Researchers at the National Institute of Standards and Technology (NIST) have measured the performance of a residential fuel cell system when subjected to various environmental and load conditions. The system, which uses natural gas as its source fuel, is capable of generating electrical power at three nominal power levels (2.5 kW, 4.0 kW, and 5.0 kW) while providing thermal energy for user-supplied loads. Testing was conducted to determine the influence of ambient temperature, relative humidity, electrical load, and thermal load on system performance. Steady-state and transient tests were conducted. The steady-state tests were performed in accordance with the American Society of Mechanical Engineering (ASME) Fuel Cell Power Systems Performance Test Code (PTC-50) for fuel cell power systems. The results of the investigation are being used to develop a proposed rating procedure for residential fuel cell units.