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Need help with Kill-A-Watt meter

Discussion in 'Electronic Basics' started by Peabody, Feb 28, 2013.

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  1. Peabody

    Peabody Guest

    I got the Kill-A-Watt P4400 meter today, and just need a little
    help with what the display values mean. (This is all AC, and I
    never quite imprinted on AC.)

    So what I need to know about any device is the rate of KWH it
    consumes - because that's what I get charged for. The button on
    the right displays the total KWH the device has consumed since it
    was plugged into the KAW, and the total elapsed time. From that
    you can calculate how many KWH it will consume per day, month or

    But for devices that don't cycle, I assume I could just look at the
    instantaneous Watts value and calculate from there. Right?

    However, it also has:

    RMS Volts
    RMS Amps
    Power Factor

    Are any of these useful for calculating a device's cost to operate?
    If so, how would I use them? It seems to me all I need is Watts,
    or KWH over time for cycling devices.

  2. For your task you don't need all of these:
    Just read the Watts.

  3. Ecnerwal

    Ecnerwal Guest

    If that's all you want to know, spiffy, ignore the rest.
    No, really, "Ethel, don't look..."
    "...but it was too late. She'd already been zowwed."

    Volts is pretty much "is power service OK." Hardly ever needed, though
    it might help you check that a portable generator was in range if you
    were using one. Quite rare for it to be out of range if you have power
    at all.

    You forgot Hz, which is frequency, again, mostly useful for checking a
    generator. If the power frequency provided to your house by a major
    utility is actually off, go buy a lottery ticket. If the KAW readout
    says it's off by a little, it's the KAW's low tolerances 99.9999% of the

    VoltAmps are volts times amps, and how different that is from watts is
    power factor. For the home user (and KAW is a home-user oriented device)
    it's primarily of use to figure out what-size UPS you should buy for
    your computer and/or network hardware. A 100W device with a 200VA (0.50
    power factor) will need at least a 200VA rated UPS to run it.

    Larger power customers actually have incentive to improve power factor
    (closer to 1.00 is better) in the form of lower costs. Home users
    typically get a free ride. If you have a generator or UPS, improving
    your power factor will be in your self interest for the items you run
    from those power sources.

    If you haven't already noticed - the main reason you want to pay
    attention to the KAW clock is that the KAW resets on the briefest of
    power interruptions. So if you leave it for two weeks, make sure it
    counted for two weeks. It would be nice if it would hold time for a
    minute or so, but it doesn't.

    After the initial surge of checking everything, it may get more sporadic
    use, but it is nice to have. Most recently used mine to see what a new
    computer build was actually using as it ran. When I had it plugged into
    my desktop, I could tell when newswatcher hung on exit, as the power use
    would rise. Of course, after a few seconds of that the fan noise
    increases, and if you've grown to expect the occasional hang-on-close
    you look for it, but it was amusing to note.

    It does make me yearn for a 240V version from time to time. I actually
    put a Murata readout (does not total KWH, but provides volts/amps/watts
    and PF) on my well-pump for monitoring. I can roughly equate the water
    depth to the amps/watts, and hope to be able to spot signs that the pump
    is going bad before it actually dies (though I also plan to put a new
    pump on the shelf after 5-7 years or so, in order not to be panicked
    into buying one in a hurry at high cost.)
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