Wind Mills are Generators that utilize the energy of the wind to produce EU . It is the hardest type of generators to deal with, depending greatly upon both placement (obstructions and height) and luck (wind strength). However if well managed it is the most powerful green generator. It's output can vary (~0->11EU/t) depending on these factors: height , number of surrounding blocks and wind strength.
the calculation of the output is the following : power = s * (y - 64 - c) / 750; 's' is Wind Strength, 'y' is Block Height, and 'c' is number of blocks (not including the windmill itself) in a 9x9x7 area around the windmill.