Standby/Offline Uninterruptible Power Supply (UPS) systems, offer a cost-effective solution for protecting electronics from power outages and fluctuations. However, they aren’t typically compatible with generator power, and this is primarily due to the way they operate. Standby UPS systems stay ‘offline’ until they detect a power interruption, at which point they switch from utility power to battery backup (using a relay). Because this type of UPS waits for a complete power loss before kicking in, the transition isn’t seamless, and there’s a brief delay, called ‘transfer time’, before the UPS turns on the inverter and outputs power to the outlets.

Generators also take some time to start up and stabilize after a power outage, and this delay can exceed the transfer time of a standby UPS. This mismatch can cause the UPS to switch back and forth between battery and generator power, leading to potential damage to the UPS or the equipment it’s protecting. Furthermore, standby UPS systems are designed to provide power for a short duration—just enough to properly shut down equipment—and their design doesn’t favor the extended runtime that a generator could provide. Also, the power produced by generators can sometimes be ‘dirtier’ than utility power, with more frequency and voltage variations, and standby UPSs will struggle to handle this. Most of them can not handle it and will behave as if there’s no input power. Thus, for environments where generators are used, a more advanced UPS type, like a line interactive or a double-conversion (online) UPS, would be a more suitable choice. If you’re planning to use your UPS with a generator, always best to check ahead of time before buying it to make sure it will work as intended.

Examples of generator compatible line interactive UPSs:

APC Smart-UPS 750VA
APC Smart-UPS 1000VA
APC Smart-UPS 1500VA
* Remember to set the power quality to low/poor if you’ll be using the UPS with a generator.