Model 3

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Watt-Hours per Mile vs. Miles per KWH

edited November -1 in Model 3
Like many on the forums, while waiting for M3D delivery I'm reading and learning. One puzzling question keeps coming to mind after seeing numerous posts referring to the "wh/mile" metric. It seems backwards.

ICE vehicles have used MPG (miles per gallon) as the fuel efficiency metric for as long as I can remember. Miles is the travel distance and a gallon of gas or diesel is the fuel metric.

Just curious as to why folks are flipping the numbers around with EVs, i.e. using fuel as the numerator and travel distance as the denominator. I'm assuming there is a good reason why and would like to better understand.

What is the some underlying reasoning here? Wouldn't it make more sense for the EV market to stick to a fuel efficiency calculation everyone already understands?

For example, let's say the average M3 with the long range battery (with roughly 70 KWH usable capacity) can drive roughly 300 miles between charges. This would equate to a "fuel" efficiency rating of roughly 4.2 miles per KWH, right? Why would the market not use this more simple terminology rather than wh/mi?
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Comments

  • edited July 2018
    Wh/mi or wh/km is a more efficient way to convert energy to cost. MPG is a backward metric and should end with ICE-age.
  • edited November -1
    Wh/mi gives you reasonable accuracy without resorting to multiple decimal places. E.g. 248 Wh/mi = 4.0322 mi/KWh and 249 Wh/mi = 4.0160 mi/KWh. People are more comfortable with whole numbers.
  • edited July 2018
    I think Bolt and Leaf use miles/kWh , from what I have seen in these forum comments. So yes the other ratio is also being used.

    But Tesla has chosen to present wh/mile, so that is what going to be mostly discussed in these forum.
  • edited July 2018
    Yeah I wish they hadn’t flipped it. In this case the ice method is more practical. This has been discussed in the forum too. Someone mentioned that outside the us even for ice cars they use fuel units per distance unit too but I think it’s very impractical.
  • edited July 2018
    Because if you use miles/kwh you get somewhere around 4 mi/kwh so people are going to think wow it only gets 4mpg the is horrible. Where using Wh/mile you get numbers like 240 and people think that is great.
  • edited November -1
    @Patrick, Quote: "ICE vehicles [in the United States of America] have used MPG (miles per gallon) as the fuel efficiency metric for as long as I can remember.

    There--fixed that for ya. You've got the U.S.-centric blinders on. All of the countries that use the metric system use the inverse number of liters per 100km for their efficiency rating, which is a fuel consumption number, just like watt hours per mile.
  • It makes more sense to compute the energy required to go a set distance than the distance available with a set amount of energy because it encourages people to think of efficiency rather than range.

    When people think of efficiency they are more likely to become more efficient drivers. And that is better for us all. And the environment.
  • edited July 2018
    When the EPA lists the fuel efficiency of an EV, it expresses it as MPGe, how many miles it will go on the equivalent energy in a gallon of gasoline. Our Model 3 is listed as 130 (136 city/123 highway). But next to that on the window tag is "26 kW-hr per 100 miles", which of course is 260 Wh/mile.
  • edited July 2018
    Ya. I like the option to show miles / kw. It makes more sense to me.

    The bolt. eGolf. Fiat. All show it in mi/kw.
  • edited July 2018
    @Daryl Funny you should mention this: "the window tag is "26 kW-hr per 100 miles", which of course is 260 Wh/mile" which has had me wondering for a while...

    At 260 kw per 100 and a 310 mile range (310 * 0.26 = 80.6 kwh battery) but that is not right. So... what gives on these numbers? Anyone know?
  • edited July 2018
    Outside the US, most countries use a similar metric for ICE cars too ... units of energy / units of distance (i.e. liters / 100km). After being used to mpg, it is a little confusing, but in the end, I don’t really see one as being better than the other. They are just different.
  • edited July 2018
    I like dalesmith’s answer. On my commute, I usually keep an eye on my efficiency and can usually achieve around 200. I like the instant feedback on efficiency per mile.
  • edited July 2018
    @TexasBob - Same confusion. I've been assuming: 75kwh battery and 310 advertised range = 242 Wh/mi as the "standard" driving style.

    It's been hard for me to achieve 242 Wh/mi unless I use Chill mode.
  • edited July 2018
    Driving in traffic at a very low speed makes it unfortunately easy to get below 240.
  • edited July 2018
    First, the USA should go metric. Seriously, get out of the 16th century. Yeah I know that one’s been done to death.

    Second, Wh/km is the “correct” measure. @ravisundaramam “ I think Bolt and Leaf use miles/kWh” - nope, at least not in the LEAF. It measures and displays Wh/km. Well actually it displays kWh/km which is less useful, since my LEAF does 0.14 kWh/km which is far less user friendly than 140 Wh/km.

    Finally, the nice thing about Wh/km is that your goal is to get that number as close to zero as possible instead of as close to infinity as possible.
  • @ebmcs03 “Ya. I like the option to show miles / kw. It makes more sense to me.”

    It makes sense to you because that’s what you’re used to. In ten years of driving a Tesla your opinion may change.
  • @TexasBob “At 260 kw per 100 and a 310 mile range (310 * 0.26 = 80.6 kwh battery) but that is not right. So... what gives on these numbers? Anyone know?”

    260 kw per 100? Did you mean 260 kwh/100 mi? If so, that is a horrendous driving efficiency. That would be 2,600 wh/mi. Or a range of about 29 miles per full charge.

    310 miles is an estimated range. 75 kwh is the actual battery capacity. Your driving efficiency determines your actual range. If you are getting an efficiency of 2600 wh/mi on a 75 kwh battery your range would be 28.8 miles.

    To get the rated range of 310 miles you would need an efficiency of about 242 wh/mi (24.2 kwh/100 mi)
  • @bryan gets it. Almost instant feedback of one’s driving efficiency (wh/mi) can make it easier for one to become a more efficient driver. And that will save one money.

    If the metric is mi/kwh the number is too small to use to adjust one’s driving to become more efficient. Unless it is stated to several decimal places it doesn’t change fast enough.

    Wh/mi metric is a much nicer feedback if your goal is efficiency.
  • edited July 2018
    Thanks to all for the comments and clarifications - most appreciated! I knew there had to be some good reasons behind the new terminology....

    Into the future we go!
  • edited July 2018
    I still like to have both options. Just like any other settings. Let us choose. Bar vs psi. I’m vs miles. Miles vs %. All are options in the settings. Why not include miles / kw.
  • edited July 2018
    @Rocky wrote, “All of the countries that use the metric system use the inverse number of liters per 100km for their efficiency rating”

    Wrong—Japan uses kpl—km/lt—kilometers per meter, known as ねんぴ (nempi). Fixed that for ya...
  • edited July 2018
    About 1650 miles since taking possession July 3rd. About 1000 miles was a roadtrip, 75-80 MPH velocity, rest is city driving. Average currently 249w/Mile. My daily trips around Chandler/ Phoenix area in 105-117 degrees with AC neutralizing the summer temps I seem to be average in the 220-240W/MI range. The numbers I've always seen are the 75KW or 75000 watts/242=ab out 309.9Miles.
  • edited July 2018
    *meter—>liter ;-)
  • edited July 2018
    @TexasBob
    EPA figure is based on efficiency from the wall, not the battery, so there are frictional losses of charging included as well.
  • edited July 2018
    This kind of discussion inevitably becomes a pissing match about units. As an engineer I use both SI and Imperial on a daily basis, sometimes a mixture of the two. In the end the unit of measure doesn't matter and it comes down to what the user is accustomed to using. One system is not superior to another, they're just numbers.

    What is important in this kind of thing is your reference point. That is how we gain understanding/insight into what the numbers are telling us. Some of us will have to gain a new reference point as we get accustomed to a new unit of measure. It won't be better or worse, it will just be different. :)
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