The company I work for designs and makes 3 phase (single phase too now) energy meters so I have a fair bit of experience with this sort of thing.

We bought a couple of cheap meters that used a wireless link back to a monitor with an LCD screen display kWh or $ spent. They used a clamp for one line of the power cord. However they did not measure voltage at all. You "programmed" in the voltage from a few presets.

If you use a current clamp, you need to measure the voltage also. You also need to measure the phase angle since kWh = volts x current x power factor (cos phi) x time (hours))

Domestic power is charged in kWh so you need to know the phase angle of your load to know how much you'll really pay.

Those plug in ones might measure phase angle and voltage but I'm not sure. Cheapo ones possibly assume a power factor of 1. If it can display system voltage and phase angle you're a sure bet it will do things right.