Need to know amperage or wattage and if it is single phase or three phase to complete the calculation.

For example, if it draws 5000 watts for one hour, that would be 5 KWH.

If it draws 50 amps single phase at 480 volts for one hour it would be 24 KWH.

If is was 50 amps 3 phase 480 volt, for one hour it would be 41.52 KWH.

So, as you can see, we need much more detail to provide an accurate answer.

We need to know the amperage of the welder first, that will give us the watts. Watts = Amps X Volts