The operation of UPS' can be divided into two basic types:
The double conversion continuously changes the input from AC into DC, and then converts the DC to clean regulated AC which is used to power the load continuously. The standby power unit powers the load from the input AC normally, starting the inverter only when the input AC is lost.
The standby power unit is a good choice when the load is not extremely critical and when the power is relatively clean, free from spikes, and has infrequent momentary interruptions. Since the standby power unit requires some time to turn on, it offers little or no protection from spikes and flickers which are passed directly through to the load. Even a 1 millisecond transfer time will not protect the load from damaging voltage spikes. When a standby power unit fails, the failure only becomes apparent when the unit is required to supply backup power and does not. The most stressful period of operation, and therefore the most likely time to have a failure, is during the shock of starting up the unit which is exactly the time that the unit is needed most.
The standby power unit should never be used when it is to be powered by a backup engine-generator unit. The generator's frequency deviations will be passed directly to the critical load or will cause the standby UPS to operate from battery even when the generator is up and running.
By contrast, the on-line double conversion unit the DC bus absorbs any input spikes, flickers, sags, surges, brownouts, or frequency variations, and continues providing clean regulated power to the load without discharging the batteries. In a blackout, the inverter continues to supply the load from the batteries without having to switch on. Failures in the unit, if they occur, will most likely occur when the AC is available and the load will be transferred automatically to the input AC without interruption.