Heavy-duty electric powertrains provide a potential solution to the high emissions and low fuel economy of trucks, buses, and other heavy-duty vehicles. However, the high-capacity batteries needed to power these vehicles are both cost and weight prohibitive. One possible method of supplementing battery power is to mount flexible solar panel modules to the roof of these vehicles, thereby allowing for a smaller battery (reducing battery cost and weight) or extended vehicle range. Electric buses identified as the type of vehicle that would derive the most benefit from roof-mounted solar panels due to their low operating speed (including frequent idling) and large available surface area. In this paper, the performance of an electric bus with combined battery and photovoltaic power sources is simulated on the Orange County Bus Cycle for average weather in Davis, CA. Lithium-ion batteries and flexible solar panel modules are compared in terms of vehicle performance as well as cost per kilogram and cost per kilowatt-hour. The proposed procedure shows how, for a desired operation time, one can determine the amount of stored energy that can be replaced by solar panel modules. The cost-benefit of removing battery cells is found and compared to the cost of adding solar panel modules. The study shows immediate cost savings, followed by additional savings from reduced consumption of energy from the grid. Additionally, the simulation demonstrates how vehicle-mounted solar panels can extend battery lifespan by reducing the battery discharge rate and maximum discharge percentage.