Keeping the computer on:

When you buy a UPS, pay attention to the max Watt output (not the max VA output). Most will list their devices primarily by VA, but that isn’t something that is terribly relevant for you or most buyers. Buy one with a fair margin of Watt output above the pull of everything you will have plugged into it.

If you aren’t sure how much wattage your computer uses, buy something like a Kill-A-Watt meter and then ramp up load of all devices to the max. A 600W or 800W UPS is generally where you want to be looking for something like a gaming PC, and 400W for a reasonable desktop server.

Keeping the computer running for a length of time:

Now that you have a UPS that can keep the computer on. How long can it keep the computer running? Generally if you are pulling around the maximum wattage for a UPS, that will be about 10-20min; long enough to get through power blips or get everything saved and shut down (buy a UPS with a USB link to the computer so it shows up as a battery and the computer will be able to automatically shut itself off when the battery is low). If you need more than that amount of time, make sure you pay attention to the battery capacity of the UPS as well.

Though, usually if you need more capacity than that you are looking into producing a custom solution (batteries aren’t hard to chain together) or a UPS and generator combo, where the UPS just needs to last long enough for a generator to come online.