The reduction of pollutant emissions is an important advantage of fuel cells in small stationary power systems, particularly in combined heat and power generation (CHP). In fact, fuel cells are one of the future practical solutions for micro-CHP systems (5-10 kW) in the domestic environment. This paper reports numerical and experimental results of 5 kWe CHP H-Power units running on natural gas. The natural gas is reformed locally to produce hydrogen. The numerical model of the units allows the simulation of the operation of the reformer, the cooler-shift, the prox (preferential oxidation reactor) and of the PEMFC. The PEMFC polarization curve is determined by a subroutine taking into account mass and charge transfers and the condensation of liquid water in the electrodes membrane assembly (EMA). The molar flux of species through the anode and cathode porous backing layers is calculated analytically by means of the Stefan-Maxwell equations. The gas channels are assumed parallel to the EMA, and the effects of concentration variation are investigated. The relevance of the numerical results is first validated by comparison with experimental data; then, the performances of the system are analyzed in terms of electrical efficiency. Although the measured electrical performances are disappointing, the numerical simulations and analyses of experimental data suggest promising improvements. The model is employed to evaluate the best possible electrical efficiency and to establish the ideal operation domain of the installations. (C) 2007 Elsevier Ltd. All rights reserved.