If I have a Nominal System Voltage for a project of 120KV how do I pick the right kVA size for a UPS (120volts

Answer 1

UPS and/or station battery ratings are determined from the power required and the duration (hence energy) required.

Station batteries usually have a fairly complex load profile calculation, involving discrete periods of low output and high output.

UPS systems are usually sized to carry the minimum required load to allow ridethrough or safe shutdown of a process.

The 'nominal system voltage' of 120 kV provides no indication of the information necessary for sizing a UPS.

Answer 2

If you are truely confusing voltage with power,
you've got no business dealing with 120KV.
in any capacity.