A 5 kW proton exchange membrane fuel cell (PEMFC) with a reformer has been installed and tested at the Pontifical Catholic University of Rio de Janeiro (PUC-Rio), Brazil, aiming the experimental determination of its performance and co-generation potential to increase the fuel chemical energy usage. The unit uses a fuel processor to convert energy from natural gas into hydrogen rich reformate. The fuel cell is totally instrumented, supplying data for calculating the overall system efficiency (total efficiency), reformer efficiency, stack efficiency, conversion efficiency (DC/AC), and co- generation potential, at previously set up output powers of 2,5 kW. This study details the equations required for calculating the parameters, both theoretically, from thermodynamics and electrochemics points of view, and experimentally, from mass and energy balances, comparing the results. Steady state data were taken, resulting in reformer, stack, conversion and total average efficiencies, together with the calculated standard deviation. It was also found that the energy loss in the reformer and in the stack are approximately the same. It was also showed that the reformer and stack degradation reduce the system life, which also has an efficiency lower what is stated by the manufacturer. The fuel chemical energy usage potential was estimated by calculating the heat rejected by the stack and the heat rejected in the reformer, giving a value of 71,3%.