The economic viability of trackless trams, also known as Autonomous Rail Rapid Transit (ART), heavily depends on efficient energy management, particularly in optimizing charging costs. This paper proposes a two-stage methodology to determine the optimal fleet size and charging expenses for an ART line, incorporating opportunity charging and time-of-use (TOU) electricity pricing. We evaluate and compare three different charging strategies: optimal opportunity charging, optimistic opportunity charging, and pessimistic opportunity charging. Through a case study of the Yibin ART T1 mainline, we demonstrate the effectiveness of our two-stage approach and quantify the cost advantages of the optimal strategy over alternative methods. Results indicate that the optimal opportunity charging strategy reduces total operational costs by 22.24% and 9.58% compared to the pessimistic approach and optimistic approach, respectively. The study highlights the critical role of optimal charging scheduling in enhancing ART system economic sustainability.