Wind power for the Worm's Head at Rhossili

 The picture above shows Worm's Head, a tidal island at Rhossili on the Gower. The coastguard station below had a small wind turbine. The wind speed was said to be about 20mph average that day, a stiff breeze.

Conventional wisdom is to assume that the air stops dead when it hits the blades and to assume that all of the kinetic energy of the air is transferred to the blades. Let's estimate that the diameter is 50cm. 20mph = 32 km/h (5 miles is 8 km). 32 km/h = 32 x1000/3600 = 8.9 m/s. So in 1 second, a volume of pi x 0.25squared x 8.9m is stopped. That's 1.7 cubic metres of air. Let's go with air having a density around 1kg per cubic metre. Going for 1 significant figure, then 2kg of air is stopped every second. Kinetic energy = 1/2 x mass x speed squared = 81J. Let's say the turbine is 30% efficient. That works out at no more than 30W.