
A rooftop wind turbine is actually a much more compact unit than a standard wind turbine. The reason for this is that since it is already on the roof, it doesn’t need the substructure to elevate the turbine to a suitably windy elevation. Thus, the only thing you need to put into a rooftop unit is the turbine and blades to propel it.
A rooftop wind turbine is a lower output turbine, on average, than freestanding turbines are. This is because a house isn’t generally made to put a few tons on the roof, and freestanding turbines don’t need to have interior space for habitation. The last reason is a noise restriction. The higher output a turbine has, the noisier it is. This is because noise dampening reduces the effectiveness of a turbine by diverting power away from the rotational force of the blades.
A smaller scale rooftop wind turbine can produce a modest amount of power, not enough to power an entire household, but enough to take a chunk out of the energy bills of that house. That is the niche they have evolved to fill, and they do so admirably.