In this paper, a mathematical model for calculating regenerative braking (RB) energy considering the time-varying efficiency of a common electric vehicle (EV) is presented, taking into account environmental conditions. The contribution factor (CF) for estimating the impact of the RB system on the total consumed energy is calculated. To validate the calculated CF, a Blade Electron EV (Converted Hyundai Gets) was driven along a selected route in Australia and the measured CF is then compared with that of the calculated one. The results clearly demonstrate the accuracy of the mathematical model. The effect of the change in the elevation and speed limit profiles along the route on the total consumed energy using the RB system is also discussed. Finally, the estimated state of charge (SoC) of the battery is then compared with the measured SoC along the selected route.