This paper provides a detailed comparative analysis of optimal sizing of battery-only, ultracapacitor-only, and battery-ultracapacitor hybrid energy storage systems (ESSs) for a plug-in electric city bus (PECB). It is shown how the configuration affects the optimal size of the ESS. The problem of optimal sizing of the storage system for the PECB is formulated and solved for a specific set of vehicle parameters, battery, and ultracapacitor cell data, as well as a specific daily drive cycle. A modified particle swarm optimization algorithm is used to solve the optimal sizing problem. The optimization platform developed by the authors and used for this study is highly flexible and accepts different component data, vehicle parameters, and drive cycles as the input for determining the optimal sizes of the storage system.