IoT systems operate at low power domain to reduce energy consumption. Switched-capacitor converters can achieve high conversion efficiency while meeting the load requirements. Our proposed work optimizes the conversion efficiency of the switched-capacitor converters in two stages: In the early planning stage, we optimize the capacitance allocation among multiple SCCs considering the load requirements in both the active state and sleep state; In the runtime stage, we propose to finely tune the switching frequency and switch width according to load requirements of the active state. Results show that our proposed work could improve the energy efficiency by around 13% and 3% in two stages, respectively.