National Wind Power's North Hoyle offshore wind farm lies 7.5km off the north Wales coast between Rhyl and Prestatyn and comprises 30 turbines.
The facility is capable of generating enough electricity to meet the annual needs of 50,000 homes across the country.
British Wind Energy Association chief executive Marcus Rand called the event a landmark for the UK wind industry. "We are confident that by the turn of the decade there will be many more offshore wind farms around the UK," he added.
Powergen Renewables' Scroby Sands project is expected to be switched on next year. Another 11 offshore wind farms also hold the necessary consents for development to proceed.