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Thread: Please check my math

  1. Back To Top    #1

    Please check my math

    Wanting to make sure I understand how to calculate energy use correctly

    My pump draws 12 amps at 115 volts.


    12 x 115 = 1380 watts.

    1380 / 1000 = 1.38 kwh

    My electricity costs $.08618 per kwh, so

    1.38 x .08618 = $.1189384 per hour to run.

    12 hours daily would cost 12 x .1189384 = $1.43 or x 30 = $42.81 monthly.

    24 hours would be double, or $2.86 daily and $85.80 monthly.


  2. Back To Top    #2
    JasonLion's Avatar
    Join Date
    May 2007
    Silver Spring, MD
    That looks right. The only thing to keep in mind is that pumps almost never run at exactly their rated current, they generally use less that that, but can very rarely use more than that. Without an amp meter anything you figure will be an approximation, though usually a fairly good approximation.
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  3. Back To Top    #3
    Good point. I actually just happen to be related to a union electrician with all the gadgets, so should be easy to check.


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    mas985's Avatar
    Join Date
    May 2007
    Pleasanton, CA
    The easiest and most accurate measurement you can make is with your electric companies power meter on the side of your house. If it is digital, then just read the difference between the motor on and off. If it is the old style like mine, count the time it takes to make several rotations (and one rotation with the pump off) and use this formula:

    Watts = kh * 3600 / (average seconds for one rotation)

    Watts pump = Watts on - Watts off

    kh is the meter kilowatt hour constant usually printed on the meter dial (mine is 7.2).

    The problem with an amp meter is that it does not measure the phase difference between the voltage and current (i.e. power factor). A true AC power meter will but they are expensive and hard to come by.

    Really what matters most is what the electric company is using to charge you.
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  5. Back To Top    #5
    Thanks Mark.

    Our meter is not digital.

    I'll try out your formula some time soon.

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