My experience has been that current UPSes tend to give a pretty limited runtime, more-limited than you might think, and are surprisingly-expensive for the capacity that they do provide.

The reason one historically really wanted a home UPS wasn't necessarily to run a machine through a power outage, but to provide time to save any work and shut that machine down. We used to use filesystems that could become corrupt if they weren't brought down cleanly. You could maybe run a machine for minutes, not many hours.

So it was important to warn (and maybe auto-shut down). UPSes are good at that. You have beeping alarms, software to auto-shut-down a machine cleanly when the battery gets low if someone isn't around, software to notify a single attached computer about the battery level.

But with the combination of filesystems that don't do that plus software that auto-saves, that's less critical. What I think most people want is just more runtime, having a shot at making it through short power losses.

I think that today, if I wanted to provide longer-running resistance to power outages, I'd probably look at one of two things:

• For a device or two, "power stations". These sometimes get called "solar generators", despite not generating anything, because manufacturers are marketing them as an alternative to gasoline/diesel generators for power outages. All-in-one box with batteries, charge controller, and inverter in one little case. This kind of thing. They won't beep to warn you that the power is out, and they won't have software (or interface) to tell a computer to automatically shut down.

• For larger capacity, a "grid tie solar system". These tend to be extensible, with separate 12V (or 24V or 48V) batteries or the like and separate boxes for the battery charge controller and inverter. You don't actually need to hook any solar panels or other power sources up to it (though, hey, maybe you decide that you want to do so), as the battery charge controller will charge the batteries from the grid as long as the grid is providing power. You can select a size for and replace the batteries and inverter and charge controller separately, and you aren't constrained to use a single vendor for all the components.

I would look to see that the inverter in question provides "pure sine" output. Some inexpensive inverters provide a square wave, and some devices don't much like this.

EDIT: Ah, just saw your new comment:

Power outages are usually very short (few seconds max).

You may not care about keeping the system running through longer outages, then, unless you're also worried about longer ones.

Noise! They are louder then you think.

Ability to mute the alarm.

Bonus points if you can connect it to a computer to automatically shut it down cleanly.

Optional, battery expansion out of the back

Keep in mind, UPS's should be in well ventilated areas, they can off gas hydrogen and other explosive fumes depending on the battery type

even though i think that the psu of the 3d printer might get you over the switching time offline usv's have, i'd go with an online usv (voltage & freqency independent)
one missed step and your print might be gone.

depending on your outages you might go for a smaller battery, because you'll have to replace them more often with an online usv.

Whatever would support the combined max wattage of the devices you will connect to it. Ideally something that can handle 50-100% more power. You can control some UPSes via USB or network, so you can hook it up to that mini pc or Pi (why are they separate?) and run NUT on it. You could technically pause a print/shut down a computer/Pi if an outage is more than x seconds to reduce power usage and get through one that you otherwise couldn’t.

Please don't confuse a generator with a ups. Ups isn't meant to run devices for long periods. Generators are

The biggest issue is battery size: If the heatbed cools down the print fails. The heating is the part that takes the most power so the battery has to be large enough to support the entire remaining print duration or power outage.

I can’t help much on the power draw side of this question, but one thing to look out for with a UPS is some sort of communication option. (Usually NUT over ethernet, but there are some USB options too.) Most modern UPS brands will have a plugin you can install on your Raspberry Pi and Mini PC that allows your UPS to signal, “Hey, I’ve got 3% of battery life, you actually need to gracefully shut down now.” It’s mostly useful for NAS applications with spinning drives, but it could help save your Pi’s SD card potentially.

It’s a pretty standard feature these days, but the cheapest of the cheap will omit it.

You might actually want to consider a DJI Power 500 or 1000. They're actually portable power banks but can supposedly work in UPS mode, and will run for WAY longer than a traditional lead-acid battery UPS. See also Ecoflow's options.