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Trying to determine true power of household devices

  1. Aug 3, 2011 #1
    Trying to determine "true power" of household devices

    Hello, I hope this is the right forum to post these questions in.

    I am trying to determine the power usage of all the devices in my household to try and lower our electric bill (we apparently use way more power than our similar-sized neighbors, not good!)

    For AC circuits and devices, I know about apparent power, true power and reactive power, power factor, phase angles, inductance and so on, I did take 3 courses in college physics, though I admit I'm rusty on electromagnetism. My problem is one of practical application.

    I have a Kill-A-Watt meter which can directly measure watts, volt-amps, power factor, voltage, current, frequency, and kWh. I have used the meter for some devices, such as my HDTV and my PC.

    However I have some devices which I cannot plug the meter into, either because they are simply too high power of a device for the meter, or it would be greatly inconvenient to plug the device in (moving large appliances around), etc.

    Therefore I would like to be able to calculate the true power of some devices simply by looking at the manufacturer's tag and doing some simple calcs. For instance the central air conditioner, which is extremely high power. This is where I have run into problems.

    My questions:

    1) If I have only voltage and current listed for an AC device, and I cannot directly measure the power, how do I calculate anything other than its "apparent power" (volt-amps)?

    I need to know the TRUE power which the utility is going to charge me for, that's my only real goal.

    Most devices' tags mention only voltage, current and frequency, and while some indicate the power of the device in watts (more on that in the next question), most do not.

    2) Why do some devices use less power than one would expect them to from the tag info?

    Example 1- I plugged the Kill-A-Watt meter into a simple fan I have, which indicates 120 V, 0.5 A on the tag. One would expect the volt-amps to thus be 120 * 0.5 = 60 VA. However, the meter indicated VA of around 33! The true power was 31 W (using 0.26 A instead of 0.5 A). Why was it only using around half the power listed?

    Example 2- another fan with a tag indicating 45 W, was actually using 41 W. (I don't mind a handful of Watts like this, but would still like to know why there is a difference.)

    Note- I had the fans on the highest speed setting.

    In the case of Example 1, I would have mistakenly determined the fan to be using much more power than it's actually using had I not connected the meter; you can see how this is important for high power devices especially, e.g.'s washer and dryer, pool pump, space heater and so on.

    EDIT- SORRY! I just figured out it's the RMS thing. I feel stupid.

    3) Do the following equations apply only to DC and AC "apparent power"?:

    P = V*I = (I^2)*R = (V^2)/R
    V = I*R

    (If any of them applied to finding AC true power then I could use the tag info alone to find the resistance of the device and work from there.)

    4) Do I need to take into consideration whether a device is 1-phase, 3-phase, etc.?

    Final note- I currently can NOT simply go look at my home's electric meter outside with various devices turned off and on, as we just had a "smart meter" installed and it doesn't seem to be giving me a useful readout. It has a blinking digital display which is showing very little information, and the city power company is still in the process of integrating the new meters with end-user devices and such.

    Thanks, and sorry for the very long, detailed "questions"!
    Last edited: Aug 3, 2011
  2. jcsd
  3. Aug 3, 2011 #2


    User Avatar
    Gold Member

    Re: Trying to determine "true power" of household devices

    With a regular meter you could have tested each electrical device by counting the spin disk rotation. Who is to say that your smart meter isn't defective and how could you ever tell.
  4. Aug 3, 2011 #3
  5. Aug 4, 2011 #4
    Re: Trying to determine "true power" of household devices

    Using your portable tester or as 256 bits says the spin dial on the mains meter will give you an accurate reading.

    It is important to know that anything with an electric motor will be subject to start inrush currents. These can be up to several times the running currents.
    So a device which switches a motor on and off such as AC, refregerator pump, etc will draw periodic peaks as the device motor turn on.
    Lighting also has this effect to a lesser degree.

    go well
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