AbstractBattery electric buses (BEBs) are being planned in urban areas around the world to curb emission. An efficiently designed electric bus system offers the dual advantage of clean public transportation and reduced private vehicle use. However, electric (e) buses are costly and require large investment in charging infrastructure, and replacing an entire fleet of diesel buses with e-buses in one shot is financially infeasible for most, if not all, transit agencies. We propose a framework for partial electrification in the initial phase of a multiphase approach to bus electrification. Given a budget, our optimization model identifies routes (and their battery sizes) and terminals (and number and size of fast charges at them) for electrification and accounts for electricity charges, while maximizing the total mileage (e-mileage) of electric buses on the selected routes. The model was applied to the bus network of New Delhi, India, one of the largest metropolitan areas in the world. Sensitivity analysis highlights the importance of two key parameters (opportunity charging duration and budget) for the e-bus system design and their impact on the daily e-mileage, the primary electrification measure in our study. Model results will help urban transit agencies in developing a plan for transitioning to a sustainable e-bus system in a phased manner and will also help in assessing the needs for future electrification phases.