One of the most significant challenges for wind power is its intermittency. The speed at which the wind blows is not constant, and this hampers the ability of wind turbines to generate a consistent output and to cover predicted peaks in demand for electricity. Only 10% of the UK's wind generating capacity can be assumed to be available to help meet peak demand for electricity.
This assumption – which is used by National Grid when forecasting supply and demand – is deliberately conservative. In 2010, the load factor of the UK's onshore wind farms was about 21% – that is, they generated around 21% of their theoretical maximum electrical output. The load factor of the country's offshore wind farms in 2010 was about 29%.
Most wind turbines are designed to reach their maximum electrical output at a wind speed of about 30 miles per hour (mph). Few areas of the UK consistently experience wind speeds as high as this. When the wind drops below about 9mph turbines generally can't generate electricity, but especially high winds can damage them. Consequently, most are designed to brake automatically when the wind speed goes above 56mph.
These are the reliability challenges facing wind. Read about the possible solutions.