This document summarizes a presentation given at an AIChE conference that proposed a new definition of impeller efficiency based on the kinetic energy imparted to the fluid rather than traditional definitions. It discussed two common efficiency definitions and introduced a new "kinetic energy number" metric. The presentation used computational fluid dynamics simulations to compare the efficiency of different impeller types in an experimental tank, finding that a pitched blade turbine imparted 12% more kinetic energy while requiring 15% less power than a hydrofoil impeller, demonstrating its higher efficiency for that system based on the first law of thermodynamics. However, the hydrofoil produced a more homogeneous flow distribution.