Comprehensive global decarbonization requires that transportation services cease to rely on fossil fuels for power generation. This paper develops a generic, time-driven life-cycle cost model for mobility services to address two closely related questions central to the emergence of clean energy transportation services: (i) the utilization rates (hours of operation) that determine how alternative drivetrains rank in terms of their cost, and (ii) the cost-efficient share of clean energy drivetrains in a vehicle fleet composed of competing drivetrains. The model compares alternative drivetrains with different environmental and economic characteristics in terms of their life-cycle cost for any given duty cycle. The critical utilization rate that equates any two drivetrains in terms of their life-cycle cost is shown to also provide the optimization criterion for the efficient mix of vehicles in a fleet. This model framework is then calibrated in the context of urban transit buses, on the basis of actual cost- and operational data for an entire bus fleet. In particular, our analysis highlights how the economic comparison between diesel and battery-electric transit buses depends on the specifics of the duty cycle (route) to be served. While electric buses entail substantially higher upfront acquisition costs, the results show that they obtain lower life-cycle costs once utilization rates exceed only 20% of the annual hours, even for less favorable duty cycles. At the same time, the current economics of the service profile examined in our study still calls for the overall fleet to have a one-third share of diesel drivetrains.