An uninterruptible power supply (UPS) system with energy feedback to chargers and sinusoidal output charges a battery pack through a first charger under a mains mode. Under a battery mode, the UPS boosts DC voltage outputted from the battery pack to a higher voltage level through a DC-to-DC conversion module, converts the DC voltage to a sinusoidal AC voltage through a DC-to-AC conversion module, and supplies the sinusoidal AC voltage to a load. When the load has energy storage elements and discharged energy occurs in a power supply loop, a micro-controller unit can control the discharged power to charge the battery pack through a second charger, thereby solving the issue of the discharged power from the energy storage elements of the load and enhancing the operational efficiency of the UPS.