite simply: the more people in a place who cycle, the safer it becomes for everyone."
Sustrans Scotland has been working with local authorities to address some of the specific concerns.
It contributed to a consultation on improving one of Edinburgh's busiest junctions, at Sheriffhall roundabout.
A Transport Scotland spokesman said: "The Scottish government remains committed to undertaking improvements at Sheriffhall roundabout having announced the preferred route option earlier this year.
"As part of the scheme, grade separation at Sheriffhall will improve access for pedestrians and cyclists compared to the existing arrangement.
"By allowing Edinburgh City Bypass traffic to pass over it will significantly reduce congestion and improve accessibility for all modes of transport including pedestrians and cyclists."
Work to expand one of Kent's three offshore wind farms has got under way.
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A university student has proposed to his girlfriend during a graduation ceremony.
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A list of the country's top 20 cycling collision hotspots has been drawn up by the charity Sustrans Scotland.