Wind energy is assessed thermodynamically, from resource and technology perspectives. The thermodynamic characteristics of wind are considered. Wind speed is affected by air temperature and pressure and has an effect on wind turbine performance, based on wind chill effect and Bernoulli's equation. The wind chill effect leads to temperature differences that suggest enthalpy and entropy components must be considered in a thermodynamic analysis. The wind pressure effect based on Bernoulli's equation affects the entropy of wind. These components have not previously been considered in evaluations of wind turbine efficiency for electricity generation. A new efficiency formula for wind energy systems is described, which provides important information about the system. It is seen that average differences between energy and exergy efficiencies are approximately 40% at low wind speeds and up to approximately 55% at high wind speeds. Copyright © 2005 John Wiley & Sons, Ltd.