Wind energy. Windmills covering the landscape of a 'wind farm'. These windmills are used to generate electricity on a commercial scale. In the wind farm seen here, the 4000 wind turbines can produce a total of 400 megawatts (MW) of electricity, about 20% of a conventional power station. Wind farms, being subject to the weather for their output, may be seen as an adjunct to normal supplies rather than as a direct relacement for 'fossil' fuels such as coal. Photographed at Altamont Pass, California, USA.