UPS backup question.

Unless it's true-online (which it's highly unlikely an office UPS is), there's going to be a small power glitch both when the UPS kicks in and back out, but that's not why your iMac had shut down--it shouldn't be bothered by the small flicker, nor is it able to somehow detect it.

If I'm reading right that you didn't have the USB cable connecting the iMac to the UPS when this happened, and assuming that the power was out for more than a couple of minutes, it's pretty simple: The battery ran down, at which point the power was cut to the iMac.

With no communication between the UPS and iMac, the iMac had no way to know that the UPS's battery was running out (or even that it was running on a battery at all), and the UPS had no way of telling it this, so it chugged along until the battery was drained and it was forced to cut power.

Now that the iMac is connected to and communicating with the UPS, it should automatically shut itself down, which is an improvement over a sudden power off though you may still lose unsaved work if something is open and you're not there to manually save and shut down.

Now, if the outage wasn't long enough to run down the battery, it's possible that either the UPS is too small to handle the iMac's load, in which case it'd just shut down immediately, or that its battery is going (they last a few years), and so while it was once able to handle the load it isn't now.

Easy enough to test--just yank the cord out of the wall and see what happens. If the iMac stays on and warns you, happy days. If it dies immediately and/or the UPS starts complaining, you're going to need to either replace the batteries or get a bigger UPS.

 

Sounds like the UPS is too under capacity for the computer. Need larger UPS that can handle more load.

No. Not if your UPS has enough capacity.