Spain had to shut down some of its wind turbines for one day at the end of December as wet and windy weather caused a surge in green electricity generation at a time of low demand, according to grid operator Red Electrica. The country's thousands of wind turbines supplied a new record of 54.1% of demand, forcing gas- and coal-fired power plants to run at minimum output to avoid system overload as hydropower companies drained brimming reservoirs. "High wind output in the early hours of this morning, together with the high level of hydropower generation, due to reservoirs opening up after recent rains, forced the control centre to cut thermal power to a technical minimum," Red Electrica said in a statement." Spain has invested heavily in wind power generation over the last decade to cut carbon emissions and reduce its reliance on imported fuel. It now has over 18,000 MW of turbines installed, out of a total power generation capacity of about 93,000 MW, and first produced over half of its electricity with them early on 9 November 2009. Wind turbines are seen as a key technology for producing electricity without emitting climate-warming carbon. But the Spanish experience highlights the difficulties for grid and other plant operators in balancing the system when the wind blows hard and there is little demand, especially early in the morning.