Extensive residential demonstration programs and the needs for innovative and reliable back up power systems are driving the development and diffusion of small stationary fuel cell power systems. Low temperature polymer electrolyte fuel cell (PEFC) power systems are particularly suitable for back up and UPS applications due to short start up times, whereas for small cogenerative residential applications both PEFC and solid oxide fuel cells (SOFCs) are emerging as promising technologies. The technical and economical viabilities of fuel cell based systems have been already demonstrated for a few niche applications such as back up system with high autonomy. Nevertheless fuel cell technologies are not yet mature. Durability and reliability are of great concern and have to be specifically addressed. Real world experiences and extensive laboratory testing are paramount for the development of reliable products, as well as to harmonize and refine codes and standards required for the market entry. This paper presents and discusses the results of a 3 year experience on commercial PEFC 1 kWe units. Basic characterization, cycling, and steady state endurance testing data are analyzed herein with a focus on power system performance, reliability, and degradation issues. End user and system integrator testing approaches were applied. Power system response to load demand and electrical efficiency were measured following as much as possible the prescriptions of codes and standards. The influences of operating and environmental conditions on system efficiency were investigated as well. Positive results were achieved and, in particular, system availability resulted extremely high. Steady state long term endurance tests evidenced, however, critical durability and safety issues to be improved.