How To Give An Electric An MPG When They Don't Run On Gas?

Cruising the electric cars on the showroom floor, consumers could soon be faced with an array of new numbers and stats on the piece of paper in the car window. Until now we’ve just had the traditional city vs highway MPG, but how do you give a rating that makes sense to car that doesn’t run on gas?

The EPA says they’re coming out with a new rating that will cover electrics. Manufacturers expect it will be some sort of mpg equivalency.

“Right now it looks like there’s going to be a lot on the label,” G.M.’s vice chair of global product operations told the New York Times. “They’re trying to figure out what are all the variables that customers are going to see out there.”

They better figure it out soon, the cars are set to hit the road this December.

Plug-In Cars Pose Riddle for E.P.A. [NYT via Pat’s Papers]

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  1. parv says:

    Is “miles/watt” too obvious to use?

    • nrich239 says:

      I was thinking MPC – miles per charge. And give the 2 sets, Flat / Hilly

      • PunditGuy says:

        I was thinking about something like this, but then it needs to account for the battery losing its capacity over time. I want to know how far I’ll be able to drive on a full charge after the battery is 5 years old, with whatever would be a reasonable amount of charging done in that time.

        • Loias supports harsher punishments against corporations says:

          I agree, an estimated battery life needs to be given. Based on average consumer mileage per year or some such.

          What would be really nice is a graph of battery capacity over time given that information. Some batteries have a constant decline in capacity while other might baloon near their end.

        • parv says:

          Another thing to account for would be degradation, if any, in charge holding capacity of a battery set due to “quick charging”.

      • cosmic.charlie says:

        You would get the same distance if it was flat/hilly as long as you ended at the same elevation.

        Conservation of energy…

        • bonzombiekitty says:

          Which isn’t true when it comes to cars in most situations. When you go down hill, you break from going too fast. The act of breaking means not all the potential energy you gained by climbing the hill is transferred into forward momentum.

          If you didn’t care about how fast you go, and you didn’t have to worry about things like transmissions assisting in limiting speed (assume constant acceleration), then yeah, if you went down as much as you went up, the mileage would be the same as if you went the same distance on a level surface (yes, that’s grossly simplified). However, we DO care about how fast we go. You can’t exactly go speeding down a curvy mountain pass at 200MPH. You have to convert a good deal of that potential energy you got climbing that hill into heat (via braking) to keep the car at a controllable speed.

          • cosmic.charlie says:

            Where did anyone said anything about a curvy mountain pass until you brought it up?

            Hills /= Mountains

            Also, it is my understanding that most electric vehicles will have regenerative braking which means that you will return some of that energy to the energy storage device (abet will some losses) for the trip back up the other side of the “curvy mountain pass.”

            And there are ways to cheat the game to get the best possible ratings. That is why cars don’t usually get near the estimated fuel economy. My point (which you failed to miss) was that there is that the flat vs. hilly comparison was moot. Thanks for the rebuttal.

          • AnthonyC says:

            In an electric or hybrid vehicle, regenerative braking takes care of a large fraction of the loss associated with going uphill and downhill.

      • Talisker says:

        I agree. Only way it makes sense.

    • pecan 3.14159265 says:

      That’s the convention currently being adopted by a lot of companies, like Tesla. It just makes sense. People wanted to know miles per gallon so they could figure out how long they could drive until they ran out of gas. People who buy EVs want to know how long a single charge can last. I think the EPA should just use that kind of standard as an equivalent to the conventional “miles per gallon.”

    • Snowblind says:


      We are going metric!

      • ztoop says:

        While that may be metric that isn’t SI. meter/ joule is. Or use cgs: cm/erg.

        • AnthonyC says:

          I like Planck units. Yeah, it makes my top speed seem puny, but then energy consumption looks tiny, too!

          Or maybe go all out, and measure furlongs per slug-fathom-squared-per-fortnight-squared.

          Point is, the unit doesn’t matter, as long as everyone knows what it means. Case in point: parv (1st comment) didn’t know what a watt is, at least not well enough. Luckily, it seems he does now.

    • Limewater says:

      No, it’s not obvious because it doesn’t make any sense. It would need to be something like (distance)/kilowatt-hour or (distance)/MegaJoule or something. A watt is a unit of power, not energy.

      • parv says:

        Thanks to you & DanRydell.

        I indeed was thinking of power, not energy. If had thought about battery efficiency I might have scribed joules instead.

    • DanRydell says:

      Your suggestion illustrates the problem with using electrical terminology to provide the efficiency rating – most people don’t understand electrical terminology, including you. Watt is a rate of energy conversion, you need to pair it with a unit of time to end up with a measure of energy consumed. This is why your electric bill lists your usage in kilowatt HOURS.

      Miles per kWh might work, because most people know their approximate electric cost per kWh. The problem is, Miles per kWh would be a very small number. You can’t brag that your car gets 3.5 miles per kWh, you’d just sound silly.

      • parv says:

        If the number is comparively large, then good; else bad. This is not a rating of fluid fuel combustion engine rating after all.

      • fs2k2isfun says:

        Or you could do it European style (i.e. liters/100 km), so kWh/100 miles or something like that to give a larger unit. Consumers would have to learn to look for a smaller number, instead of a bigger one like with MPG.

      • PanCake BuTT says:

        All comments below yours are futile, & it should have not gone any further than your post. Your insight & slight snob attitude is welcoming & sort of relieving. Good stuff ! No sarcasm!

      • bd_ says:

        How about using miles per terajoule? (1 mile/kWh = 277 mile/TJ)

  2. raydee wandered off on a tangent and got lost says:

    Now all I can think of is the Simpsons “Stonecutters” episode… “Who holds back the electric car, who made Steve Gutenberg a star?” … yeah, it is stuck in your head now too. :D

  3. pecan 3.14159265 says:

    I agree there should be a way of describing the equivalent miles per gallon (as comparing the EV to the conventional gas vehicle) but the Volt isn’t a purely electric vehicle so the moniker “mpg” kind of does still apply. After the electric battery runs out of juice, it depends on gas to run the electric battery – so it still does depend on gas. I saw a Volt recently – I must admit, it’s shiny.

  4. Southern says:

    If it’s electric, they they may come out with an equivalent, but there’s only going to be one real number that I care about (other than price, of course) – and that’s how many miles do I get on a full charge.

    Other numbers (like how long it takes to charge, how much electricity it takes to charge, etc) would fit into my decision somewhere, but I ain’t buying no car that only goes

    That’s why I like the VOLT so much. :)

    Maybe they’ll come out with something like a “KwPM” (Kilowatt per mile) formula.

    • Southern says:

      Sorry, the site must have eaten my “less than” symbol thinking it was an HTML command.. I meant to say “I ain’t buying no car that only goes “less than” 100 miles on a full charge. At that point it’s just a commute car, and is worthless to me as a family/vacation car. I’d still have to maintain multiple vehicles.” (the rest of the message showed up.)

      • pecan 3.14159265 says:

        The Tesla Model S is supposed to get 300 miles per charge. I want a Model S so badly…I don’t have the money, and we don’t even need a car, but IMO, it’s the most exciting electric vehicle because it’s a luxury vehicle that a lot of people can still afford. People want a car that they can justify as a multi-purpose vehicle. The fact that the Model S can seat six or seven people means it’s just as useful as a small van or a small SUV, and it’s a respectable vehicle for a night on the town.

        • Southern says:

          300 Miles I could justify considering, as that would/should last me for a full day of in-town driving. :-)

    • RickN says:

      Same here regarding miles per charge. I also think, given past mileage claims, that any miles per charge need be taken with extreme skepticism.

      Maybe they get 300 mpc under their perfectly controlled conditions, but I won’t. Just like I never get the claimed 20 mpg in city driving with my car.

  5. Loias supports harsher punishments against corporations says:

    Is it that hard to label based on the origin of energy, be it gas, electricity, or garbage (Go Mr. Fusion!)?

    35 miles per gallon
    52 miles per watt

    And when it’s a hybrid system, the rules don’t change much. What’s the mileage on energy source A only, B, only, and when both are engaged?

    • DanRydell says:

      Yes it is difficult, because many, many people (like you) don’t understand why “miles per watt” doesn’t make sense.

      • Southern says:

        I don’t know “why” it doesn’t make sense.. We know that in the case of, say, a 100 Watt light bulb, that it will “use” 1KWh in 10 hours, which costs us $.10 (or whatever we’re paying per KWh)

        The conversion might not be so easy in an “MPG” formula, but it shouldn’t be impossible. I.E., if the range on a full charge of an EV is 100 miles, and it takes “100KWh” to fully charge the battery, couldn’t we assume that the car uses “1KWh” per mile, or 1,000 watts per mile?

  6. TuxthePenguin says:

    This is a raging debate? We know how much energy is in a standard gallon of standard unleaded gas. Use that as your base… what’s so difficult about that?

    Oh… that it might not show that great of MPGe improvement compared to a typical internal combustion engine?

    • Tim says:

      No, the issue is that if your car doesn’t use gas, you really don’t care how efficient it is in terms of gas usage.

      It’d be like if I said an air conditioner uses the equivalent of 100 ears of corn per hour. It doesn’t actually use ears of corn, it uses electricity … I want to know how much electricity it uses.

      • TuxthePenguin says:

        Technically you are correct. But most Americans understand fuel efficiency in terms of MPG. So, in order to minimize the confusion and hasten adoption, you want to couch it in terms similiar – MPG equivalent.

        Its why we’re never going to get rid of the QWERTY keyboard – people know it and are used to it. So why reinvent the wheel?

        • Tim says:

          You’re right, but the reason why people think that is because people care about how much gas they’ll have to buy. People, for the most part, don’t care that their car is environmentally friendly (or if they do, it’s secondary to how much gas they’ll buy). They want to know how much they’ll spend on gas.

          With an electric vehicle, they’ll instead have to spend money on electricity. So logically, it would make sense to tell them how much electricity it would use.

          I get the feeling that you’re trying to prove that electric vehicles are less energy-efficient than ones with internal combustion engines. This discussion is not about that.

    • ARP says:

      We could potentially do that, but I don’t think the number would be that helpful. A gallon of gas has about 132×10^6 joules of energy and a joule is a watt/second….etc. I guess we could convert it to kw/h equivalents.

      On top of that you’d need to factor in the cost of gas v. cost of electricity. In the end, I think you’d just want to know how much to go how far. $1=5miles. In that arena, electric has gas beat.

  7. legolex says:

    I had to read that title about 4 times before understanding it. Der.

    • zifnab0 says:

      That’s because the title doesn’t make any sense.

      The proper title should be “How to compare electric vehicle efficiencies when the standard automobile metric (MPG) doesn’t apply?”

  8. Applekid ┬──┬ ノ( ゜-゜ノ) says:

    I get 40 rods to the hogshead.

  9. Thaddeus says:

    She’ll do 300 hectares on a single tank of kerosene.

    What country is this car from?

    It .. no longer exists….. Poot it in ‘H’!

  10. fortymegafonzies says:

    I’m not really sure that rating the efficiency of an electric is really even worthwhile. It costs about $2-3 or so to fully charge a Volt and drive 40 miles. Even if it were fully 100% less efficient it would still only cost $4-6 to charge it and drive the same 40 miles. Sure, it would cost twice as much, but it’s still a pretty trivial sum and most people probably wouldn’t fret over $2.

  11. ARP says:

    I think this would be pretty easy- miles per Kw.

    What I think they need to improve is hybrid mileages (e.g. Volt) How do they measure it? How many miles you can go with a full tank and a full charge / gallons of gas the tank holds? I think they should at least put in a secondary “all gas” mileage.

  12. raybury says:

    How about coal burned per mile?

    My other complaint about all-electrics is, what do you do if you run out of power? You can’t ask someone to bring you a gas can, unless they also bring a generator with it.

    • Clearly says:

      You ask them to bring a tow truck.

    • hoi-polloi says:

      At least with the Leaf, all you’ll need is an extension cord and some time. From their site:

      It will charge on a regular 110/120V 20-Amp dedicated outlet. This is considered a “trickle charge,” which means it would charge at a slower rate. For home charging, we recommend a home charging dock on a dedicated 220/240V, 40A circuit.

      In some cases, that sounds easier than finding an open gas station in the middle of the night.

      • Chmeeee says:

        The Leaf needs 16 hours for a full charge.

        • ARP says:

          But if you run low on power 10 miles from your house, you probably only need 30 minutes to get enough power to get home.

          Electrics are not designed for long commutes or cross country trips, but they can serve your needs 75%+ of the time. The average commute to work and back is about 16 miles each way.

          Let’s up that to 25 miles that we encompass most of the bell curve. So, 50 + an extra 15 miles for errands on the way + 20 miles in power for AC/Heat/Accessories/reduced batter capacity= 85 miles. You still have a 15 mile margin.

        • hoi-polloi says:

          The last time I checked, people who ran out of gas don’t generally go fetch 10 gallons they can be sure to top off the tank. You’re getting a couple gallons to make due. I’ve not done a great deal of research on the charge time with various types of outlets, but you may only need a relatively short charge to get home. This strikes me as a small issue, given how rarely most people run out of gas and that these will likely be used as commuting cars.

    • webweazel says:

      See, this is what bugs me the most about pure electric cars. Godforbid you run out, you can’t just schlep down to the local station and come back with a can of electrons. Or a generator on a looooong cord.

      I still think the best idea is the one where the gas engine charges up the batteries. You don’t have to do all that plugging and unplugging all the time. (and considering how every time I need the car, the hubs has it down to 1/8 tank where the gas light is always on-GRR) It would probably never be plugged in enough. Good luck finding a fill station with an outlet. Wasn’t there another story where some guy got fired for plugging in his car at his job or something?

      Nah, for pure electric, I’d stick to a golf cart.

      • Cantras says:

        There are garages with plug ins for heater blocks in trucks. Not quite the same as a gas station, and it won’t top you up, but I’d think you could plug in while you were at work or shopping and get a smidgen extra juice.

      • hoi-polloi says:

        There’s obviously an infrastructure that needs to be built to support this. You can get some answers and anticipated options from the Leaf site:

        You can do a slow charge the car with 110V outlet, but a 220V outlet (already used for electric dryers and the like) will be used for primary charging. Eventually, the goal seems to be 480V rapid-charging stations. I’m curious to know how long that would take.

        This will certainly take some anticipation and planning, but it’s just a matter of adjustment. A more obvious display and showing you’re ‘in the red’ earlier may help avoid full drains. Besides, how many of us are accustomed to routinely charge our cell phones or other electronics overnight? I find this pretty exciting.

    • Chmeeee says:

      Since the Volt’s not an all-electric, you don’t have to worry. With an all-electric, you better be damn careful to watch that range and try to avoid planning trips that will leave you close to the max range.

    • ARP says:

      Even the most inefficient coal plant is more efficient than an ICE at scale. Also, most people don’t get 100% of their power from a coal source. In fact, the coasts get a significant amount from “clean” sources. So, while there is still an environmental impact, its much less than an ICE. This is a common conservative straw-man attack, as nobody in their right mind thinks that electrics are 100% clean, they’re just much cleaner than ICE’s.

      Re- running out of power. Well first, off, you’d know that you’re running low on power because your guage says you are. How often have you run out of gas? I never have, but I know it happens sometimes. Second, someone could potentially “jump” you, just like when you have a dead battery. They’re not going to top you off, but they can probably get you enough juice to get to an electric source.

      • wackydan says:

        I think “jumping” an all electric isn’t going to be in the future. I do not think that the manufacturers have thought of that, and probably because of safety more than anything else.

    • brianary says:

      Not a good measure for those of us in the Northwest that get their power hydroelectrically.

  13. Firesoul1 says:

    Miles Per Charge? Then the computer will tell the driver how many miles can the batteries go (depending how much energy is being used at the time).

    • wrjohnston91283 says:

      But that’s not comparible between different cars with differen batteries. It’s good to know, but its not a comparable.

      I do agree, miles per kilowatth hour does seem to be the best.

  14. JohnDeere says:

    you convert fuel cost to your electrical cost right.

  15. UnicornMaster says:

    I would say use a MPG equivalent. For example if this Plug In Electric gets 200 miles per $5 charge in electricity, and gas is $2.50 a gallon then the Electric car gets 100 MPG equivalent. That way consumers can make more informed decisions about how much it costs to run and compare it to conventional vehicles.

  16. twritersf says:

    The most important figure will be the expected range on a full charge. It’s not like you can pull over and fill up a battery in a few minutes when it runs out of juice, so knowing how far you can go–especially if city and highway styles of driving create a significant difference–will be an incredibly important piece of information for buyers.

    • bonzombiekitty says:

      While that’s important, that’s not a good replacement for MPG. Car A and car B could each go 100 miles on a charge. But car A’s battery & motor are less efficient and is thus twice the size of car B. So it costs twice as much to “fill up” car A as it does for car B.

  17. crabbygeek says:

    How about MPC… Miles Per Charge (obviously one a full charge, not a partial) …

    • fs2k2isfun says:

      How do you account for different sized batteries? Miles per charge would be a measure of range, not efficiency.

      • crabbygeek says:

        Good point, i wasn’t thinking from the EPA’s point of view but from an owner or potential owner evaluating the car…

  18. mikeyo says:

    They are probably also going to quote how much horsepower it has, even though it doesn’t run on horses.

  19. Tongsy says:

    Solution should be to figure out how many miles it can go on a charge, figure out how much that charge costs, and figure out how much gas you could buy with how much money you spent on that charge.

    That way you could have a “MPG equivalent”

    • PowerIsNotTheSameAsEnergy says:

      That will change when the price of gas vs price of energy changes, like when you drive from New Jersey to Virginia. Or if you just wait 6 months.

    • NeverLetMeDown says:

      The problem with using cost of power and cost of gas to convert from MPG to an electric metric is that both those numbers vary, by region and over time, and their correlation varies by region and over time.

      Assume, for the sake of argument, that 1kwh will move the Volt two miles (no idea what the right # is, just as an example). In Hawaii, for the price of a gallon of gas ($3.47), you can buy 12.5kwh of electricity, implying 12.5*2=25MPG. In Washington state, the price of a gallon of gas will buy you 38.5kwh, implying 77MPG.

  20. Talisker says:

    At this point, it’s more important to the buyers to know how far they are going to be able to go on a typical charge rather than what the consumption of energy is per mile.

  21. unimus says:

    Well, judging by the figures released by GM since 2006, I’d say it’s a few hundred. Or whatever the marketing department thinks it should be.

  22. ScandalMgr says:

    All we want as consumers is $/mile, because consumerists are supposed to be myopic like that.

    Please do not consider environmental impacts from climate change (rising oceans) or habitat loss impacts to endangered species due to energy infrastructure and distribution (Gulf Oil Spill anyone?), for roads & bridges, etc.

    If consumers had to pay all those external environmental costs, instead of foisting them on our grandchildren, either they would not have kids, or they would walk or take a bicycle.

    Have a happy toxic legacy, commuters. Our great grandchildren will not be thanking us.

    • vastrightwing says:

      Exactly, all I want to know is how much the vehicle will cost me per mile. This is how I use a vehicle: I drive it from point A to point B and it will cost me this much.

      They do this already with air conditioners, hot water heaters and what not. If the label says based on $0.21/Kwh & $2.75/gal of gas, this vehicle will cost you this much to go 25 miles, 100 miles & 300 miles. That should do it.

      (ps) Those other costs are being paid for indirectly by all of us. That is the way it is. Society subsidizes many things for the benefit of society as a whole. You can make moral judgements about them if you like, however, your judgement changes nothing.

  23. dush says:

    It should have range/charge. Simple

  24. human_shield says:

    I think it needs 2 ratings. 1, a range per charge. 100mpc. The second, efficiency rating showing how many kwh it takes to fully charge.

    Everyone should know how much they pay per kwh from their electric bill, so if I know how many kwh it takes to charge, I know how much it will cost me to go the full range in miles.

    • hoi-polloi says:

      Exactly. If I’m buying an electric car, both cost per charge and average distance per charge are important. I’d also want an upfront idea of battery life and replacement battery cost. If you like to own cars for a significant period of time, that will be an issue.

  25. vastrightwing says:

    This spreadsheet calculates the total cost of driving an electric vehicle over a specified number of miles. It also allows you to compare it to a gas vehicle.

    A4=Total Miles
    A5=Total Cost
    A10=Total Miles
    A11=Total Cost

  26. chaquesuivant says:

    Here is a converter for horsepower to kilowatt:

    Maybe this formula might be part of it?

    I haven’t read all comments yet, my apologies if someone posted this earlier.

  27. common_sense84 says:

    Simple price per mile.

  28. blinky says:

    33 sounds about right, from what I hear.

  29. Jurph says:

    I see two metrics covering pretty much every question an average consumer will ask about efficiency and cost:

    City/Highway Max Range (full charge + full gas tank) takes care of the question “how often do I need to do XYZ?”

    City and Highway Dollars-per-mile, based on the previous year’s most expensive 30-day-average $/kWh and $/gal prices. It also helps estimate the daily operating costs.

    Yes, the energy companies could try to manipulate the 30-day averages… but if they do, consumers win!

  30. Kevin says:

    MPG isn’t an effective measurement to begin with. Time needs to be factored in as well.

  31. PBallRaven says:

    It would be average KWH / Mile. Your electric bill is charged by the KWH. How many KWH would you use to get 1 miles worth of range out of the car. Of course, you have to take into account the efficiency of the charger and a whole host of other things…

    Once they have cars that use generic battery packs that can be swapped out in a couple of minutes, and you can pull into a station and have your car’s pack replaced automatically by a machine, will these cars really be pratical.

  32. f5alcon says:

    we know the energy density of gasoline, so we create a term for something that is miles per unit of energy consumed it wouldnt matter if that unit of energy came from gasoline, diesel, electric, the amount of energy it would take to move the car is the same

  33. AnthonyC says:

    You need exactly 3 numbers to get all the useful info:
    1. Miles per charge
    2. Battery capacity (or miles per kWh, either way)
    3. mpg when running on gas.

    The problem with what they’ve done so far is that they pull ridiculous stunts, like driving 40 miles on the battery and 10 miles on gas (using .25 gallons), and claiming they got 200 mpg.

    If I had to guess, I’d say that once again tech is ahead of law. As long as things like CAFE standards are phrased in terms of mpg, GM needs a way to get credit for the fact that under normal driving habits, the Volt (which gets 35-50mpg when running on gas, depending on who you ask) will use ~80% less gas than a similarly efficient car that doesn’t running 40 miles all-electric.

  34. KyBash says:

    The most important number is miles per charge.

    Since the other number is only going to be used to compare electrics to electrics and not electrics to gas, they could use an average cost per year, like what is on the yellow stickers on new appliances. 12,000 miles per year / miles per kwh * national average cost per kwh.

    Nothing is going to be accurate because there are so many variables, but it would provide a basis for comparison.

  35. Extractor says:

    The more important question should be how these electric cars are going to pay their fair share for road maintainance. We do it now in the form of gas taxes which they dont use. How will these pay for all the wear and tear they produce on our roads.

  36. sopmodm14 says:

    uhh, they should jsut use charge times

    like your car battery readings

  37. webweazel says:

    Where are the flywheel cars at? I had heard of the technology being used like 15 years ago. Where is the development of these now? This is a very interesting read on the subject: