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.

  • 1
    If you just want to know whether an AC unit is on, an ammeter will suffice. A typical household energy monitor with it's measuring clamp round the single circuit in or near the main panel (rather than round the house supply incoming live wire) will achieve this. May 30, 2017 at 8:38
  • How about a small sign by the door "Please turn off A/C when leaving to help us keep our rates low"? May 30, 2017 at 17:15
  • 1
    @MarkStewart That's a great suggestion. I don't know why I haven't done that already. Thank you. May 30, 2017 at 17:39
  • So you want something connected to the circuit, that you can glance at to determine if the A/C is on? If so, a simple ammeter will do just fine.
    – Tester101
    May 31, 2017 at 11:24
  • @Tester101 Yes, that's what I want. Though I've connected one now, and the dial barely moves. it seems to go from 10 to 40 for some reason. Perhaps I need one with a smaller range. May 31, 2017 at 14:51

1 Answer 1


Without a doubt, it is the Kill-A-Watt meter. You want to measure Watts. And also Watt-hours.

Here is what they look like:

enter image description here

They are available from many vendors. Worth every penny.

Every brand offers both instantaneous use, but also cumulative and max.

Since most work the same, I suggest you shop around, assure yourself they all do about the same thing, then look at prices and speed of delivery to make your decision

  • These are American devices. I'm located in India. Also, I need something that can do remote monitoring. I think these kind of devices are designed to be plugged in between the device and the wall, which would not work for my purpose. Also, I'm not sure what kind of amperage these devices can handle. May 30, 2017 at 8:26
  • Leaving the A/C unit running at a normal temperature creates an cooler, dehumidified, environment in the living space as apposed to running the unit only while the area is being occupied. That being said, if the rental is only for short term rentals (a few hours), then you may want to turn off the unit when nobody is paying the rent. If this is the case, I would install a remote switch to shut off the A/C unit when nobody is paying the rent. Monitoring the electrical use would only tell you that the unit is running
    – d.george
    May 30, 2017 at 11:51
  • Hi @d.george Well, it's short term, but not that short term. Occupancy is measured in a multiple of days. And it's a small area - I think it cools down quite fast once the A/C is on. I've heard the viewpoint that is makes sense to leave an A/C running for half a day because it would save the energy to cool it when you get back, but I don't buy it. It may make sense for short periods. May 30, 2017 at 17:47

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