The wattmeter senses both voltage and current.So when a light is on how does the meter know how much was unused if the meter doesnt read outgoing on the hot and the rest coming back on the neutral. Probably a dumb question but one I dont know.
When the light is off no current flows.
The meter cannot calculate watts if all it senses is current and the voltage at one point.
It has to know the voltage across the load (or supply) for that.
To state it another way, there is not energy going out one wire and coming back through the other. There is just the energy delivered as a total by the two, three, or four wire circuit.