Note the last image of the article is not correct, if you want full redundancy you need two independent power feeds. How independent depends on your needs of course. You might just want to run it from a different circuit but the same utility power. If you've got insane needs and megabucks you can talk to the utility about being fed from two substations or at the extreme end you can get one feed from the utility and you can make your own second feed. Traditionally we did this with water turbine working from a river but today I might look into solar and perhaps molten salt.

As an example, way back then when this was a very lucrative business, we placed the servers for a premium number erotic call in an industrial park on the border of two districts in Budapest because that's where we could get two independent power feeds without running our own lines. Internet connection wise, one was a simple leased line the other was a microwave connection to very far away. Short of bombing the entire site it was fair impossible for the installation to go offline and -- for the six years I knew about it, it never did. Note the site served German callers, that's where the big bucks came from.

Another thing they don't mention is that when utility AC returns, each UPS in the chain tries to recharge, which can be a significant chunk of power. Any given UPS is sized so that its downstream load plus its own recharging power doesn't overdraw the circuit, but add two recharging powers and it's much more likely.

I don't think this is an issue with double-conversion UPSs, since their input power is fixed by their rectifier size and they'll simply charge more slowly instead, but with standby type, it's very much a concern.

I had an underpowered battery backup for my desk, and was using it for my gaming PC, two laptops, and two external monitors, and all my peripherals. (It didnt occur to me I was overrunning it, otherwise my rant here is nonsensical)

The monitors both had the same general issue - they would fail to find a signal every few weeks, and I'd find that waiting a few hours with them turned off would help.

I have pages of notes here. Another way to get them to work would be booting up the windows computer, which would seem to 'trick' the monitor into getting a signal on hdmi, and then I could switch to display port for the mac laptops to be used.

Anyways it's all crazy rambling notes, with copious timestamps, looking for patterns. I have an IR heat thermometer from the kitchen and my monitor vents would regularly have air over 160F coming out the tops when the monitors would not even post the vendor logo after a hard power reset.

I removed the battery backup and it's been months now with zero blips. So the only obvious takeaway I have is that overrunning a battery is a completely worthless endeavor.

I know from experience that plugging an old school digital alarm clock into a UPS does not work. The clock drifts badly. Turns out those use the AC cycle rate as their clock.

I've never found a straight answer on whether it's ok to daisy chain surge protectors. Anyone know? I've been trying to avoid it based on a warning I don't understand, but it can be tough to find power bars without surge protection.