The project reached full output two months earlier than expected. West of Duddon Sands began producing power in January.
Scottish Power Renewables, a subsidiary of Iberdrola, and Dong Energy formed a joint venture in 2010 to construct the project.
It consists of 108 Siemens 3.6MW turbines, installation of which was completed in June. The companies put the early completion down to the use of two installation vessels, supplied by A2Sea and Swire Blue Ocean, working together at the site.
UK energy secretary Ed Davey officially opened the project alongside Iberdrola chairman Ignacio Galan and Dong executive vice president for wind power, Samuel Leupold.
Leupold said, "The UK is an important market for our company and we have committed investments totalling more than £5 billion (EUR 6.4 billion) here over the last decade.
"We expect to invest a similar sum again by 2020, with another offshore wind farm under construction, and four more in planning or development."