Andras

But did you take power factor into consideration?
The inverter actually goes to the trouble of calculating Root Mean Square power, your Sonof may not be. Measuring voltage and current with a multimeter and multiplying them results in apparent power (Volt Amps), not real power (Watts), if the power factor is other than unity. The inverters themselves put a moderate capacitor across the utility input, resulting in hundreds of VAR. Granted, that's not enough on its own to make the difference, but the result probably comes from your load (and the capacitor is likely cancelling part of that inductive reactance).