The first annual report into the performance of a £67m wind farm off the Norfolk coast has been published.
Problems with gearboxes and generators hit the wind farm
The 30-turbine Scroby Sands wind farm built off the coast of Great Yarmouth generated less energy than had been expected.
The report from the Department of Trade and Industry showed the wind farm experienced mechanical problems which significantly lowered production.
Owners E.ON-UK said lessons would be learned from the development.
The figure was revealed in the government's first annual report into the wind farm.
Last year, 27 intermediate speed and 12 high speed gearbox bearings needed replacing, along with four generators.
The report said this had "serious implications for resources, costs and downtime".
Jason Scagell, Director of E.ON UK Renewables, said: "We were very happy with Scroby's performance in the first half of the year, particularly as the operation was in its infancy, but the second half was less satisfactory due to a number of defects with the gearboxes and the generators.
"That meant we had to carry out an extensive programme of modifications but, despite that, the wind farm still generated 153GWh, which is around 90% of our forecast annual output.
"Scroby Sands was and remains a pioneering project from which lessons will be learned."