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Why would a standby UPS fail to power devices when there's no power outage?

I have an ordinary consumer-grade standby UPS with, I believe, ample power for the devices connected to it.[^1]  It's five years old.  For the second time in about a month, I've found my computer powered off and the UPS apparently without power (indicator light was off).  Both times, I pressed the power button until the UPS's light came on, then restarted the computer and carried on without apparent problems.  In neither case was there a power outage or discernible power disruption (nothing else in the house, including another UPS, had problems).

My first thought was that the UPS's battery had failed (I know they are not immortal), but then I read up on the difference between standby and line-interactive UPSs and found that my mental model was wrong.  I thought the house current fed the UPS which then fed the computer, and if the battery in between was bad that would cause problems downstream.  According to several articles I read ([example](https://blog.tripplite.com/what-type-of-ups-do-i-need)), that's actually how a *line-interactive* UPS works and a standby UPS, in contrast, *switches to* the battery during a power outage but otherwise just passes house current through to the devices plugged into it.  If that is the case, then even if the UPS's battery is failing, I don't understand why that would matter when the house power isn't out.

In the end I want to ensure that I have reliable power to my computer, which might mean replacing this UPS, and I almost asked this question on Power Users, but I'm asking on Electrical Engineering because I'd like to understand *how* a UPS works and *why* I'm seeing this behavior.  What could cause a UPS to shut itself down and stop passing power through?  For example, if it can't keep the battery charged, does a UPS shut down entirely rather than lead you to believe you have backup power?  (If so, is that universal, or is it something I can screen for in my next UPS?)  Are there diagnostics I could do as an ordinary user without any special electrical-testing tools to understand what's happening?

[^1]: The UPS spec says 800VA 450W.  It's powering a Mac Mini, a monitor (that sleeps when not in use), one external hard drive (Time Machine), a USB hub (keyboard, headset), a network hub, and an infrequently-used printer.  This was the biggest UPS I found on the ordinary consumer market at the time I bought it (2018).  I don't understand enough about voltage and wattage to evaluate this collection of stuff against this UPS, so maybe I am in fact overloading it?