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Why does Tesla use Wh/mi instead of miles per KWh?

Discussion in 'Model S: Battery & Charging' started by GasKilla, Dec 15, 2015.

  1. GasKilla

    GasKilla No Gas Know Peace

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    MP/kWh seems a lot easier for me (most people?) to understand because it correlates well with MPGs. Thoughts?
     
  2. phigment

    phigment Member

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    Wh/km is easier for me because it correlates with l/100km
     
  3. green1

    green1 Active Member

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    except L/100km was always a ridiculous measurement anyway. It should have been km/L However I suspect L/100km came about because way back when we first went metric people didn't like that the numbers for km/L looked significantly worse than the mpg they were used to, so they came up with this ridiculous system to make numbers that felt more comfortable.

    km/kwh would be much more convenient being that we fill in kwh and usually want to know how far that will take us.
     
  4. andrewket

    andrewket 2014 S P85DL, 2016 X P90DL (soon 100)

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    Shrug. Wh/mile works for me.
     
  5. JeffS

    JeffS Member

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    I am with the OP on this one. "Miles per the-thing-you-buy" seems like it would be so much easier as a standard. Miles per Gallon or miles per kWh.

    The thing that makes Wh/mi nice though is it's far more granular a measurement. 333Wh/mi is 3mi/kWh. 250 is 4. 200 is 5. 500 is 2. Big jumps. A measure more granular is good.

    When I do a day at 292Wh/mi, I know that's better than rated 303. And in winter when I see 350s pretty regularly, I know its cold outside. All of those round to 3mi/kWh. But they are all quite different.
     
  6. green1

    green1 Active Member

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    Decimal places are a possibility, just sayin'....
     
  7. HankLloydRight

    HankLloydRight Fluxing

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    Why would M/kWh need to be integers? One or two decimal places would make the units comparable in granularity.
     
  8. JeffS

    JeffS Member

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    I think most would agree that most non geeks spend their time in whole numbers. I lean toward geek fairly steeply. So a decimal place is a rock solid answer. But I think, on a bell curve, the middle 80% will be "whole numbers". The right-most deviations will appreciate an ever increasing number of sig figs. And the std deviations on the left will be comfortable somewhere between "Huh...say what?" and "ghlrhurggfluyews...drool" for efficiency measurements. Its that middle 80. Whole numbers.
     
  9. HankLloydRight

    HankLloydRight Fluxing

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    So gas prices would be better for that 80% as $1/gallon, $2/gallon or $3/gallon?
     
  10. JeffS

    JeffS Member

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    Yep... That is the perfect argument. Currency vs mileage numbers. A unit of currency that has ALWAYS included decimals vs mileage numbers that have NEVER included decimals. Totally perfect. Splits hair while drops mic.

    And if you look up...no...higher...no...still higher than that... See that thing up there? That was the humor in my post. I threw it too high. My bad. Sorry.
     
  11. stopcrazypp

    stopcrazypp Well-Known Member

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    The argument for Wh/mi as for L/100km is because it focuses on the amount of energy/fuel consumed rather than the miles. This makes it far easier to calculate relative costs.

    Don't want to rehash, but this article goes into detail about it (g/100m vs mpg):
    http://www.popularmechanics.com/cars/a12367/4324986/

    A fundamental example: 18 mpg car to 28 mpg car vs a 34 mpg car to 50 mpg car.
    Which example saves more gas?

    Many people would pick the latter using mpg, but the right answer is actually the former and it becomes far easier to see using g/100m:
    5.5g/100m to 3.6g/100m vs 2.9g/100m to 2g/100m.
     
  12. GasKilla

    GasKilla No Gas Know Peace

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    This makes sense. I'm coming from a leaf so it's hard for me to make sense of the way Tesla represents "MP-thing", but in the defense of mi/kWh it is always represented with one decimal place so you'd see 3.5, 2.9 etc
     
  13. CmdrThor

    CmdrThor Member

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    I have a LEAF which reports mi/kWh which I thought was just fine because I was used to mpg of ICE vehicles. Then I read a post which posed the following question. You drive 30 miles to your destination at 2 mi/kWh. You return to your origin at 20 mi/kWh. What was your average mi/kWh for the trip? Without looking at it too deeply you would think the answer is 11 mi/kWh. But that is not the case. You used 15 kWh on the way there, and 1.5 kWh on the way back. Therefore you traveled 60 miles using 16.5 kWh averaging 3.6 mi/kWh.

    If you invert all that to Wh/mi that's 500 Wh/mi on the way there, 50 Wh/mi on the way back, average of 275 Wh/mi as you would expect.

    I'm sure the auto industry likes distance/fuel because the numbers start to get bigger fast and people think they have a much more efficient vehicle. But fuel/distance makes a lot more sense in determining how far you are going to be able to go which is very important in an electric vehicle.
     
  14. ItsNotAboutTheMoney

    ItsNotAboutTheMoney Active Member

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    L/100km is likely used so that you aren't having to use leading zeroes, as you would with L/km.

    l/100km is used in European countries, because they're rational, and figure that a measure of consumption is better for comparison than a measure of range.
     
  15. green1

    green1 Active Member

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    Except the more common question is "I have X kwh, how far can I go" rather than "I have to go X distance, how many kwh do I need?" This is due to the fact that the car holds a set number of kwh, whereas the distance they take you varies.
    Meaning that I want to know distance per energy (km/kwh), not the other way around (wh/mi)
     
  16. ItsNotAboutTheMoney

    ItsNotAboutTheMoney Active Member

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    X kWh, xkm/kWh, d = X * x.
    X kWh, xkWh/km, d = X / x.

    I don't see what the problem is. Either way you're having to do multiplication in your head.

    (In reality, people waited until the car beeped and filled up, or estimated how far they could go when the needle was in that position.)
     
  17. David99

    David99 Active Member

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    Miles/gallon is only easier than unit/100 because people in the US (and that's the vast majority here) are used to it. Either way makes absolutely the same sense. "unit per 100" is the exact same as percentage. It's 'something' per 100. We all agree that everyone in the world is pretty familiar and comfortable with that concept. So it would make just as much sense to measure energy usage in an EV in one way or another.

    With gas car you need to do some math to figure out how far you can go or how much you need for a trip. Either way wasn't straight forward. EVs tell you pretty accurately how many miles you have left, so it's much easier these days.

    Tesla is kind of inconsistent here. Energy usage is shown in Wh per miles, but nowhere is the amount of energy you have available shown. You can see the remaining capacity only as a percentage or as 'rated/ideal miles', but not as energy (kWh). I know they avoid it because the amount available to you varies depending on battery condition. It would be very confusing when the car shows 50 kWh and later, when it gets cold, all of a sudden it's less.
     
  18. AWDtsla

    AWDtsla Active Member

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    I am constantly switching to the settings menu to change units between energy and distance. Was the screen not big enough to put 2 numbers on it? How about no setting and just both numbers all the time.
     
  19. liuping

    liuping Active Member

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    When Traveling, Wh/Mile is more useful for me. I need to go X miles, so how much charge (i.e. how much time at the supercharger) do I need.
     
  20. Ben W

    Ben W P85 #61, Roadster #108

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    Well, yes... This is why the Tesla dash prominently and always displays the "how far can I go" figure (in Rated Miles), while the Wh/mi statistic is relegated to the trip meters. But there is very rarely a need to explicitly see the Miles/kWh figure. Even while at a charger, the relevant measure is "Miles per hour of charging", which is shown on the dash, and not "miles / kWh", which is tangential and requires extra calculations to do anything meaningful with.

    The main logic behind using Wh/mi (or l/100km) is that it makes gauging relative efficiencies much more intuitive. The number of miles the typical person drives per year is relatively constant; the much more important factor is the cost of the fuel. Suppose you have two cars that you put similar mileage on: a 10mpg Hummer H2 and a 30mpg Civic. And suppose you want to trade up to either a 12mpg Hummer H3, or a 50mpg Prius. Which will save you more money?

    Perhaps surprisingly, replacing the 10mpg Hummer H2 with the 12mpg Hummer H3 will save you significantly more money (in fuel costs) than upgrading the 30mpg Civic to the 50mpg Prius.

    This is more obvious when you look at it in terms of gallons per 100 miles:

    Hummer H2: 10 gallons / 100mi
    Hummer H3: 8.33 gallons / 100mi
    Civic: 3.33 gallons / 100mi
    Prius: 2 gallons / 100mi

    So switching from Hummer H2 to H3 will save 1.66 gallons / 100mi, whereas switching from Civic to Prius will only save 1.33 gallons / 100mi.

    What I'd really like to see is a car that uses seat sensors to accurately track and display Wh / passenger-mile. (or for an ICE, gallons / passenger-mile.) This gets at the fact that a fully occupied SUV can actually be more efficient (in energy/passenger-mile) than a solo-driven Tesla. It's unlikely that it would make everyone suddenly start carpooling, but it would bring to your awareness just how much difference carpooling can make to overall efficiency.
     

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