As transit authorities begin adopting Battery Electric Buses (BEBs), they must address the challenge of extended battery charging sessions while maintaining fixed bus schedules and managing the cost of energy. This paper proposes a novel technique for minimizing the monthly cost of energy for operating electric bus fleets in the presence of time-varying uncontrolled loads. The charging problem is cast as a constrained bin packing problem and expressed as a mixed integer linear program (MILP). Among other things, we show that the proposed method significantly decreases demand on power infrastructure by temporally balancing the loads from buses with uncontrolled loads.