Fuel cell technology is an emerging, environmentally friendly energy conversion technology for use in mobile and stationary applications. A contribution to the understanding of fuel cell system efficiency and operation under full and part load is presented in this paper. An analytical, three-parameter model, independent of specific fuel cell system, is developed and important performance parameters defined and discussed. Three useful properties of fuel cells are documented: (i) their ability to produce electric energy with constant - or even increased - efficiency at reduced power (enhanced part power efficiency), (ii) their ability to respond instantaneously to changes in power delivery demands (instantaneous load-following properties), and (iii) their theoretical ability to deliver an exhaust gas consisting of almost pure CO2 (intrinsic CO2 separation).