We propose an impedance spectroscopy (IS)-based model to determine the dimensionless figure of merit (zT) of a commercialized BiTe-based thermoelectric module in the time and frequency domains. In this method, the transient response of the resistance is measured for different current ranges and an apparent current dependence of the measured resistance is observed in the steady state. We successfully explain the experimental results using the model wherein the dependence is caused by the heat balance between the Peltier heat and Joule heat. In addition, a necessary condition of the balance is required to reproduce the experimental value of zT theoretically. Furthermore, we experimentally determined zT using the measured resistance in the time domain and we applied the IS-based model in the frequency domain for comparison. In the time and frequency domains, we obtained zT = 0.842 ± 0.006 and 0.834 ± 0.001, respectively, by applying the appropriate current for neglecting the influence of the Joule heat; a negligible difference was obtained in the results, as verified via temperature dependent estimation. Through this method and the corresponding analysis, we achieved a comprehensive understanding on how to measure zT and the associated error in the measurement, accurately and precisely, during the experiment. We conclude that zT can be determined precisely in the time domain within several minutes using the proposed method that applies an appropriate current across identical thermoelectric modules and elements.