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Model X: Average Wh/mile Tracker

Discussion in 'Model X: Battery & Charging' started by MattXowner, Apr 24, 2016.

  1. Harvey Danger

    Harvey Danger Member

    Joined:
    Tuesday
    Messages:
    10
    Location:
    The Pacific Northwest
    Just sold my 2016 X 75D with 20" wheels. 35,000 mi. Final Trip B (never reset, see pic) : 350 Wh/mi

    IMG_000573.jpg
     
  2. Royalpopat

    Royalpopat Member

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    Messages:
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    Location:
    New Zealand
    Does the car calculate usage based on what the motors are truely using? or it is based on how much the battery has drained in the duration?
     
  3. Harvey Danger

    Harvey Danger Member

    Joined:
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    That's a really great question! Naturally it has been discussed at length over the years. I'll see if I can sum up what I've learned in a sentence or two.

    The consumption figures people are reporting in this thread generally come from the "trip meter", and *those* values come from a Tesla firmware algorithm that only counts energy going out of the battery (and, during regenerative braking, into the battery) WHILE THE CAR IS IN 'DRIVE'.

    Let's ignore the possibility for the moment that the computer misses some energy transfers that it *should* be catching. Some say the trip meter value is *always* slightly lower than it should be due to the practical difficulty of accurately recording these flows in real-time , especially with older MCU's struggling with memory issues.

    But even setting that aside, the 'only while in drive' criterion also implies a whole lot of real-world energy transfers are not counted in this figure even when it is working as intended.

    At a minimum, it misses any energy consumption while the car is unattended, aka vampire drain.

    Importantly for anyone trying to track electricity *expenses*, it also ignores inefficiencies associated with recharging. You will pay your utility company for more kwh than what you see reported by the car after a charging session because of this discrepancy.

    [Anyone with better info on this should feel free to jump in of course.]

    Anyway, if you wanted to be really conservative and consider true 'wall to wheels' efficiencies, your car is probably under reporting it's true electricity consumption by a meaningful amount. When you add up all the possible errors, I would say a good conservative estimate would be a 20% error, at least until someone tells me otherwise.

    To recap: after driving a certain number of miles the car will report a certain number of kwh used. These numbers are reported in the trip meter. (The car also reports, in the trip meter, for convenience, the result anyone would get from the simple calculation of dividing that many kwh by that many miles. This quotient is given in wh/mi. People love to talk about this number, but just be aware it is not directly measured by the car, it is simply derived from other measured numbers.)

    The number of kwh reported will certainly be less than the amount of real electrical energy actually consumed by the car during the time it took to go all those miles. The difference could be as much as 20%.

    PS

    Real world example: over the last five years my car went 35,000 miles and WHILE IT WAS MOVING its computer tried to detect how much energy was used and came up with a total of 12,250 kwh. (This comes to 350 wh/mi)

    Assuming I only charged at home (and never supercharged), I might naively expect to pay only for the 12,250 kwh reported by the car.

    If electricity in my part of the world is 13¢/kwh, that would only cost $1592. What a great deal! But if I had a way of determining what my utility actually charged me (which is difficult my electric bill includes lots of other usage), I would probably discover it was something like 20% more than 12250 kwh.

    I would not be shocked if i was actually charged (12250kwh increased by 20%) times (13¢ per kwh) = $1911

    Obviously if you are supercharging a lot (either at more or less than home rates) these estimates would need to be adjusted, but this is the general idea...

    Again, I'd love to hear if anyone has solid info that materially contradicts any of this. 'Sharing is caring.'
     
  4. Harvey Danger

    Harvey Danger Member

    Joined:
    Tuesday
    Messages:
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    Location:
    The Pacific Northwest
    Sorry, I got carried away there. You can tell I love this topic. But it was a crazy long response to a simple question. Here's a more succinct answer:

    It tries to, yes.

    No.
     

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