WHY SHOULD ONLY AN ONLINE UPS BE USED WITH A STANDBY GENERATOR?

An online UPS accepts input power with relatively wide variations in voltage and frequency, a common occurrence in power produced by standby generators. The true on-line (double conversion) technology provided by an on-line UPS handles these variations by converting the input power from AC to DC and then converting DC to AC output power. Generators should be equipped with an electronic governor to minimize frequency variations. Always check the frequency range of the generator output as the use of a mechanical governor does allow for large changes in frequency to reach the load. A wide frequency swing may cause the UPS to switch to the battery more frequently.

An offline or line-interactive UPS is not recommended for use with a standby generator. An off line UPS passes utility power straight through to the load. When a variation is detected, it can protect the load from the frequency variations of the standby generator by transferring to battery power. Occasionally, the input frequency will match the specifications of the offline UPS and it will transfer back from battery. These occurrences are infrequent and short-lived, but the battery may not have sufficient time to recharge. It will support the load only until it is completely depleted and then shut down the load.

A line-interactive UPS faces the same issue as the offline. The power conditioning (tap switching) functions of the line-interactive units focus on correcting voltage variations and have no effect on frequency variations. The line-interactive unit reacts to out-of-spec frequencies similar to the offline UPS.

The same input frequency variations that would cause an offline or a line-interactive UPS to transfer to battery are of little concern or have no effect on the online UPS. Online UPS's compensate for generator frequency variations while prolonging battery life.