The working process of backup UPS is: when the power supply of the grid is normal, on the one hand, the grid charges the battery pack through the transformer to the charger; On the other hand, it is supplied to the load through the transformer and bypass switch (K connected to point B). The current flow of electrical energy is as follows:

At this point, the energy flow is the same as that of online UPS, except that the process of converting it into battery power transmission is different from online UPS. In other words, online UPS does not have a conversion time when the power grid is abnormally converted to battery power supply, while backup UPS has a certain conversion time, which is the same as the conversion bypass time in online UPS. Generally, it is desirable to have a shorter conversion time.

From the above, it can be seen that a backup UPS refers to a system in which the power grid directly delivers power to the load through a bypass switch during normal power supply, while also charging the UPS battery. The electricity sent to the load is from the power grid that has not been processed and treated by UPS, and the power supply quality is significantly inferior to that of online UPS. When there is an abnormality in the power supply of the grid, the inverter inside the UPS is activated to convert the DC energy provided by the battery into AC energy and send it to the load.

Although the basic structure of backup UPS and online UPS is roughly the same, the output of online UPS is of better AC quality than that of backup UPS during normal power supply in the grid. This is mainly because the output of online UPS is stable voltage and frequency, while backup UPS at most adopts rough voltage regulation for output without other processing functions such as frequency stabilization. Moreover, in the event of abnormal power supply to the grid or when the battery pack begins to provide energy to the inverter, the online UPS has no conversion time, while the backup UPS has a certain conversion time. Therefore, from the perspective of working mode and power supply quality, the performance of online UPS is better than that of backup UPS during the conversion process from grid power supply to battery pack power supply. Some manufacturers of backup UPS have added grid filtering devices, and some have added some taps on the output transformer to achieve simple voltage regulation of the output, improving the performance of their products. However, there is still a certain gap compared to online UPS. However, the cost of backup UPS is lower than that of online UPS, so small capacity backup UPS has also been widely used.