How big is your power supply? For example, if your power supply is rated for up to 750 watts and has a 85%+ efficiency rating, it could not possibly consume more than 900 watts. That's one way to decide which UPS would work for you (preparing for the worst possible power consumption).
Don't worry about the VA rating. That's a volt amp rating for highly inductive or capacitive loads; and since your PSU's power factor is almost certainly unknown, going by a VA rating is useless to you.
Some other ways to determine minimum UPS size needed is to use a "Kill A Watt" meter which you can find many places online, use an online power usage calculator (such as EVGA provides here:
https://www.evga.com/power-meter/ ), or see the max power consumed as displayed on your current UPS (if equipped).