To reduce the impact of climate change, the Japanese economy has set mitigation goals that include the decarbonisation of the energy sector and the electrification of transport. As a result, zero-emission vehicles could change the electricity demand curve, and it is thus necessary for them to be integrated into energy system models to estimate their impact and any opportunities or challenges they represent to grid stability. While previous studies have integrated single-vehicle technologies in the simulation of country-level energy grids, the present study improves on available models by integrating a country-level energy system model with a transmission grid, while considering two different drivetrains and improving on the diversity of the vehicle movement patterns considered. The simulation model results highlight that the electricity demand of each drivetrain is distinct, with a midday peak for battery electric vehicles and less pronounced morning and afternoon peaks for fuel cell electric vehicles. An important conclusion is that the infrastructure setup and associated use rules can be expected to significantly impact transport demand curves, indicating the need to further investigate how policy changes can impact the overall configuration of the energy mix.