The online UPS consists of a rectifier, battery, and inverter, which is consistent with the AC mains and load. The AC power input to the online UPS is first sent to the rectifier circuit, which then converts it into a DC power supply. Then, this DC power supply is used to charge the battery, which is supplied to the inverter circuit and then to the load.

There is a transfer switch, but it is always in the ON position. Therefore, at any time, the power transmitted to the load will come from the combination of rectifier and inverter circuits. The main advantage of using an online UPS is that in the event of a power outage, there is no need to switch between the main power line path and the battery backup path。