Share, discuss, troubleshoot, try not to blind yourself.
You need a UPS that meets your wattage requirements for the amount of time you want to keep things on in the event of a loss of power.
Without the amount of time it's impossible to give any recommendations.
Here's a calculator to help you get your answer
Keep in mind that the wattage the laser is rated for might be lower than the total power draw of the system, so I'd get a watt meter and monitor your peak watts.
Thank you! I'm definitely a newbie, does a 120w CO2 laser only draw 120w?
More directly answered, most CO2 lasers are rated in optical output power. There are a lot of inefficiencies before that, so e.g. the 85% efficient power supply, the (wild-ass-guess) 40% efficient laser, then you have other electrical loads like probably at least a dozen watts of fans, substantial power for motors, lights, controllers, air pumps if you have one...
I would expect a "120w" CO2 laser to have peak power consumption of at least 500w. You can use a kill-a-watt to check out the actual consumption while cutting, frankly I'd probably go with a 750-1200 kVA ups just for substantial fudge-factor.
Curious if you've already bought the UPS and if so how it's gone?