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Thread: Power Line Monitor

  1. #1
    Join Date
    Feb 2006
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    Default Power Line Monitor

    I'm trying to compare the actual power consumption of my house to what the PoCo indicates. The house has a smart meter and my monthly bill shows the power used each day.

    I have a four channel scope and a couple of current transformers and can connect / data log on my computer.

    Should I measure the V to I phase to calculate the kw with the measured power factor or just sample the rms Voltage and Current and ignore the phase?

    Thinking that I'd log every 30 seconds. Sound ok or should log more often?

    Thanks!

    Rob

  2. #2
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    In home meters do NOT consider power factor. They only measure watts.
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  3. #3
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    Unless you measure the same way they do, your results will not be the same.
    Sampling every 30 seconds will not be accurate. If you have a well, you could in theory miss most of your well pump cycles. You wont be able to account for motor startup inrush (fridge, freezers, washer, dryer, AC, well, HVAC, microwave (has large caps to charge), etc all have large inrushes.

    If you think you are being billed incorrectly, you can ask to have your meter tested and calibrated. They usually charge you for this if the meter tests as accurate.

    If you are just trying to get more information on how you are using your power then get something like a kill-a-watt EZ plug in usage meter. They are cheap, just move it around your house every day or 2 to different outlets.

    They also make devices that can legally attach to the outside of your meter to give you real time power usage. They can see the wheel moving on analog meters and most digital meters still have something that flashes or moves that some of these devices can detect. Download a installation manual before you buy to see if it's going to be compatible since you have a more modern meter.
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  4. #4
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    Ok, no power factor.

    Sample faster to catch inrush, so that means >20ms.

    I've suspect for a long time (years) that my meter isn't accurate. I was recently away for two months. House was shut down except for fridge, dehumidifier, and AC fixed at 85. My bill showed a significant increase of power consumed every weekend. Outside ambient temps didn't mirror the usage so its not being burned up by the AC. The other interesting thing is the total energy used was almost the same as when I'm home! I called the PoCo and they're "investigating". Its been two weeks and they haven't done anything yet...

    The smart meter cycles through a whole bunch of displays but I haven't been able to figure out what its showing other than the one of cumulative usage.

    Rob

  5. #5
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    VA will tell you something. Watts will always be less or equal to VA depending on the actual power factor. So if you measure 20KVA-hours and the meter says 25KW-hours, there is a problem. I think sampling every 5 to 10 seconds would be accurate enough for what you want to do even not knowing what the power factor is. Inrushes cost almost nothing. It is those little 5w wall warts and TV/appliances that never really turn off and run 24/7 that add up and cost you. A 5 watt load, running 24/7 will cost you about 35 cents a month at 10 cents per kw-hr. You many have 20 or more of these.

    I think you can do a 90% solution with the tool you describe, a clamp on amp meter, and a kill-a-watt plug in tester. Use the kill-a-watt to find those little power suckers you didn't know you have when a circuit shows some unexpected load. You need to be able to measure milliamps on a circuit, as 5 watts at 120V is 42 milliamps (.042 amps). Many clamp on meters can't go that low.

    Do you have a neighbor who has tapped into an outside outlet??? Underground feeders/circuits can also suck power if they are damaged and leaking into the earth. Turning things off like you did on vacation is the way to start. So is measuring the load when you think things everything is turned off -vs- flipping the breakers off and measuring again.
    Last edited by suemarkp; August 11th, 2015 at 12:50 AM. Reason: Change a KVA to KW
    Mark
    Kent, WA

  6. #6
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    I'm looking at the whole house not individual wall-warts. Nothing small would have changed while the house was vacant. My clamp on current transformers are on the feeds between the meter and panel.

    Neighbor was the only thing I could think of, other than the power meter being flaky.

  7. #7
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    I realize that, but if the house seems to be using 10 amps more than you think, you begin to investigate where that power is going. So that's what the clamp on meter is for and then the kill-a-watt to find the culprits on each circuit. Maybe your meter is really off (in which case the main metering is all you need), or maybe you have things sucking power that are unexpected.

    You can get home power monitoring products that do the main service lines and individual circuits or feeders. I think a co-worker has the Ted Pro http://www.theenergydetective.com/home Looks fancy, but kind of a lot of money. But gives you real time monitoring of every circuit in your panel if you can afford all the CT's.

    Oh, and it is possible the power company has the wrong meter ID for your house. I think that is probably the most common error so you effectively get someone else's bill and they get yours.
    Last edited by suemarkp; August 11th, 2015 at 12:54 AM.
    Mark
    Kent, WA

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