Scottish and Southern Energy has sold a 50% stake in the Greater Gabbard offshore windfarm to a subsidiary of the German utility RWE for £308m.

SSE acquired a 50% stake in the project, which will be the world's largest offshore windfarm, when it bought Airtricity this year. It took its stake to 100% when it bought out Airtricity's then partner, Fluor International, after the latter won the contract to build the 500-megawatt development. SSE said then that it intended to sell 50% of the project, which will cost £1.3bn to develop, and yesterday announced it had tied up a £308m deal with npower renewables, a subsidiary of RWE Innogy.

Under the terms of the deal SSE will be the operator of the windfarm in its development and operational stages.

Ian Marchant, chief executive of SSE, said local renewable-energy projects were increasingly important to Britain's security of supply. "This year has been characterised by high and volatile prices for fossil fuels and by political uncertainty in key oil- and gas-producing regions."

SSE paid £40m for the Fluor stake but said the higher sale price to npower reflected costs incurred in developing the project. Greater Gabbard, which is 15 miles (25km) off the Suffolk coast, will have 140 turbines and the power generated will be brought ashore through three undersea cables to a substation to be built near Sizewell. SSE and npower will share equally the electricity provided. Onshore work has already begun and work on the offshore elements is expected to start next year. The partners expect development to be finished by 2011, with the first electricity available in 2010.

RWE operates a 60MW offshore windfarm at North Hoyle off the north Wales coast. It is building a 90MW windfarm offshore at Rhyl Flats to come on stream next year, and awaits the go-ahead for a 750MW installation at Gwynt y Môr.