Tower height is crucial for two reasons: (1) Adding tower height is the cheapest way to gain greater and more consistent energy yield. (2) Turbulence is greatly reduced with height. Turbulence is the equivalent of rocks on a highway. Sudden shifts in wind direction cause vibrations and stresses that greatly reduce reliability as well as energy output.
NOTE: The AIR-X Wind Turbine by Southwest Windpower can be mounted onto a building in some cases, because it is small and vibration-dampened. Consider this only if the building's roof clears surrounding obstacles by at least 15 feet, and gets high exposure to steady, not gusty, prevailing winds. In some cases, there can be an acceleration of prevailing wind over the rooftop that can add to the wind energy potential.