The internal resistance of the wattmeter and the multimeter (right hand side) is known, it is about 4k per volt in AC mode so about 500k (multimeter) if it operate on 120 volts.
Here is my attempt:
The power dissipates in the multimeter is negligible, duo to high impedance value. I'm not quite sure if this assumption is correct.
The input power that i read on wattmeter should be equal to core losses and copper losses the latter one is negligible (open circuit test), but i get stuck here, i don't know how to filter the effect of multimeter and the wattmeter.