Proton exchange membrane (PEM) fuel cells are a maturing technology and has in recent years entered the market of backup power systems for e.g. telecommunication applications. Here they provide emission-free and long load support during grid failures and outages. Monitoring performance indicators of PEM fuel cell systems in the field can contribute to improved lifetime, maintenance scheduling and thereby economic competitiveness. This paper establishes some performance indicators and extracts these from a number of sites, that are all installed and operating in the field. The extracted performance indicators include stack operating time, operating time at non-optimum temperature, consumed reactants, voltage decay, and state of health estimation.