The OTP pump is used as a water pump(38w) and a skimmer pump(21w). By swapping out the impeller you will have a difference in wattage. This is common across the board with most skimmer pumps because they are water pumps that are modded with a pinwheel or needle wheel impeller. When you place in a pinwheel impeller the wattage decreases. Basically the more air that is injected by the pump the less wattage the pump will draw. If you place a watt meeter on your skimmer pump you will see that the pump will draw 21w or less in saltwater. sorry for the confusion.