The accuracy of electric energy metering devices is related to the fairness and justice of the trade settlement and the economic security of power grid operation. Published relevant technical standards have specified the error control requirements of electricity meters and transformers, while there is still a lack of accurate and quantitative definition of the comprehensive error performance index for the electric energy metering devices composed of electricity meters, transformers and their circuits. Based on the measurement uncertainty theory, this paper discusses the calculation and evaluation methods of standard uncertainty component, combined standard uncertainty, and the overall expanded uncertainty of electric energy metering devices under different typical conditions and the determination methods of the allowable comprehensive error limit, which provides technical basis and method for the comprehensive error performance evaluation of electric energy metering device, as well as the optimal combination and selection of electricity meters and transformers before they are installed.