For when you know the load you want to put on a UPS or a Power Station, you know how long you need to be able to run on battery, and you want to find out how big of a battery you need. You've come to the right place!
Output voltage of 110V is assumed on Ah calculations.
Device efficiency is assumed to be 90%, for some wiggle-room.
Do not try to run anything mission-critical like your grandmother's ventilator or something solely based on these approximate results, because that's just a recipe for disaster.
You can calculate battery runtime with the UPS/Power Station Runtime Calculator