APC work well. I have a 600-series connected to my NAS.

Important to pick one that has USB output to the computer (more expensive) - so that it can automatically command a system shutdown as its internal battery reaches low power conditions.

Uninterruptible power supply, or UPS.

When it comes time to replace the battery, I suggest avoiding the cheap aftermarket ones. They don’t last very long.

CyberPower CP1500AVRLCD (various version iterations) UPS units have been cheap, reliable go-to choices in many instances.

they are stepped square-wave units, but are used quite a bit in home-lab setups and are pretty well supported by NUT if you are on linux.

edit: I have used them for years to replace APC units. only issue has been cosmetic input voltage vs output voltage reporting mismatch via USB on very early v1 units.

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.

The problem with UPS is the fact that you have to replace the battery reasonably often. It can get quite expensive. What you might want to consider is a power conditioner. Furman makes some that are excellent and pyle Makes some great affordable ones.

Regardless of which direction you go, don’t cheap out.