As you can’t measure wind energy directly you have to calculate it from parameters which are measurable. The most important measurable parameter is the wind speed, which is measured by the most accurate anemometers according to the MEASNET standard. Currently there is just a handful of different models of anemometers available which fulfill these standards.
Besides well-known and approved cup anemometers the German TR6 guideline also allows ultrasonic anemometers for bankable energy yield studies. Ultrasonics are a good alternative for icing conditions, as they usually allow proper heating of the sensor without compromising its accuracy.