Wind speed varies with time, from nothing on calm days to occasional violent gusts. If the wind speed at a site is recorded over a year, it will be seen to vary about a mean wind speed value. This is the annual mean wind speed (AMWS) and is an indication of how much wind energy is available. In Europe, the AMWS could be as low as 4 m/s (9.0 mph) for an inland site to around 8 m/s (13 mph) or higher on the most exposed sites. In general, you need an AMWS of at least 5 m/s to be able to generate a reasonable amount of energy, and ideally more than 5.5 m/s The SENWEI turbine works well at low wind speeds at 2.5m/s, but because the energy available in the wind is a function of the cube of its speed, there is very little energy available to be harvested at wind speeds less than 3 m/s.