Abstract: This paper presents a step-by-step heating power calculation method from cell level to system level for a 5 MW/6 min lithium iron phosphate-based hybrid supercapacitor energy storage system operating under thermal power-frequency regulation conditions. Based on the rated power of 5 MW and the voltage range of 1035–1425 V, the maximum currents at the cabin, cluster, and cell levels are derived. Combined with the cell internal resistance, the peak heating power at the cell level and the total peak heating power at the cabin level are obtained. On this basis, a typical frequency regulation condition with 6 charge-discharge cycles per hour is established, with a full-power operation duty ratio of 60%. Taking the equivalent thermal load as the baseline, the thermal accumulation coefficient, engineering redundancy coefficient, and environmental correction coefficient are introduced to determine the required continuous cooling power of the liquid cooling system, ensuring sufficient capacity to handle instantaneous peak heating under the extreme SOC 0% condition. The proposed calculation procedure can serve as a reference for the thermal design of similar energy storage systems.