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Disagreement of kWh of the Tesla X and the electricity meter

Discussion in 'Model X: Battery & Charging' started by guyG, Jan 7, 2018.

  1. guyG

    guyG Member

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    I observe a significant disagreement of the displayed kWh of the Tesla X and my electricity meter at home. About 20% more consumption on the meter than on the display of the Tesla.

    No other devices than the car are connected to that meter.

    Does Tesla (intentionally?..) underestimate the kWh retrieved?

    Has anybody similar observations?…



    guyG
     
  2. NewTMSMan

    NewTMSMan Active Member

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    What meter are you using and how are you measuring the kWh used for charging from the electrical line vs. what is received in the battery? I see reasonable agreement between my Aeon Labs power meter on the lines I use to charge my Teslas and the power tracked in TeslaFi for charging accounting for charging efficiency (typically around 90%, but have seen it as low as 70% for a top off charge with a cold battery).
     
  3. guyG

    guyG Member

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    The meter is a Eltako meter.

    The kWh of my Tesla is the addition of the kWh displayed by the car when the charging session is finished. I have noted these figures during 2 months.

    So it is not a isolated finding, but a reliable one spread and averaged over several weeks.
     
  4. NewTMSMan

    NewTMSMan Active Member

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    Percent difference?

    EDIT: Never mind, just realized you posted 20% in your original post. I have seen 20% difference, but overall over the past few months I have averaged 88% charge efficiency on my Model S and 90% on my Model X using a Tesla HPWC.

    It may be using a different charger the efficiency is lower, cannot comment on European charging really, sorry.
     
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  5. minderbinder

    minderbinder Member

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    2 things account for that:

    1 - The charging display in the car measures energy going into the battery. It doesn't account for losses in the AC/DC conversion in the charger or power line losses getting there. The charger is only 85%-ish efficient.

    2 - I don't believe that display counts energy that goes to heat the battery, so when it's cold you won't see that energy use in the car either.

    I have a separate meter as well, and my total energy use at the meter is about 20% higher than the total logged in the car. Efficiency varies with the season - better in summer, worse in winter (due to point #2).
     
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  6. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    15 to 20% higher is normal because of the conversion losses and the car not measuring energy when the car is not moving.
     
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  7. guyG

    guyG Member

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    Interesting answers!

    So 15-20% difference seems not so unusual.

    Any idea if a HPWC makes a significant improvement?
     
  8. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    No. It won't. The HPWC has some advantages, but it doesn't affect the AC/DC conversion losses because that occurs in the on-board charger. The DC current is what's measured in the car.
     
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  9. guyG

    guyG Member

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    OK, thanks for your answer!
     
  10. Troy

    Troy Active Member

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    That's normal. The chargers convert AC power to DC. Some energy is lost during this conversion. Here are some numbers from EPA documents. I have picked the Model 3 as an example because the EPA document happens to show wall consumption and usable capacity. The MS/MX documents show only wall consumption.

    Model 3 LR, usable capacity: 78,270 Wh (Source, see footer on page 6 here)
    Model 3 LR wall consumption: 89,404 Wh (middle of the same page)
    Charging efficiency= 78,270 Wh/89,404 Wh= 87.5%
     
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  11. mknox

    mknox Well-Known Member

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    My observations are as follows (I have a separate sub-meter on the charging circuit as well):

    • Tesla only meters energy coming out of the battery pack while the car is "On" for use in efficiency and kWh consumed displays;
    • Tesla does not consider input energy when charging the car in the calculations. They do show the kWh added on the charging screen when you charge, but that seems to be energy measured going into the main pack and does not account for other use such as running the HVAC;
    • Tesla does not consider standby/vampire losses at all in any of the calculations.
    I see a large discrepancy between what the car reports (kWh) and what I actually meter myself as well.
     
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  12. jdw

    jdw Supporting Member

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    There is no difference between a HPWC and any comparable 208/240V charging adapter or method. There is a significant difference between 110V charging and 208/240V however - 110V charging is noticeably less efficient.
     
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  13. David99

    David99 Active Member

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    Yes 20% seems realistic. The chargers are about 90% efficient, but of course the car needs some power as well while charging. Any use of the cabin heater or AC or even battery heater adds up as well. The car's energy counter only measures what goes into the battery. Charger inefficiencies and all the other things are not calculated. Of course what matters is what you take from the grid. I wish Tesla would keep track of that.
     
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  14. hacer

    hacer Member

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    It could, it depends on what you're currently using to charge. For example, the on board charger is much less efficient when given 120V (in the US) compared to 240V. There is also a variable efficiency that depends on many factors but current is one of them. I've seen 97% efficiency charging at 240V @ 25A in warm (not hot) weather. See Ideal Charge Rate??. The efficiency can also depend on the amount of charging - if you charge for a short period of time you can avoid the overhead of cooling and fan operation that would happen when charging longer at certain temperatures.
     
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  15. hacer

    hacer Member

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    They actually measure it (instantaneously) but don't integrate it or display it for you. It's possible to use the REST API to query the instantaneous voltage and current coming from the wall every few seconds and integrate it yourself. I think one (or more) of the pay Tesla apps actually does this.
     
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  16. David99

    David99 Active Member

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    #16 David99, Jan 7, 2018
    Last edited: Jan 7, 2018
    Yes I let Teslafi do that for me.

    I checked my totals for December and the charge efficiency overall is 88%. Not bad at all.
     
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  17. Need

    Need Active Member

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    I just got my first ToU electric bill so I can see how many kWh I used to charge the MX in super off peak. It shows 550 kWh. I do use the dish washer and the washer at night, not sure how much energy those uses, but I think most of the 550 were on charging the MX. I have just over 1000 miles on it. That means it was about 500 Wh per mile on meter, but on the car it shows about 350. Does it mean I have like 30% loss in the transfer or am I missing something? I am using 240v charging with the UMC. Or maybe the car uses a lot of juice doing the ModelXmas shows? LOL.

    On the good side, if I didn't switch to ToU, I would have to pay $50 more this month.
     
  18. Need

    Need Active Member

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    Nevermind, I think I figured out what's wrong. I guess I use a lot more juice at night outside of charging the MX. Last month, before I got the MX, I used 600 kWh. This month 1040. Outside of the MX, nothing has changed much. So the MX really only used 440. That would put me at about 20% loss.
     
  19. mknox

    mknox Well-Known Member

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    The Model S/X only registers energy use when the car is "On". It does not account for anything that happens when the car is off, including activating the HVAC remotely. It also does not account for standby losses as the car sits nor for charger losses at it charges.

    I have had a home sub-meter on my car's charging circuit for 5 years and can tell you assuredly, the car uses a lot more juice that the display says.

    But in their defense, the car is trying to tell you what your efficiency is / will be while underway in order to predict range etc. You just have to remember that the total kWh value is total kWh drawn from the pack when the car is on. Not what was put into the pack.
     
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