This paper presents a new methodology for calculating the total cost of ownership (TCO) for the deployment of battery-electric buses in public transportation. The model considers multiple parameters and their dynamics, complementing the existing body of knowledge on TCO models. The model integrates internal and external cost categories, accounting for cost dynamics over time and the allocation of these costs among different stakeholders, including public transport operators and local authorities. Unlike static models, our dynamic framework captures the evolution of costs throughout the project lifecycle by incorporating forecasted values for variables such as operational expenditures, energy prices, maintenance, and environmental costs. Furthermore, the model includes externalities such as emissions and noise pollution costs, which are often overlooked in traditional TCO assessments. Based on data obtained from public transport operators, we applied the TCO model to a real-life long-term conversion scenario in southern Poland. The main research findings emphasize the importance of operating and external costs in the overall TCO structure, with the latter accounting for up to 30% of the total TCO. Their inclusion in the model is crucial because they are typically not considered in TCO models.