Relative Generator and Rotor Size
The comparison between generator size and rotor size is important. The smaller the generator (in terms of rated power output) the less wind is needed to turn it. A large wind turbine rotor (capturing a lot of wind energy) coupled with a small generator will therefore produce electricity almost non stop as it will work in even light winds. However, it will not produce very much power as in strong winds it the power available from the rotor will exceed the rated maximum power output of the generator..
A larger generator, by comparison will be efficient at very high wind speeds, but inactive at low wind speeds as the force in the wind is insufficient to turn the rotor against the inertia caused by the generator.
Manufacturers will use the distribution of wind speeds and the potential energy content of the wind at different speeds to established the ideal balance between the size of the rotor blades and the size of the generator. An advantage can occasionally be gained by fitting two or more generators to a wind turbine, but it really has to be determined on a case by case basis as to whether it would be worthwhile to do this, depending on the electricity price.
Wind shear is an engineering term for the variation in wind speed with height. On sites with high shear the wind speed increases significantly with height. Because of wind shear, a taller, more costly tower will generally produce more energy. Again, a taller tower may be worth the additional expense but this depends on the individual circumstances and can only be fully determined by on-site wind assessment. A further consideration in tower height is that extra height is required to accommodate bigger diameter rotors. The trend towards taller turbines is generally due to this latter consideration.