Abstract A redefinition of the fuel cell efficiency at the fuel cell stack level has been proposed for polymer electrolyte membrane fuel cells. The new definition takes into account not only the electrical efficiency of the stack but also the theoretical energy expenditures for bringing the stack feed streams to conditions required by the stack as well as the loss of fuel in the stack. A proposed general formula for the new stack efficiency has been adapted to three practical cases: the stationary combined heat and power, power-only mobile, and direct methanol fuel cell applications. The redefined stack efficiency has been determined experimentally for a practically sized H 2 -fueled PEMFC stack and compared with the customary electric, or thermal, efficiency of this stack. Calculations have shown that the new stack efficiency can be very different from the electric efficiency, carrying additional information useful to a fuel cell system integrator.