As for energy storage system, power converter is one of the most significant components. However, the repetitive thermal stress leads to great concern of the power converter reliability, this is because of the variation of power generation and power demand. This paper proposed to assess the power converter reliability in the energy storage system with the data of long-term power load and solar irradiance. Our research shows that the IGBT is more easily to fail than the Diode in the energy storage system. Additionally, the effect of the junction temperature swing with low frequency, triggered by the mission profile variation, brings a high degree of the consumed lifetime of power converter. Thus, the regulation of the junction temperature swing with low frequency deserves more attention in thermal control, and so does the reliability improvement of power converter.