I had the same question as on this page. Since it wasn't mentioned on that page, I'm assuming that placing two 120VAC Killl-A-Watts in series in order to measure 240VAC will not work. So my question is why won't it work? (If this had worked I would have added cost of usage from each unit together)
The Kill-A-Watt needs to measure both voltage and current.
Internally current sensing is on one of the AC power lines. If you place two in series, you will only sense current on one of the units; it will also only sense (about) half the AC power voltage. Therefore this unit will read about ½ the total power. The other unit will only be useful for forcing the V to be equally shared, and since its sensed current will be 0, it will indicate 0 power.
The units are not rated for over 120 V -- insulation, conductor spacings, and overall quality of the construction. In addition, the power socket & plug are built for US AC power systems. It is not safe to use this for higher voltages or without the correct plugs and sockets.
Putting both Kill-A-Watts in series will not allow 120V instruments to be used on 240V. They will both see the same voltage.
Since the load current is flowing through both units they will both measure the same current.
As they are both sensing the same current and voltage they will measure the same power.
There will be minor differences in the power measured as the upstream unit will be measuring the power required to operate the downstream unit.
If you are simply trying to measure a single 230 vac cct you can do it with a single 120 vac kill-a-watt.
Halve the voltage with a resistive divider.
Double the current by, if a resistive current sensor, doubling the sense resistor value.
If a CT put two turns on the ct sense input.