Wikipedia
Airwatt or air watt is a measurement unit of the effectiveness of vacuum cleaners which refers to airflow and the amount of power (watts) a vacuum cleaner produces and uses. It can also be referred to as a measurement of the energy per unit time of the air flowing through an opening, which is related to the energy that electricity carries through the power cable (wattage).
The airwatt is a useful measurement of vacuum cleaner motor efficiency, since the power carried by a fluid flow (in the case of a typical house vacuum the fluid is air) is equal to pressure times volumetric flow rate. The airwatt relates to actual airflow, while part of the electrical power (watts) consumed by a vacuum cleaner is dissipated into heat due to necessarily imperfect efficiency; two vacuum cleaners of the same airwattage have essentially the same suction, while devices of the same electrical wattage may have a difference in efficiency and thus have substantially different airwattage.