First let me make sure it's clear that I am NOT trying to extend runtime by connecting two UPSs in series. That's been asked a million times on various forums, and is not what I'm trying to accomplish.
I've had 3 UPS units fail on me in the last 12-18 months, and I'm starting to wonder if it's the power flickers that are doing them in. My power rarely goes out for more than a minute or five, but before it does, it always violently flickers for a few seconds. Those flickers are hell on my unprotected equipment, and I'm wondering if that's what has caused my UPSs to die prematurely (the newest one barely lasted 5 months).
The old ones still function and still seem to do automatic voltage regulation, but none of them last more than 1-2 seconds once they switch to battery. I've tested the batteries, and they're fine; they were also all replaced about 9 months ago.
So, what I'm hoping is that the old ones can sit upstream of the new UPSs to take the brunt of any rapid brownout /surges and keep my new UPSs healthy. They're all pure sine wave and similarly rated.
Thoughts? Warnings/cautions?
In my experience, outages are exceptions that need to be checked. In a different location I was subject to all manner of shenanigans when I contacted the power company I was asked to stand on my porch and report where a wire crossed the road. It turned out that their map was out of date and I was on the wrong side of a connection. Once they updated their map, no more issues.
In my current location there was an issue with one of the phases in the street. After discovering this they moved me to another phase.
Then I reported another issue a few years later, which turned out to be a fault in the substation.
I don't know in which country you live, but here in Western Australia I've been happy with my local power company. They're responsive, fix faults faster than they say they will and come out if there is an actual problem.