Nissan Leaf EPA Rating Hugely Flawed

Update: True MPGe is closer to 36, see below.  The 36 actually comes from the government's own research and rule-making, which they have chosen to ignore.

The EPA has done the fuel economy rating for the all-electric Nissan Leaf.  I see two major problems with it, but first, here is the window sticker, from this article

Problem #1:  Greenhouse gas estimate is a total crock.  Zero?

The Greenhouse gas rating, in the bottom right corner, is that the car produces ZERO greenhouse gasses.  While I suppose this is technically true, it is wildly misleading.  In almost every case, the production of the electricity to charge the car does create greenhouse gasses.  One might argue the answer is zero in the Pacific Northwest where most power is hydro, but even in heavy hydro/nuclear areas, the incremental marginal demand is typically picked up by natural gas turbines.  And in the Midwest, the Leaf will basically be coal powered, and studies have shown it to create potentially more CO2 than burning gasoline.  I understand that this metric is hard, because it depends on where you are and even what time of day you charge the car, but the EPA in all this complexity chose to use the one number - zero - that is least likely to be the correct answer.

Problems #2:  Apples and oranges comparison of electricity and gasoline.

To understand the problem, look at the methodology:

So, how does the EPA calculate mpg for an electric car? Nissan's presser says the EPA uses a formula where 33.7 kWhs are equivalent to one gallon of gasoline energy

To get 33.7 kWhs to one gallon, they have basically done a conversion through BTUs -- ie 1 KWh = 3412 BTU and one gallon of gasoline releases 115,000 BTU of energy in combustion.

Am I the only one that sees the problem?  They are comparing apples and oranges.  The gasoline number is a potential energy number -- which given inefficiencies (not to mention the second law of thermodynamics) we can never fully capture as useful work out of the fuel.  They are measuring the potential energy in the gasoline before we start to try to convert it to a useful form.  However, with electricity, they are measuring the energy after we have already done much of this conversion and suffered most of the losses.

They are therefore giving the electric vehicle a huge break.  When we measure mpg on a traditional car, the efficiency takes a hit due to conversion efficiencies and heat losses in combustion.  The same thing happens when we generate electricity, but the electric car in this measurement is not being saddled with these losses while the traditional car does have to bear these costs.  Measuring how efficient the Leaf is at using electricity from an electric outlet is roughly equivalent to measuring how efficient my car is at using the energy in the drive shaft.

An apples to apples comparison would compare the traditional car's MPG with the Leaf's miles per gallon of gasoline (or gasoline equivalent) that would have to be burned to generate the electricity it uses.  Even if a power plant were operating at 50% efficiency (which I think is actually high and ignores transmission losses) this reduces the Leaf's MPG down to 50, which is good but in line with several very efficient traditional cars.

Update: I have new numbers, which in part help respond to the first commenter.   The short answer to his comment is that there is a big difference between handwaving away10% you missed and handwaving away 70%.  I agree that the EPA numbers for the Leaf are valid "tank-to-wheel" numbers (meaning how efficiently does the car use the energy in its tank).  The question is, whether tank-to-wheel has any meaning at all.  My article above is basically an argument for why it is not valid.  Here is an extreme example -- what if we ran cars off of replaceable flywheels that were spun up by third parties and then put in our cars already energized.  These would be highly efficient on a tank to wheel basis, as we just need to transmit what is already mechanical energy to the wheels.  But does ignoring the energy costs and inefficiencies in spinning these things up offline really make sense?

We can go to the government itself to solve this.  In this rule-making document, the DOE defines some key numbers we need here.

They define petroleum refining and distribution efficiency as .83, meaning it takes 1 gallon of gas out of the well to get .83 in your tank.

For electricity, they define two numbers that must be multiplied together.  The fossil fuel electrical generation efficiency is .328 and the transmission efficiency is .924, for a net of .303.

Note the big freaking difference between .83 and .303, which is why to call it all handwaving is disingenuous.  Sure, we often handwave away the fossil fuel cost of getting gas in our cars, but the fossil fuel cost of getting electricity in the batteries is four times higher.   The government even does the math, multiplying the 33.7 Kwh/gal used above by .303 and dividing by .83 to get an apples to apples well to wheels mpge number for electric vehicles of 12.3 Kwh/gal.

So a total apples to apples comparison factor already exists, and the government chose not to use it for the window stickers.  This is probably because it would have given the Nissan Leaf an mpge of 36, not bad but fairly pedestrian for such an overhyped technology.  And at some level the Leaf is irrelevant.  This entire process has likely been tilted to make the Government Motors Volt look better.


  1. MikeinAppalacia:

    A state-of-the-art natural gas combined cycle generating station can have a heat rate of about 7000 btu/kwh measured at the generator terminals, about 50% eff. Net of plant auxilaries, transmission, and distribution down to a 240 volt outlet, that would be about 40%. I think a reasonable estimate of coverter/charger efficiency would be on the order of 95-98%. So, the equivalent would tend to be a little less than 50 mpg on gasoline?

  2. kebko:

    Another little bit biasing electric vs. gas cars. Using electricity is basically a way to fill up your car without paying gas taxes, so much of the cost savings is just free-riding.

  3. Tim:

    Hold on a second. *Your* basis of comparison is flawed, not the EPA's. The basis of the comparison between gas and electricity assumes that the energy source has already been delivered to the vehicle's storage unit; not from the point of generation. The mileage numbers don't factor in externalities.

    So, the basis of comparison is this:

    Given a raw amount of potential energy (measured in BTUs) in the vehicle's storage unit; how far can the vehicle travel -- including *all* losses.

    Combustion losses are part and parcel of an internal combustion engine, just like resistive losses are something that the Leaf has to deal with. In fact, combustion efficiency is one of the ways that powertrain engineers improve fuel economy.

    And the same rule applies to CO2 emissions. Given a set potential energy in the tank, how much CO2 is emitted out of the tailpipe going down the road. Yes, the Leaf handwaves CO2 emissions from electricity generation, just like a standard IC engine vehicle handwaves CO2 emissions associated from converting crude(and corn) into gasoline and transporting that gasoline to the storage tank at the service station.

  4. DrTorch:

    I'm trying to understand your second point. I think I get what you're complaint is, the top left '99' is misleading in your opinion.

    I get what you're saying, but my complaint is that it's just a stupid number. They actually provide the correct number: dollars/year in fuel costs. It would be better for the miles driven to be explicitly defined.

    Doing that, you can back out an mpg based on fuel consumption. If it is 10,000 mi/year, and gas costs are $2.75, then you get "49 mpg". Surprisingly close to Mike's estimate based on efficiencies.

    If it's 12,000 mi/year, then it's as if you're getting nearly "59 mpg."

  5. Tim:

    Generally, MPG isn't the right metric to use at all. The better metric is gallons (or gallons equivelent) per set distance. The MPG metric isn't linear; whereas gallons per mile is. In other words, going from 10MPG to 20MPG is a bigger efficiency step than going from 20MPG to 30MPg; which is more of a step than going from 30MPG to 40MPG.

  6. Frederick:

    One other point not considered the loss of energy in storage. All batteries self discharge. With lead acid batteries the loss is approximately 10 percent of charge per month, regardless of the level of charge in the battery. As the electric car is generally stored with a full charge to a minimum of 1/2 charge (if commuting you need at least 1/2 charge to get home), that means you must also reduce the efficiency of the electric car by an additional factor, which would depend on how much energy you use. For the occasional commuter or short distance commuter, who only used say 4 complete charges per month, it would mean an additional 2 percent loss. For the person who uses the full charge of a battery 25 days of each month, it would mean a .4 percent additional loss in efficiency.

    Of course there is some gasoline loss due to evaporation, given all car fuel systems are nominally sealed the loss due to evaporation is much smaller than either of the above figures.

  7. Stan:

    I think this is stupid. When I look at mpg stickers I think what kind of mileage am I going to get, how far can I drive without filling up? So in the first place an MPG equivalent sticker is absurd with this view, since there is no gas.

    But if it must be so, then 2 things:
    simply stating .34kwh per mile would suffice.
    it's not about saving the planet or what it took to make the energy available, its about the consumer's efficiency.

    So even if it took a million child laborers and a decimated rainforest to produce a charged battery, if that batter delivers .34kwh per mile then that's what it should say.

  8. clean and efficient:

    Generally, the estimates I use focus more on BTU/output, that is, BTUs needed to produce a fixed outcome, say, miles, or ton-miles, if you want to adjust for vehicle size. Use 'gross' BTUs (coal, crude oil, natural gas) or 'net' BTUs (gasoline, electricity, natural gas). I haven't done any numbers for awhile, but back-of-the-envelope suggests to me that electricity needs a carbon price, say, even $5/ton, to create a compelling advantage, along with a strong shift in consumer preferences. Thermal efficiencies I use are 40% for electricity generation, 15% for internal combustion. Agreed that CCCT generation technology can probably get 50% efficient. I don't know how to usefully estimate nuclear 'heat rates.'

    All of the above focuses on energy efficiency only. Much Further consideration is needed for the infrastructure needed to support production (drilling/mining, refining)and delivery (trains, tankers, pipelines, power lines). Add a healthy bunch of 'national security' to the equation, too.

  9. greg:

    I agree that it's just a stupid number.

    The information that needs to be conveyed to the consumer is essentially the cost to operate the vehicle on a daily/weekly basis (there are longer term maintenance issues, but that's not what I'm thinking about on a new vehicle). So MPG was a rough way of telling me the operating costs. Since my electricity is metered out in kWh, why not Miles/kWh?

    That would still leave the losses inherent in the vehicle's conversion of that power into forward motion, but again, that is the case for both gas and electric.

  10. greg:

    In other words, the 34 should be the prominent number, but it still seams like a back-assward way of displaying the information. I guess 2.9 MKH just sounds like too small a number.

  11. smcg:

    By way of comparison, I recently drove 9,000km in Europe (family of four and luggage) over all types of roads/conditions and the 2l turbo diesel I was driving returned 4.7l/100km. That is near-as-dammit 50mpg, and without the costs and impact of the electrics.

    FWIW - I reckon that hybrids are the go if they use regenerative braking, allow use of a smaller IC engine (power level not that much more than cruise plus charging loads), and have the electric motor give about 10 minutes "boost" for hills and acceleration. That way the vehicle doesn't need to cart about the ridiculous battery loads we see in Prii (for instance). If the IC engine goes with turbo-diesel approach per above, they are already about as efficient as an electric or Prius in any case.

  12. Roy:

    Nobody has reminded us (so I will) of a significant omitted factor that we've recalled in prior electric car conversations. What happens if the weather outside is not comfortable? Either a heater or an A/C load will much more dramatically effect the electric powered car than it will the IC powered car.

    Caveats empor multiply.

  13. Tim:

    On the update, those calculations are how to calculate the fuel economy for the companies CAFE number; not for the window sticker. That's why this notice was published by the DoE, which is responsible for that program. The window sticker is calculated under a different program, administered by the EPA. Note the example in the appendix of the rule, where the EPA test cycles are combined; and then the E-sub-g is applied.

    But your assumption is still wrong -- it isn't the overall system cost.

    The test procedure for IC vehicles is pretty simple: A certified fuel is supplied in a certified amount, then a particular drive cycle is performed, and fuel consumption is measured. So, all the efficiency losses (combustion, rolling resistance, drag, weight, parasitics) are included in the test procedure. At no point is any petroleum efficiency factored in. Which is what you are proposing to do here, but only for EVs.

    For the purposes of the window sticker, tank-to-wheel does make sense; because you are providing information about the vehicle in comparison to other vehicles, not information about the vehicle in an overall system.

    Now, with all that; should there be some mechanism to properly reflect the overall cost and convey that information to the consumer? Probably, but it should be a different number than the sticker.

  14. cdc:

    As for the "cleanness" of electric cars, see the July issue of Scientific American...
    A mine cousin has a completely electric car fitted with pedals (a glorified self-enclosed tricycle, in fact), so the problem aptly reminded by Roy is half solved: no need for heating. But of course, cooling remains an issue...

  15. Ignoramus:

    Is there another angle to this? Auto makers are supposed to be increasing their CAFE numbers. If they can "cheat" on the calculation for an electric car, they'd get an advantage, wouldn't they?

  16. perlhaqr:

    Ignoramus: Excellent point. If car companies are supposed to have "fleet averages" of 65 mpg or whatever ridiculous number the gov't came up with by 2015, but they're also letting anyone who builds even the most pathetic sop of an electric car claim 250 mpg, that'd help them out on their SUVs that will actually sell.

  17. clean and efficient:

    Not wanting to digress or wander (too far) off-point, but I can't escape the temptation to wonder on the blurry incentives suggested by the 'labeling' issue. If energy efficiency is the goal, why not clear basic apples-to-apples information on an independently sponsored and widely advertised website? If national security is the goal, where are the incentives (long-term stable predictable tax rules and tax credits are the usual tools) for private investment in new refineries in the U.S? If promoting inept automobile company managements and corrupt labor unions are the goals, the nation may be ready to declare 'mission accomplished.'

    Without better focus, nutty statistics/information are one small piece for setting the stage for nasty consumer backlash. I get the sense that environmental advocacy is ambivalent on electric transportation (forever waiting on perfection in the form of some solar-powered fuel cell or other technology that requires enormous government $$$ subsidies), consumers for the most part want only reliable low cost transportation that does 'something' for the environment(how old-fashioned), federal politicians/bureaucrats want the hook to create a vote-attracting solution that only they can provide, oil/gas/energy providers want a stable business model (best of luck on that one), Wall Street wants super-sized profits and corresponding bonuses, and media shouts from everywhere (message delivery for some combination of the above, plus $$$ for'eyeballs' on ads). Somehow over the years, competent practicality has been marginalized by the noise. Here's hoping that it's always darkest before the light.

  18. Patrick:

    "The question is, whether tank-to-wheel has any meaning at all."

    Yes, it does, absolutely, because this is a sticker for the car buyer, and the car buyer is the one putting the money into what goes in the tank (or battery). How far can you go on KWH of electricity that you get from the plug? How many miles? That should be the number.

    Your other point, that electric generation is not GHG-free, is valid. they should make a range or an estimate based on average for the US.

  19. smurfy:

    I just want to buy one and hang a pair of bull balls on the back to keep people guessing.

  20. RRR:

    A BTU is a BTU.

    Trading them without reporting the trade is not very honest.

    But with politicos running things, I suppose we can't really expect any better can we?


    Seems as though the EPA has done such a good job their entire outfit can be cut back by 25% this next year!

  21. Russ Finley:

    A little late to your party but I just posted on this topic with some more insights:

    Consumers mostly want to compare how efficiently cars use the energy stored in their tanks or batteries. Diesel fuel take more energy to make than gasoline. How will a consumer know that a diesel Jetta will go further on a gallon than a gasoline Jetta if the sticker accounts for the extra energy used to make the diesel fuel?