How to measure the wattage and VA of Powersupplies and UPSes??

HI;

i want to measure the wattage of my own designed power supply That i've designed usign 4 LM338 ICs in parallel. moreover how can i find the wattage/VA of different powersupplies/Invertors/UPSes that use Transistors or MOSFETs in parallel. is it enough that i just add the given wattage in the Datasheets of the components to get the total wattage of the device? or there other factors are also considerable? and how to convert that wattage in to VA of the equipment.

Thank you in anticipation.

Do you mean input wattage or output wattage?
If input wattage/VA then you can simply measure the input Volts and Amps and do the sums.
If output wattage then do you mean peak wattage or continuous wattage? The latter, in particular, will depend heavily on what sort of heat-sinking and heat-dissipation arrangements the product has.

Thank you Mr Alec. i need to now all with formulas. hope you will gide me. thank you.

i need to now all with formulas

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Instantaneous wattage: W= V * I