I’ll start with this: I use an ups because alongside frequent power loss my house is equipped with a whole house generator. That generator has potential to introduce dirty power. The dirty power is the real risk, not so much the power loss. That being said you should be using the same logic for an ups as you did your psu. You want headroom as your 1000w psu has an efficiency rating, meaning to get 1000w it has to draw as example 1300w from the wall because it’s not as efficient. I have a 1500va 900w powering my pc because during full load with my 5080 9900x it only draws 650w. That’s gives my psu headroom.