Capacitors and semiconductors are the most sensitive components in power converter. Power routing is recognized as a viable software technique to ensure their useful lifetime. However, existing power routing strategies mainly focus on the unit level of solid-state transformers (SSTs). To address the reliability issue within the unit, this study proposes a power routing method based on harmonic energy control. By actively managing the low-frequency harmonic ripple current caused by the AC/DC stage, the distribution of harmonic energy within the SST unit is optimized, achieving the aging balance between DC-link capacitor and semiconductor. This approach provides increased routing flexibility with only slight control effort, without additional sampling requirements. Remarkably, this method shifts the target of power routing from fundamental energy to harmonic energy, thereby not affecting the external characteristics of the SST unit. Experiments and calculations highlight its effectiveness in achieving thermal control and lifetime balance between DC-link and DC/DC stage.