watt-hour metering

Discussion in 'Prime Plug-in Charging' started by Old Bear, Oct 19, 2017.

  1. Old Bear

    Old Bear Senior Member

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    I believe some EVSE units provide information about power consumption, but mine does not. Which is too bad because I'd like to know how many kWh I am using to charge my vehicle each month.

    With this in mind, I decided to see whether there were any affordable watt-hour meters and was surprised to discover that there is an abundant supply of small digital meters which can be purchased from a number of sources online for about $12 to $15.


    These units are about 3-1/2" across, so they could be mounted on a modified standard US switch plate. They connect to 120/240vac which provides power to the internal clock, display back-light, etc. They use an inductive loop placed around the power feed to measure current and perform the math to display watt-hours. There is a push button to reset the accumulated kWh to zero, so you can measure over the course of a charging cycle, a day, a month, etc. You just need to keep a log of your results.

    Has anyone tried using something like this?
     
    #1 Old Bear, Oct 19, 2017
    Last edited: Oct 19, 2017
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  2. hill

    hill High Fiber Member

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    What you're describing shows AC input voltage. That's okay, if that's what you're looking for. But there's loss too - not being accounted for via such appliances, when AC still needs to be converted to DC storage. Some plugins account for this, to show exactly what the traction pack has - as usable stored energy - thusly .

    2017-10-19 12.03.10.jpg
    .
     
  3. bisco

    bisco cookie crumbler

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  4. bisco

    bisco cookie crumbler

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    i use the cars readout, and add 15% for losses.
     
  5. hill

    hill High Fiber Member

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    That works - but your efficiency is likely much better than your allowance.
    .
     
  6. Old Bear

    Old Bear Senior Member

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    I'm really not looking for the car's efficiency. At least not directly. What I am looking for is the number of kWh used to charge the car at home each month so that I can figure out what it's adding to my household electric bill.

    Using this kind of metering makes no sense as a way to look at the car's power consumption because I am sometimes charging it at other locations than at home. All I'm trying to figure out is what portion of my electric bill is attributable to owning a plug-in vehicle.

    Please note that where I live the local utility makes no adjustment for peak/non-peak use -- something which would add a complication that this kind of device would not be able to track on its own.
     
  7. bisco

    bisco cookie crumbler

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    i hear you. plugging your evse into the meter should give you a fairly accurate reading. the car should track it as well (mine does) but doesn't account for losses between the wall plug, and the charger. the difference would show the losses.
     
  8. bisco

    bisco cookie crumbler

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    i got that from the pip forums back in the day, by peeps who were tracking both. not sure there was agreement though.
     
  9. Robin Mitchell

    Robin Mitchell New Member

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    I just installed that very meter in my charging outlet a week ago and so far it's working perfectly. I love the cumulative energy display, which is retained even through power failures, so I don't need to log the usage after each charging cycle.

    In my case, I am using it to tell me how much energy I'm putting in the car, since I haven't figured out how to get reliable data from the car itself. That should generally work, since I almost always charge at home, but I'm still a little grumpy that the car won't tell me how much electricity I've put in it.

    Edit: I just noticed that the original poster said the unit can be mounted in a standard switch plate. It actually can't; it's too big to fit in a single-gang box. I mounted mine sideways in a double-gang box, using a blank faceplate with a hole cut in it to fit the unit.
     
    #9 Robin Mitchell, Nov 3, 2017
    Last edited: Nov 3, 2017
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  10. Old Bear

    Old Bear Senior Member

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    You're right. It fits a "double-gang box." I have it here but have not yet installed it because we're just completing construction and I don't want to install anything that might upset the final government inspection. I don't think it's a problem, but experience tells me that one should never do anything unusual that might provoke an inspector in a bad mood. ;)
     
  11. Robin Mitchell

    Robin Mitchell New Member

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    Your years have made you wise, grasshopper :)

    By the way, I just got my first minor disappointment with that meter. The energy display starts off in Wh, as shown in your photos at the top of this thread, and then switches to kWh once you overflow that. When I charged my car last night mine overflowed, and it went straight from showing watt-hours (i.e. a resolution of .001 kWh) to showing whole kWh (a resolution of 1 kWh). I had been hoping it would go through an intermediate phase of showing fractional kWh so that I could track individual charges with reasonable accuracy when I wanted to, but it looks like the only way to do that is to reset the energy display first and lose the accumulated count. Foo.

    Still, that's a minor issue for me, and probably less for you if you're just trying to track the monthly usage against your electric bill. Just another way that life manages to be not quite perfect.
     
  12. Old Bear

    Old Bear Senior Member

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    I did finally get the meter installed. Here's what the final installation looks like:

    sub-panel-with-kWh-meter.jpg

    And here is how I installed the meter using a standard aluminum double cover plate which was fairly easy to cut:

    Mounted-Watt-Hour-Meter.jpg

    The photo was taken on the first day of the month following a reset to zero kWh. The meter is displaying energy consumption in Wh (watt-hours) and will change to kWh once the total exceeds 9999 watt-hours (9.999 kWh).

    I am using an Leviton EVR40 model 40-amp EVSE unit. In the photo above, it is supplying only 6.48 amps to meet the demand of the Prius Prime. That's only 1526 watts. Typically, the Prime draws about 13.4 amps and about 3200 watts while charging. Obviously, the Leviton EVSE is running far under it's rated capacity.

    Also worth noting is that I observe the meter showing an idle load of about 3-watts -- less than half that of a standard 7.5-watt night-light bulb -- when the EVSE is idle and not connected to the Prime. That works out to about 26 kWh per year.
     
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  13. McRascal

    McRascal New Member

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    Capture.JPG
    Are these meters measuring the current at 240V like in the attached image?
     
  14. Colorado_Hiker

    Colorado_Hiker New Member

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    So which watt meter did you actually buy? (Make and model plz!). ;)

    Jim
     
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  15. Old Bear

    Old Bear Senior Member

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    You need to understand that the donut-shaped core measures the CURRENT in a single conductor. The current is the same in both conductors of any given circuit, regardless of the voltage. That's why it's called a "circuit."

    In order to compute power consumption in watts, the meter has to know the voltage as well. Watts = Current (amps) x Voltage (volts). The meter has a separate pair of wires which must be connected to sense the voltage.

    To compute energy consumption, it also has to have some kind of clock to measure time. Watts x Time = Energy Consumption which is expressed as watt-hours or kilowatt-hours.

    Your double-loop of wire is clever but it is wrong and indicates that you don't quite understand how the current sensor works. Use Google to search for "clamp on ammeter" to learn about how this type of measuring device works.
     
  16. m8547

    m8547 Member

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    Maybe they were confused because sometimes 240V things have two "hot" and one neutral wire. But in the case of a load that's mainly 240V, the neutral current should be negligible. Sometimes it's just there to power control electronics or something minor that only needs 120V. So measuring current on just one wire is sufficient (plus of course voltage and time as you mentioned).

    Last time I thought about it, I wasn't able to derive a way to measure the total power of an arbitrary combination of 120V and 240V loads on a typical split phase system using just a single current meter. But I might have missed something.
     
  17. CharlesH

    CharlesH CA HOV Decal #5 on former PiP

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    Some 240 charging stations don't even have a neutral. My Chargepoint Home unit is like that. The plug for it has two "hot" connectors and a ground. The ground is for safety; if any current flows through it, there has been a fault somewhere and the system trips off. Unlike electric dryers, which typically have hot-hot-neutral, and in which the 120V controls run on one of the hot connectors and the neutral, the electronics in the ChargePoint Home just uses 240 V.
     
  18. hill

    hill High Fiber Member

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    not unless your Appliance has a GFI also. Plenty of people have been electrocuted (some even to death) with a ground.
    .
     
  19. CharlesH

    CharlesH CA HOV Decal #5 on former PiP

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    You're right; I meant to suggest that the ground is used in conjunction with a GFI to actually provide the protection.

    And a totally pedantic comment on the word "electrocuted". This word means killed by electricity, so everyone who has been electrocuted is dead (unless you count people whose heart has been restarted after an electric shock to be at one time dead but then revived). OTOH, one can receive a non-lethal electric shock. Sort of like everyone who has "drowned" is dead by suffocation in water.
     
    #19 CharlesH, Jan 14, 2019
    Last edited: Jan 14, 2019
  20. Old Bear

    Old Bear Senior Member

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    I think I posted this diagram a few months ago. It's a pretty much generic EVSE and vehicle charging system. Notice that there is no "neutral" wire, just a protective ground.

    EVSE-block-diagram-sm.jpg

    Also notice the EVSE's internal GFCI which monitors the charging current and stops current flow to the vehicle if there is a fault.

    All that other stuff is used for the EVSE to tell the vehicle that it's ready to deliver charging voltage and for the vehicle to confirm that there is an EVSE plugged into its charging port.
     
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