© 2022 IEEE.Due to environmental issues such as emission of carbon gases and depleting natural resources, Electric Vehicles (EVs) are attracting more attention. To make EVs viable and widespread, the batteries' high cost and performance degradation problems have to be overcome. The degradation of the battery both in terms of energy capacity and power capability is highly dependent on conditions of utilization. To understand the relationship between energy utilization and battery size, an electric vehicle model that is used to obtain energy utilization is combined with a battery ageing model that considers both calendar and cycle ageing. First, the coefficients of the battery ageing model are updated with the help of real ageing data. Secondly, the electric vehicle model is validated considering a Battery Electric Vehicle (BEV) in the market. Then, the battery size is optimized to reduce the total cost of the powertrain and consumed energy over the battery's lifespan considering Worldwide Harmonized Light Vehicles Test Procedure (WLTP). Finally, the effects of possible future improvements in the technology and alterations in user preferences on the battery size optimization are investigated.