It's not that simple. To figure out the correct (for you) size for a UPS you need more information. Critical parts of the whole picture are...
1. Idle power draw in watts (not critical to measure accurately, more just good info)
2. Max continuous draw with all drives spinning and actively being accessed, parity check with all typical VM's and containers running is a good time to measure.
3. Instantaneous surge when transitioning from idle with no drives spinning to a parity check. (Hard to measure without really special equipment, but still important to estimate)
4. Unattended shutdown time. How long does the machine take from fully active to powered off if you stop the array then power down, assuming everything is running like it normally does, no fair preemptively stopping VM's or containers.
5. Typical outage length. Do you have multiple 5 minute outages? Or just 5 second or less blinks? If the power is out for at the 1 minute mark, how likely is it to still be out hours later?
For power draw measurements, you really need a meter, something relatively cheap and simple should work fine. I can't tell what part of the world you are in from that amazon link, but here is something like what you need. link
The max capacity of your PC power supply has little if anything to do with actual draw, you really must measure how much it pulls from the wall to figure out what you need.
Keep in mind that if the UPS uses SLA batteries (almost all do), they really dislike being discharged below 50%, it's not good for their lifespan, so after you get your real load numbers you should try to stay in the first 50% of the runtime chart for the full shutdown time.
The UPS you linked may be overkill, or just right, or too small. No way to tell without finding actual usage figures.