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Help understanding Wh/mi (EPA vs. displayed)

Discussion in 'Model S' started by rscott0, Mar 17, 2017.

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  1. rscott0

    rscott0 Member

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    I've had my Model S since August, and am still having a bit of trouble getting a sense of how much energy I'm using. With my old Volt, it was easy: at the end of a trip, you could see miles used, estimated miles left. If I drove 25 miles and had an estimated 10 miles left, that's about 35 miles compared to the EPA estimate of 38: I did slightly better than the EPA average.

    With the Tesla, things get trickier. It doesn't show estimated miles left (just EPA miles left or ideal miles left, which are really just different ways of displaying % of battery left), and the battery is so much bigger that for shorter drives the math isn't as accurate.

    Issue 1: The EPA for an 85kW Model S shows 370-380Wh/mi (.37-.38 kWh/100mi), and a 265 mile range. Those numbers do not compute. 85kW divided by 265 miles is 320Wh/mi. The EPA shows a range 15% lower than what it should be -- is that because it either [1] assumes people charge to 85%, [2] 15% of the battery is "hidden" when charged to 100% (not used. or [3] something else?

    Issue 2: Here's what I saw today: I drove 14.9 miles, used 5.1kW, at 341kWh/mi. I used 18 rated miles (going from 199 to 181 rated miles, which could really be ~17.1 to 18.9 rated miles). So according to the display, I used about 20% more than the EPA estimate (I used 18 rated miles worth of electricity, but only actually drove 14.9 miles: 18/14.9=1.2). But that suggests the EPA is 284Wh/mi.

    So I'm seeing three different EPA values (~284Wh/mi, 320Wh/mi, 370-380Wh/mi), and am lost. Any ideas?
     
  2. jelloslug

    jelloslug Active Member

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    The wh/mi is the accurate measurement. The remaining range is an estimate based off of past history so it can get out of wack quickly.
     
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  3. DCGOO

    DCGOO Member

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    It varies wildly, largely depending upon the outside ambient temperature. If it is cold, you will be lucky get less than 500wh/mile, the range drops a lot. In the summer, it will all come back. But the "tank gauge" won't show that.

    Personally, I would suggest switching the display to % of capacity, it much easier to understand 24% than 64 miles left. If range becomes critical, use the estimate displayed on the energy graph which is very accurate, based upon actual energy consumed over the last 5, 15 or 30 miles (you pick). Forget the fuel tank range number, it is almost always meaningless.

     
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  4. rscott0

    rscott0 Member

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    Remaining range is not based off past history, according to the manual. Rated range is "Based on EPA testing", ideal range assumes 55MPH, flat road, etc.
     
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  5. rscott0

    rscott0 Member

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    That I get. My actual Wh/mi will vary.

    The problem is that we've got 3 different numbers that should be the exactly same, but are not.
     
  6. DCGOO

    DCGOO Member

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    The only value that can actually be measured is the energy you have already consumed, and how far you have driven consuming it. Everything else is just a guesstimate based upon history. The trip odometer may prove highly useful, especially if you use a custom setting (I use one that I reset monthly). Pick your historical reference. The energy graph can give you 3 different values, built in.
     
  7. rscott0

    rscott0 Member

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    Yes, it is a guesstimate. But why does the EPA guess 370-380 in one place, guesses 320 in another, and Tesla says the EPA guesses 284?

    Sure, sometimes we can do better than 284, and often it will be worse than 380. But with a gas car, if the EPA says 24MPG city and 30MPG highway, you don't see a 10 gallon tank with references to "200 mile range" (20MPG) or the manufacturer saying "the EPA says it is 18MPG in the city".
     
  8. amb3rgris

    amb3rgris Member

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    A couple of things on this issue:
    1. Your "S85" does not actually have 85kWh of usable electricity. There are a few threads where the exact capacity is discussed. And then there is also a portion of the battery that is kept charged for battery health, and is never accessible for vehicle use.
    2. I believe the EPA ratings use "plug to wheel" consumption of electricity. Meaning the EPA figures are looking at what you are pulling from the plug. There will be losses from things like the conversion to DC that will result in slightly less actual usable electricity ending up in your battery.
     
  9. DCGOO

    DCGOO Member

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    It is because no one has the foggiest idea of Wh/mile metric and what that actually means relative to their own use. The Maroni on my 90D says 100 MPGe, I don't think EPA cares about range.
     
  10. jelloslug

    jelloslug Active Member

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    It is though. Take a long trip and see how it changes.
     
  11. rscott0

    rscott0 Member

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    One site states 81.5kWh (e.g. it should be advertised as 81.5 not 85), with 77.5kWh usable (so that charging to 100% and driving to 0% is really charging to about 97.5% and driving down to about 2.5%, to prevent the battery from getting destroyed). But even so, the EPA numbers would suggest a usable battery size of 72kWh.

    And the plug-to-wheel makes sense, but I do not see anywhere at the EPA site or elsewhere stating that it is used (and it would be pointless: e.g. most people use 240V, but some use 120V). I've submitted a query to the EPA to see if they can clarify on my "issue #1".

    I think taking the usable battery kWh *and* plug-to-battery loss could account for the some of the discrepancy. And perhaps the "rated-and-ideal range are described wrong in the manual" could account for the rest. It would be nice to know for sure.

    For those that are stuck looking at a bunch of posts, it really boils down to this: in a gas car that gets a combined 30MPG if you get 30MPG you are getting the EPA number. What kW/mi do you need to get on a Tesla to match the EPA number? ~284, 320, or ~375?
     
  12. DCGOO

    DCGOO Member

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    #12 DCGOO, Mar 17, 2017
    Last edited: Mar 17, 2017
    The range estimate displayed in the energy graph IS based upon history, thus is pretty accurate. it uses the last 5, 15 or 30 miles driven (you pick). But that is all. Just like the stock market though, past performance (the last 15 miles) is not a guarantee of future performance (perhaps going up a mountain). But here in the Hoosier flatlands, the next 15 miles, is pretty likely to be more of the same. :cool:

    Remaining range as displayed in the IC with the battery tank icon, is simply an indication of how full the "tank" is. It really doesn't relate to range at all. That is why I set mine to show % rather than miles. "Honey, we've got a quarter tank left. plenty to get home" (best part of that is not having to stop at the gas station on the way)
     
  13. anonim1979

    anonim1979 Member

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    Calculate usable battery capacity based on rated miles values
    "85/P85/85D/P85D - ~81.5 kWh total capacity, ~77.5 kWh usable"
    "For clarification, the larger packs use a 4 kWh bottom lockout and the smaller packs use a 2.4 kWh bottom lockout. This capacity (included in the "total capacity" numbers above) is NOT usable for driving or other purposes"

    "Top lockout" is only in the software limited (like 60kWh(75kWh) ) batteries.

    Then you watch for Wh/mile you use.
    ex: 1kWh for 3 miles if 333Wh/mile
     
  14. rscott0

    rscott0 Member

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    That is very helpful. It reports that the Tesla uses 295Wh/mi as the EPA rated miles (which matches the 284Wh/mi I got, which was really somewhere between about 270-300, because of rounding). So, assuming the 295Wh/mi is hard-coded to produce the EPA-rated range, that leads us to [1] where does Tesla come up with an EPA number of 295Wh/mi when they say 370-380kW/mi, and [2] how does the EPA come up with 370-380Wh/mi and a 265 mile range (which seems to suggest a 100kWh battery)? I must be making a mistake with #2 there.
     
  15. amb3rgris

    amb3rgris Member

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    Miles per gallon gasoline equivalent - Wikipedia

    Relevant text:
    For EPA, this considers the tank-to-wheel for liquids and wall-to-wheel energy consumption for electricity, i.e. it measures the energy for which the owner usually pays. For EVs the energy cost includes the conversions from AC to charge the battery

    Also:
    Where the Energy Goes: Electric Cars

    Relevant bit from generic sample graphic: "Energy Lost in Charging Battery: 16%"
     
  16. Saghost

    Saghost Active Member

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    Keep in mind, the EPA kWh/100 miles and MPGe both include charging losses, while the car's display does not.
     
  17. jsmay311

    jsmay311 Member

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    -EPA ratings include charging losses.

    -Wh/mile numbers displayed in the car do not include charging losses.

    -There is not just one "Tesla Model S" EPA efficiency rating. There are a bunch of different ratings based on model year, battery pack size, RWD vs AWD, and regular vs performance versions.

    The 2013 S85 is rated at 0.38 kWh/mile.
    The 2017 S60D is rated at 0.32 kWh/mile.
    Big difference.

    So it depends on which specific version you have. Find and Compare Cars


    I know what it means. It means watt-hours of electricity per mile driven. Duh. ;) (Or 0.001 kilowatt-hours per mile. Or 3600 Joules per mile. Or 0.6818 joules per foot. Or 2.12 BTU per kilometer. Or 0.534 Kilocalorie per kilometer. Or 1.4x10^10 electron-volts per nano-meter. Or...) :p
     
  18. rscott0

    rscott0 Member

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    I think the recent posts explain #1: Tesla's 295kW is lower than the EPA's values because it does not factor in loss from the wall to the battery. That's unfortunate, though -- why bother having a value other than the EPA value and calling it the EPA value (to the point where they have a separate "ideal rating")? But it seems to explain that piece.

    I still cannot grasp the 370-380Wh/mile EPA rating for an 85kWh battery and an EPA range of 265 miles. The numbers suggest a range of 226 miles (85/.375). And less than 85kWh of usable battery power would suggest a range lower than 226 miles. I'm still missing something here.
     
  19. rscott0

    rscott0 Member

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    Here's what I am thinking. All I really care about is how my driving compares to the EPA. I cannot tell what my expected range is with the Tesla, like I could with the Volt. But I can see Wh/mile.

    So I will ignore the EPA kWh/100mi (Wh/mi). The EPA 265 mile range matches (I believe) what the car will show if the battery were at 100% (which is technically a bit less than 100%, but I'm willing to ignore that behind-the-scenes magic).

    The Tesla uses 295Wh/mi for its calculation, and it magically matches the EPA 265 mile range. I won't think about why the 295Wh/mi isn't what I would have thought, but instead take that as if it were the official EPA number.

    That gives me what I need: if I get that 295Wh/mi going from 100% to 0%, I've driven 265 miles. So if I drive around town and it shows 325Wh/mi, I've done slightly worse than the EPA estimate; if it shows 275Wh/mi, I've done slightly better.

    So that 295Wh/mi number is the one that drivers need to think about, if they care about EPA numbers, and how their driving compares.
     
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  20. Saghost

    Saghost Active Member

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    The amount of power that the EPA is calculating the range vs efficiency from is more than the battery's usable capacity, because of the aforementioned charging losses. The "85" number Tesla hung on the car isn't actually any of those numbers - the usable is less than that, the EPA basis is more than that.

    The EPA process is to fully charge the car, run until it stops, and fully charge it again I believe, measuring this second charging - so the range is based on the full usable charge, and the efficiency is the full usable charge with losses.

    Since both numbers are from the same test, the EPA clearly was able to feed (265*.37) 98 kWh into the UMC to fully fill the car, even though the pack only ended up with ~76 kWh usable as a result.

    What's odd is that in recent years the gap has dropped, and AFAIK the testing hasn't changed - so maybe newer Tesla chargers are more efficient? Look at the D efficiency ratings compared to the original S ratings...
     

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