The main difference between the power curve (electrical power vs. wind speed) of the large propeller-type turbines and Wind Harvesters is at the top end of the curve. This is because most traditional wind farm turbines have pitchable blades (they can be twisted at their root) to control the forces on the blade; this allows them to precisely maintain their maximum output power. Wind Harvesters do not have pitchable blades but control the maximum power by allowing the flow of air over the blades to stall in high winds; this is less precise but does not rely on bearings, actuators, and a control system; it is, therefore, simpler, more robust and less expensive. Wind Harvesters will most often come in pairs or longer arrays because they realize ~20% increase in energy output when installed one meter apart from each other. It is in this position that their predicted power performance curve and resulting Annual Energy Production nears that of the best of their taller cousins.