Guys….Do not forget about the power factor in AC circuits !
Independent measurement of line current and line voltage only gives real power in kW assuming the load is purely resistive eg an electric fire element. In this case the power factor, which is a measure of the phase relationship between voltage and current is close to 1.
For any equipment with coils and windings (inductance) eg motors, solenoids and traditional ballast type fluorescent fittings the power factor is less than 1 and it is more complex to measure the real power that the utility meter will measure.
Assuming that the rotary converter is driven by an induction motor, this may operate at a power factor of say 0.8 at full load. So in this case, using round figures if the line current is measured at 4A with the clamp meter and the voltage is 250V this gives an apparent power of 1kVA.
If the power factor is 0.8 this gives a real power of 1kVA x 0.8 = 0.8kW. A residential utility electricity meter only measures and aggregates to kWh the real kW = 0.8kW whereas you might incorrectly think this is 1kW by just measuring the line current and voltage.
For an extreme case of a purely inductive load the power factor will be close to zero. So even though 4A is flowing in the line, the electricity meter will read close to zero kW. All the current is "wattless" and just heats up the cables and utility distribution transformers without doing anything useful. The very reason why utility companies don't like poor power factor and many industrial companies use capacitive type power factor correction devices to avoid being penalised !
An additional problem is that the power factor for an induction motor gets worse as the mechanical load decreases. So a motor may take 30 to 50% of it's full load current even with no mechanical load connected and the current is mainly used to magnetise the motor. So with no load on the rotary converter you might incorrectly think that significant real power is being consumed just by measuring current and volts.
The only way is to measure the rotary converter power input is with a power meter, making sure that this measures both line current and voltage and can take account of the prevailing power factor.
Beware though that there have been a number of low cost domestic power meters which were only provided with a current probe and no voltage measurement. There was a big hoo ha with over reading of these meters in dwellings using traditional ballast type fluorescent fittings simply due to the low power factor.
I would have thought that the low cost "plug in" type power meters that can be obtained these days ought to include the voltage measurement but there is no guarantee of this though so careful reading of the specification is required to make sure that power factor is accounted for.
Lots of info out there on the interweb about power factor if there is interest eg. https://en.wikipedia.org/wiki/Power_factor
Variable speed drives are better in the fact that the "wattless" current just sloshes around between the drive and the motor and is isolated from the AC input supply by the internal DC link within the drive. However there are other supply issues introduced by VSD's such as harmonics and high frequency emissions but that is another story !
Sorry for my ramblings and hope this helps.
Adrian