I only have the pc plugged to the ups. Why would I need an 800 watt ups if the PSU max is 550 watts. Now considering the fact that in the distant future I'd like to snag an rtx 3060 maybe even 3070, I'd also have to buy a new 650w PSU. Which means that I'd need at least a 650 watt ups, right? I bought the 425 watt ups thinking that it was going to be enough for my rig, because on other forums I've read that PSUs don't really max out the wattage. Which I misunderstood I guess.
When not under load, you would be right. Under load, it's too much for your UPS. Yes, you would need around 550W at the very least, but consider that UPS of that type use lead batteries that degrade over time, after 2-3 years it won't be able to handle its rated load. The price difference is not that big, get a 900W - it gives you much better safety, and on top of that it allows you to handle longer power outages.
My rig has a 550W PSU, and I plugged a NAS, the main screen and my DSL box to an UPS - rated 900 W. More than enough to handle small outages, and when saving and enabling low power mode I can have a few dozen minutes of browsing to get infos from the power company. With the PC off and the NAS driving the UPS, I get several hours of data safety.
In fact, the last time I had problems was because the batteries had gotten too old and needed replacing. But this happens to all UPS.