The world's biggest offshore windfarm opened officially today off the UK coast but the company behind the project said that less than 20% of the £900m investment in the project has gone to British firms.

The low figure will concern ministers who have portrayed green technology as a growth sector that will help drive a recovery in the UK economy. In his speech to the Liberal Democrat conference on Tuesday, the energy and climate change minister, Chris Huhne, promised a "third industrial revolution" led by green energy.

At the official opening ceremony on the P&O ferry the Pride of Burgundy Huhne said he wanted to get Britain out of the "dunce corner" on renewable energy. He also criticised the "frankly atrocious record" on renewables that the coalition government had inherited. The UK is 25th out of 27 in the EU league table on renewable energy. Just 3% of power comes from renewable sources like wind and solar, against a target of 15% by 2020.

The biggest single contract for the Thanet farm off the coast of Kent has gone to Vestas of Denmark, the turbine manufacturer that closed its only UK blade-making facility on the Isle of Wight last year.

The 20% figure is better than the 10% reported by E.ON and its partners on the London Array scheme – another offshore windfarm – which will open in 2011, but is still an embarrassment to government.

"Where we can we have sought to use UK businesses in building the Thanet offshore windfarm and we estimate that nearly 20% of our capital expenditure has been given to businesses in the isle of Thanet and the rest of the UK," said a spokesman for Vattenfall, the company behind the Thanet project.

UK firms known to be involved include McNulty on Tyneside which did some engineering work; SubOcean of Aberdeen which laid the subsea power cables; and Noble Denton which did project management on Thanet.

And yet the profits available to suppliers to the booming offshore wind sector have more than doubled in recent years with industry estimates saying it cost £1.25m per megawatt (MW) in 2004 now up to £3m per MW today. The higher costs reflect shortages in the supply chain which enable contractors to ramp up prices. The Thanet project's costs had been expected to run to £750m but have escalated, while the even bigger London Array (630MW) is currently expected to cost over £2bn.

The lack of British content in the new offshore windfarms is an awkward reality check for successive governments, which have always talked about the number of "green" jobs that will result from renewable investment.

But industrialists said this situation would continue until the UK attracted a major turbine-making facility because the blades account for the bulk of the total project cost.

Vestas bought a plant on on the Isle of Wight expecting to benefit from onshore wind projects but ended up exporting much of what it produced to America before closing it down and concentrating on research and development.

Other big turbine makers such as Siemens and GE are looking at establishing plants in Britain but are reluctant to finally commit themselves until they are sure the deeper offshore projects from the Round Three windfarm licensing will definitely go ahead. They are worried that Huhne's promises will not be delivered at a time of public spending cuts.

Still, Thanet is an important milestone for the wind sector in Britain because it marks the point that 5MW of this kind of renewable power has now been installed and is generating carbon-free electricity.

The National Grid reported recently that Britain received 10% of its electricity from wind over one 24 hour period.

The Thanet farm, will be able to produce 300MW of electricity from 100 turbines - enough to power over 20,000 homes. It will be the biggest offshore facility of its kind until the even larger London Array, which has an eventual goal of 340 turbines, is completed. Thanet will dwarf the nearby Kentish Flats facility off Whitstable, also run by Vattenfall and using similar Vestas turbines.

RenewableUK, the wind industry trade body, argues that on average, due to better wind conditions, a wind turbine in the UK generates 50% more electricity than the same wind turbine in Germany. However, Germany already has a total of 21,315 wind turbines installed compared to little over 3,000 in the UK.

"Investing in renewable energy will boost our economy by creating new green industries and jobs – the government must ensure adequate funding and make the UK a world leader in tackling climate change."