A ceremony has been held to mark the completion of a £1bn gas-fired power station in Pembrokeshire.
RWE npower has said its Pembroke Power Station, the largest of its type in Europe, will power 3.5 million homes.
Wales Office minister Stephen Crabb welcomed the boost to the economy, with 100 long term jobs created.
However, the European Commission is investigating how the permissions for the plant were granted and whether it damages the marine environment.
Planning permission for the station was granted by the UK government in 2009, and it was granted a permit by the Environment Agency last November.
After three years of contruction work, control of the final part of the plant was handed over to the station team last week.
RWE npower described the facility as one of Europe's largest and most efficient combined cycle gas turbine plants.
It said it would provide a highly flexible and reliable source of energy.
Mr Crabb, who is the MP for Preseli Pembrokeshire, was among 200 guests at the official opening ceremony.
Speaking before what will be his first official visit since joining the Wales Office, he said: "I have no doubt that Pembroke Power Station will play a vital role in maintaining the UK's energy supplies for the future, and make its own contribution to creating economic prosperity for Wales."
Friends of the Earth Cymru has complained that the station, which takes water from the Cleddau estuary as a coolant and discharges it back at a higher temperature, could damage marine life in a special area of conservation.
Environmental campaigners have also criticised the station's technology as "second rate" when Wales should be aiming at more sustainable technology.
David Hughes, head of the European Commission office in Cardiff, confirmed that it was currently dealing with a complaint into the plant, and was in touch with the UK authorities on the matter.
"What the commission is looking at now is a complaint that the permissions that were granted for building and operating the power station were not granted in the proper way," said Mr Hughes.
"The other aspect of the investigation is a possible adverse effect on the Cleddau estuary."
Mr Hughes said the commission hoped to wind the investigation up in the next couple of months and would come to a conclusion on whether or not to proceed with, or drop, the complaint.
Environment Agency Wales said it had set out strict conditions to protect and maintain the environment as part of the permit for the power station.
"This followed extensive consultation with interested parties and detailed investigations into potential environmental impacts," said a spokesman.
Both the Wales Office and the Welsh government have said they are satisfied with the plant and the technology used.