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OR does not use this parameter, as it would imply that there would need to be a lookup table to determine the amount of air that can be pushed into the main air reservoir based upon the wattage of the compressor
Looks more related to PowerRating. While Wattage is for determination of power, taken from prime mover for compressor to work.
So was explained by Rudolf Richter in his EngManual and it sounds very plausible.