Ultracapacitor-based energy storage systems are becoming increasingly popular for their use as a secondary power source in Electric Vehicles. The sizing of the ultracapacitor-based energy storage system plays a crucial role in determining the extent to which it can support the load. Typically, the sizing of the ultracapacitor depends on the design criterion, such as desired discharge power, expected backup duration, and the minimum voltage limit. This paper focuses on developing a framework for determining an ultracapacitor-based energy storage system's (consisting of a UC stack and a bidirectional converter) optimal size that can meet the design specifications. The proposed framework considers the nature of the support provided by the ultracapacitor and the constraints associated with the interfacing converter. The paper reports the sizing details of the UC stack for constant current and constant power discharging mode based on the power processed by the converter. Additionally, the work presents the filter inductance design of the converter based on the allowable current ripple. © 2022 IEEE.