Wind speed, wind shear and turbine costs help to determine a site's wind energy potential. To predict wind turbine performance, one must know the average wind speed at a particular location and how wind speed varies at different heights and during different lengths of time. Information about variations over hours to days is essential because wind speeds typically persist for several days, reflecting weather conditions, and often exhibit daily patterns. Monthly, seasonal and annual variations also have significant effects on a wind power plant's performance. A few years of measurement data is necessary to accurately capture a site's wind energy potential.
Wind shear describes the increase in wind speed related to an increase in height.
Wind shear determines the proper height to place the turbine hub to receive the maximum wind speed over time. Turbine costs also influence size determinations. Current technology indicates the optimum tower height for large wind machines is approximately 40 to 50 meters. Developing a wind resource assessment program Conducting a survey of available wind resources is a first step for many states and utilities interested in developing wind energy. NREL also provides technical support and some funding for resource assessment programs.