Most UPS model numbers have a number in it and it's usually listing the "Volt-Amps" it can handle, not the watts. It's pretty cheesy because you'd see a big "UPS 500" plasted on the box, but that's 500 VA's and usually 300 or so watts.
volt-amps is a useful figure (though it gets overemphasized in marketing), though you need to know what it means. it specifies how much reactive load the unit can handle.
AC power is actually made up of 2 parts - real power (called P and measured in watts, this power is used and given off as heat, light, etc.) and reactive power (called Q and measured in volt-amps reactive. this power is not actually consumed, it's used to charge capacitors and inductors. it is send back at the other end of the circuit each cycle, but it still needs to be supplied), which add together (as vectors) and give you apparent power (called S and measured in volt-amps).
without correction, a switched mode power supply (like all computers use) has an pretty lousy power factor (real power divided by apparent power), potentially as low as 0.5. That means if you're drawing 300 watts, you're also drawing 520VAR and thus the UPS has to provide 600VA.
APC assumes a power factor of 0.52. cyberpower assumes 0.63.
realistically, the worst you're likely to see is about 0.7 (cheap end passive PFC, which basically just uses a big inductor to cancel out the big caps) and good supplies with active power factor correction can do 0.99.