A line-interactive UPS maintains the inverter in line and redirects the battery’s DC current path from the normal charging mode to supplying current when power is lost. In this design, the battery-to-AC power inverter is always connected to the output of the UPS. When the input AC power is normal, the inverter of the UPS is in reverse operation and provides battery charging. Once the input power fails, the transfer switch will open and the power will flow from the battery to the UPS output. This design offers additional filtering and yields reduced switching transients since the inverter is always on and connected to the output. Line interactive UPS systems are a cheaper option than online double conversion technology and will protect a critical load from power failures, power sags, power surges, under-voltage and over-voltage. Unlike the online double conversion topology, it will not protect against electrical line noise, frequency variation, switching transient and harmonic distortion.