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.
Work to expand one of Kent's three offshore wind farms has got under way.