One of the important characteristics of energy meters is their accuracy. Due to the continuous increase of energy and especially electricity tariffs, consumers are looking for accurate meters that give the right measurement. However, the use of nonlinear loads leads to current and voltage harmonics production which presents the main cause of power losses, current and voltage distortion waveforms and measurement errors. In fact, this paper presents a comparative study of two types of energy meters which are Electromechanical and Smart Meter by giving the architecture and the accuracy of each meter and discussing the error sources coming from the meters itself. Moreover, it focuses on the effects of harmonics on the energy measurement and put attention on the importance of using an error function to estimate the error measurement value of the energy meter. Simulation results present the behavior of the two types of meters under harmonics, give an error function using System Identification Toolbox which is a linear ARX model and finally, prove that electronic meter gives less error and presents better performance under distorted current and voltage waveforms than the electromechanical meter.