Electricity network operator National Grid paid 12.8 million pounds to wind farm operators last year to compensate them for switching off their turbines when the grid was overloaded during stormy days, data showed Wednesday.

The highest sum paid on a single day last year was 1.7 million pounds as 14 wind farm owners were compensated on September 11 for switching off a total of 4,650 megawatt-hours of electricity capacity, statistics compiled by the Renewable Energy Foundation (REF) showed.

Britain's electricity network has struggled to accommodate the strong growth in wind farms in recent years and the network operator has been forced to implement grid balancing mechanisms, such as switching off wind farms, to maintain secure supply when the network is overloaded with wind power.

Ultimately, the cost of balancing the power system is carried by the energy user.

National Grid said the balancing cost was 708 million pounds in the 2010-11 period, less than one percent of household energy bills.

The introduction of opaque trading arrangements to manage wind power is a very unwelcome step in the wrong direction and must be reversed without delay, said John Constable, director of REF.

Record breaking wind speeds measured at the beginning of this year has already cost the grid 1.2 million pounds in 2012, the data showed.

On January 7, National Grid paid over half a million pounds to five wind plant operators to cut output from their facilities, a few days after heavy storms interrupted power supply to over 100,000 homes in Britain.

(Reporting by Karolin Schaps, editing by William Hardy)