What specs should I look out for UPS

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.

This is absolutely the correct information. I sold UPSs for years and the general consensus is that if you want more than 5 to 10 minutes of power, then you want to go with an actual generator solution
Saying that, UPSs tend to be used in industrial environments to keep machinery running until the generator starts up and can accept the load.
And in the case of a 3d printer after so many seconds of now power what id use one for would be to cut off the bed and hotend, list the head and then run the part cooling fan full blast until power went out.
The print would be ruined but less likely to have heat creep and jams when the power goes out leaving the existing melted plastic to wick up into the heat break when the cooling fan goes out. And less likely, but technically possibly, the nozzle, still not sitting on one piece of plastic with enough heat to scorch or worse.
Yeah, I only care to keep the printer running for just a few seconds but thanks for the input
Computers and 3D printers use switch mode power supplies. They don't care if the inverter provides a square wave or a sine wave.
What sort of grid-tie systems are you thinking of?
I ask because I've been considering something like that and am finding nothing but things which have an internet connection back to the manufacturer. Some US states apparently prohibit charging batteries via the grid outside of certain scenarios. Abiding by that is fine, I'm not going to charge illegally. But I'd still like a device that is 100% not controlled remotely by some other party.

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

Didn't think about noise and that's important to me so thanks, is there anything special to look for the battery to make sure it can support my 3 devices at the same time? Like amp or other electric specifications?
- Look for devices, and read their specification sheets. They will tell you how much wattage they can support and how long they can support it for. They might do it in different units you can convert to wattage.
  Do you need your devices to stay on for prolonged periods of time off the battery? Or just long enough to shut down? That will have a huge impact on how expensive your UPS will be.
  You can add up the wattage of all of your devices, and see what the maximum is. Or you can get a $3 power meter and measure it empirically. Most UPS's will set off an alarm if you're drawing more power than they could support if they had to switch to the battery
- A note on noise in case there is some misunderstanding.
  The UPS is silent during normal operation (at least, all of them I've ever seen are, maybe super heavy duty ones will have a cooling fan but I've never seen one and even that can't be too loud), the only time it will make noise is when the power goes out and it switches to battery or if you are pulling too much power to keep the battery fully charged it will beep to warn you.
  If you are expecting a few outages that last a few seconds, you probably don't need the beep warning, so as the other person said a mute option would be nice as the beep is usually very loud, my first UPS did not have this option so I just opened it and cut the lead to the speaker..
How safe is it to keep a lead acid battery UPS in a relatively/loosely closed space about the size of 8 of those UPSs?
- A typical UPS uses sealed lead acid batteries. Those don't vent hydrogen unless something goes really wrong like severe overcharge or a short circuit.
  Just make sure it has enough ventilation to keep it from overheating. They are usually around 80% efficient, so it will produce around 40 watts of heat with your load while it's on battery.
- just have some ventilation so any hydrogen can escape (remember its lighter then air, so it will go up and out)

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.

First time I heard of online usv (also English isn't my main language), will have to check this out
- mine neither.
  that's why i wrote usv, it's the german equivalent.
  i meant online ups.

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.

"why are they separated", not sure what you meant, mini pc is acting as a server for a few usage and pi is dedicated to klipper.
- Yeah, that explains stuff. I thought you might have had some stuff on Pi that you could’ve been running on the PC. In that case I’d recommend that you run the NUT on RPi, and set it to pause print on a power failure, possibly change CPU scaling (i.e. to “powersave” CPU governor if you’re running Linux) on the mini PC, or even possibly shut it down in an event when the power outage lasts longer than a minute.

Shouldn't be an issue, power outage here are mostly about one second (5 at max) and even if my printer is being halted, my glass bed stick so much that my print don't pop off by themselves, I always use a scraper even when completely cool down.

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

Should have added in my post that power outage don't last more than a few seconds so ups should be fine

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.

I have nvme setup on my pi 4 (I know it's overkill for that purpose but already had pi setup like this prior to klipper), that being said I will definitely look at getting one with communication control even if power never goes down for more than a couple of seconds

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.