Hi
If there is a transformer in your welding machine you may calculate transformers power. I found one very approximate equation for calculating real power of a transformer.
P=A*A, where A is cross section of transformers core.
I usually test capacitors with current clamps. I use Fluke 345 which can actually measure the amount of active and reactive power consumed by a capacitor. You can compare measured data with data on capacitors nameplate.
If you have ordinary clamp meter you may assume that all current is reactive. With use of reactive current, voltage and nominal capacity you can calculate actual reactive power of capacitor. But note that capacitors power is proportional to voltage squared (U*U).
I hope that this information is useful.
Best regards,
Miha