A prototype pump performance converted from its model pump performance shows an increase in efficiency compared with that of the model pump. This paper discusses if this increase results in an increase in pump head or in a decrease in input power, based on the relationship between tangential force on impeller blades, head loss and input power. It was clarified that a part of the efficiency increase brings about input power decrease, as the tangential force on the blades constitutes a part of input power of a pump.