Energy conversion efficiency constitutes a critical parameter in the operation of charging facilities and serves as a pivotal focus of research in the domain of remote metering for these facilities. However, currently, the primary method for evaluating the energy efficiency of charging modules remains the direct method. This approach is challenging to measure indirectly due to the complex circuit structure and the characteristics of the components involved. An indirect measurement method for assessing the efficiency of charging modules based on grey box theory is proposed in this paper. Firstly, an accurate calculation model is established through the analysis of component losses. Next, the parameters of the complex model are processed using a parameter fusion method, resulting in an indirect measurement model of efficiency that utilizes the voltage and current outputs of the charging module as variables. Finally, the Levenberg-Marquardt (L-M) algorithm is employed to solve for the model parameters. The experimental verification of typical 15 kW and 20 kW charging modules demonstrates that the error of this method is less than 1.3% under low load conditions, and less than 0.5% under high load conditions. The indirect measurement method proposed in this paper exhibits high measurement accuracy and can be directly applied to most standard charging modules. It possesses good universality and can effectively meet the indirect measurement requirements for the conversion efficiency of charging modules.