###### Abstract  
Due to its considerable impact on climate, bus air conditioning systems are being pushed to take a new and sustainable path. Electric buses relying on transcritical CO\({}_{2}\) air conditioning units are perceived to be eco-friendly and future-proof solutions to achieving such a target. However, in order to have highly efficient air conditioning systems, the CO\({}_{2}\) charge needs to be optimized. In this paper the energy and exergy-based analyses were performed to investigate the effect of normalized refrigerant charge on the system performance by using a test rig of a transcritical CO\({}_{2}\) air conditioning unit for an 8 m electric bus. Results showed that the normalized refrigerant charge range of 0.248-0.336 was recommended in order to ensure the maximum coefficient of performance (COP). In addition, in sufficient charge conditions, the optimal COP, cooling capacity and exergy efficiency were 1.716, 18.97 kW and 29.79%, respectively, under the standard refrigeration condition of 35 \({}^{\circ}\)C/27 \({}^{\circ}\)C. As the ambient temperature rose from 35 \({}^{\circ}\)C to 40 \({}^{\circ}\)C, the COP, cooling capacity and exergy efficiency decreased by 16.03%, 10.90% and 12.22%, respectively. Furthermore, the exergy efficiency was found not to be sensitive to slightly insufficient charge, whereas overcharge was observed to be even beneficial to exergy efficiency under the condition of ensuring the maximum COP. In addition, insufficient refrigerant charging seriously affected the irreversible losses in the indoor and outdoor heat exchangers, whereas slight overcharge had little effect on the component exergy efficiency. Finally, the need to improve the CO\({}_{2}\) compressor efficiency to enhance the system performance was revealed. t 10 May 2021 14 June 202 have considerable advantages in heating performance, especially at low-temperature heating conditions, which can solve the problem of large power consumption of electric vehicles in winter [5; 6; 7; 8]. Therefore, transcritical CO\({}_{2}\) systems have great potential to become future-proof solutions for electric vehicle air conditioning units. There are many factors that affect the coefficient of performance (COP) of the transcritical CO\({}_{2}\) systems [9; 10; 11], refrigerant charge being one of the most crucial ones [12]. Compared with the traditional vapor compression systems using HFCs, the performance of transcritical CO\({}_{2}\) systems is more sensitive to the refrigerant charge [13]. [PERSON] et al. [14] experimentally investigated the cooling performance of a CO\({}_{2}\) heat pump unit by varying the refrigerant charge amount, electronic expansion valve (EEV) opening degree, compressor frequency and outdoor fan speed at various outdoor temperatures. The results showed that the optimum normalized charge is 0.712 to achieve the maximum COP at the standard cooling condition (outdoor temperature of 35 \({}^{\circ}\)C, indoor temperature of 27 \({}^{\circ}\)C) with the compressor frequency of 45 Hz and a superheating degree of 5 K. The performance of a transcritical system was experimentally evaluated under different refrigerant charge amounts by [PERSON] et al. [15]. The influence of the refrigerant charge on COP, cooling capacity, compression ratio and suction line superheat was analyzed in detail and the maximum COP was assessed at the optimal refrigerant charge of 7.5 kg. [PERSON] et al. [12] presented a steady-state simulation model developed for a transcritical CO\({}_{2}\) air conditioning system. The authors found that the variation in COP is marginal for a charge variation of \(\pm\)18% from the optimum value. However, to the best of the authors' knowledge, there is only limited research on refrigerant charge of CO\({}_{2}\) mobile air conditioning (MAC) systems. In addition, most of the research focuses on the systems of passenger cars. [PERSON] et al.