My UPS can't handle PC load sometimes, what's wrong?

Question

So I recently built a PC and with it, I bought a cheapish 600W UPS. My PC on PCPartPicker showed that I required around 330W to use it properly (I went with a 700W PSU). I also have my monitor and modem plugged into the UPS, so I think it would be a load of around 400W for the UPS max (for my current setup).

So, typically when I have a couple of windows open and doing no graphically intense task, the UPS does not shut down my PC, Monitor, or modem for the few seconds that my power goes out (happens frequently where I live). However, when I am playing a graphically intensive game and the GPU sucks a lot of power, a weird and unexpected thing happens when the power goes. My computer just restarts. It doesn't shut down, it just restarts. This is really annoying since I am in the middle of a game. Also, sometimes even when my UPS is connected to power, it beeps for a few minutes and then stops beeping again. Seems to me it can't handle the power load of my PC.

Have I got my thought process wrong, or is there something wrong with the UPS, since it should handle a load of up to 600W, and does so most of the time? Or is there nothing wrong with the UPS and I should buy another UPS? If it is the latter, which UPS would you suggest? I've got an RTX 2060, an i5-9400F, 16 Gb of RAM, and a Samsung 860 EVO 512 GB, B365M motherboard, and some random hard drive I had lying around.

Answer 1

To run the way you are trying to run, you need a true UPS, that is a a UPS where the PC load is supplied by the battery circuit and the AC simply keeps the battery charged.

Such UPS units come with batteries (typically Lead Acid) that can sit on charge all the time.

The sizing of the UPS should be 50 to 100 percent higher power capacity than the normally top load of the PC.

The unit you have appears to be too small as the computer may use higher power than the UPS unit can supply.

Answer 2

The symptoms you are describing sound very much like the UPS can't deliver enough power to run the GPU. It sounds like you cut the margins close and ended up with not enough power. Remember, the power numbers for your PC are power on the rails, not power at the wire--even the expensive power supplies aren't 100% efficient and the cheap ones are likely below 80% efficient. The only way to really know is to measure--this is much easier done with a consumer-level power measuring device (personally, I have used a Kill-A-Watt but I know other devices exist) to see what your PC is actually drawing under various conditions.

That being said, you need some extra capacity on a UPS. When you're running close to the limit the battery life goes way down because the UPS will drop out when the batteries can no longer deliver the power being asked for even though they have a lot of charge left.