The electrification of the transport sector plays a key role in the global energy transition and it is of great necessity to assess emissions induced by electric vehicles in the long term for effective policy-making. Typical life cycle assessment may not consider the impact of electric vehicle integration in future electricity systems adequately, or the time-dependent characteristics of electricity generation mix and EV charging patterns. The solution requires modeling methods to integrate electric vehicle into energy system models, especially with vehicle-to-grid option. However, relevant methods have not been evaluated, yet. This integration is mathematically ambitious especially for huge and heterogeneous fleets of electric vehicles and brings energy system models to their computational limits. So far, current studies have proposed several aggregation methods for the load from electric vehicle charging, which simplify the original problem but may provoke bias. In our contribution, we propose a novel method of integrating vehicle-to-grid compliant electric vehicles into energy system models and demonstrate its feasibility by comparing it with two recent others from the literature. Taking the performance of the individual modeling method as the benchmark, we improve one of the two methods from the literature with updated parameters and additional constraints. We apply all three aggregation methods in a simple energy system model for comparing and analyzing their performances from multiple aspects, that is, solution accuracy, computational complexity, parameter requirement, and their impact on greenhouse gas emissions. Finally, we discuss the reasons behind the differences and give recommendations for further research.