What is the best way to measure instantaneous energy usage in an electrical circuit? Wattmeter, ammeter, something else?
My use case is as follows: I have a room which I use for short term rental (STR). The guests sometimes leave the room and leave the (1.5 ton) air conditioner etc. running. I want to monitor whether they have done so by attaching a suitable meter to the circuit for that room. (There is a separate circuit for that room). Such a meter would then give measurements for that circuit.
Opinions on what would be most suitable are appreciated. Obvious possibilities are a wattmeter and an ammeter, which seem basically equivalent, since W=VA.
I don't know whether there are reasons to prefer one over the other. If there are, please let me know.
My location is Bombay, India.