Uncertainty of electrical efficiency measurement was investigated for a 10 kW-class SOFC system using town gas. Uncertainty of heating value measured by the gas chromatography method on a mole base was estimated as +/- 0.12% at 95% level of confidence. Micro-gas chromatography with/without CH4 quantification may be able to reduce uncertainty of measurement. Calibration and uncertainty estimation methods are proposed for flow-rate measurement of town gas with thermal mass-flow meters or controllers. By adequate calibrations for flowmeters, flow rate of town gas or natural gas at 35 standard litters per minute can be measured within relative uncertainty +/- 1.0% at 95 % level of confidence. Uncertainty of power measurement can be as low as +/- 0.14% when a precise wattmeter is used and calibrated properly. It is clarified that electrical efficiency for non-pressurized 10 kW-class SOFC systems can be measured within +/- 1.0% relative uncertainty at 95% level of confidence with the developed techniques when the SOFC systems are operated relatively stably. (c) 2008 Elsevier Ltd. All rights reserved.