Question Does (lower wattage) UPS "override" (higher wattage) PSU when electricity works fine?

Jun 21, 2021
Basically question is in the title. (all text below is with PSU 800+W and UPS 630W)

My problem is (and I havent found an answer anywhere) that I dont know if the UPS takes priority over PSU when supplying the PC when the electricity is working (as in, the time when we dont need UPS to do anything).

I have a gaming PC, just GPU, CPU and monitor combined account for 450W, with all the other things connected, i get over 700W while using a 800W or 850W PSU. Thats all fine.

BUT, If I use UPS that has lower wattage than the PSU...what is going to do all the power supplying when electricity works? PSU with its 800 or850W meaning I will never have problems when electricity is working or will it get limited to the UPSs 630W meaning it might shutdown?

Wall socket => UPS (doing nothing, just transfering what PSU needs) => PSU (supplying its own 800W) => "PC" (getting upto 800)
Wall socket => UPS (sending only 630W) => PSU (supplying only 630W) => "PC" (getting only upto 630)

I know the usual mantra "Get UPS to match your maximum power usage", I dont care about that, I really dont care about the borderline cases where it might shutdown when electricity is out, I only care about when electricity is working. What is used as "maximum" being supplied, PSUs 800 or UPSs 600W.

PS: CyberPower UT GreenPower UT1050EG-FR , the price jump between 630W to 720W or 900W where I live is about 2-3x


The second scenario is more truthful, and I think you have missed a basic concept of UPS.

The UPS is also a power conditioner/supply, your system is always running off of it. That way, when a power failure occurs, there is no change to the PSU/Computer, since it was always running off the power 'generated' by the UPS. UPS has capacitors with enough power to keep the system running for long enough for it to switch to battery power. So effectively it is two power systems in one.

If you exceed that rating there is less time on the capacitor and the PSU may lose power before the battery can take over. That is one problem.

The other would be exceeding the UPS power capability in the first place, this may cause it to just shutdown or show an overload error, or put extreme wear and tear on the internal components as they run hot for days on end.

Ideally you would get a UPS that can meet 100% demand of all the devices connected to it. But if you know your system doesn't exceed a certain rating on the average, that is fine. Most computer power supplies are overprovisioned anyway, and it is more efficient to run them somewhere near the middle of their listed rating.

You could go to the detail of getting a specific power consumption for each device, but reading the labels on the devices will get you close enough. Say 5V 2A power supply for your modem, 10W. Whatever your monitor says, plus the rated TDP of all your hardware. 65W CPU, 275W GPU, ~100W for everything else.