I was (partially) wrong

August 20, 2009

I recieved  comment form the GM spokesman, Rob Peterson, about my last two posts lambasting the supposed 230 mile per gallon Chevy Volt.  Here is Rob’s comment  in its entirety.

This is Rob Peterson from GM.

Although the Volt has a 16 kWh battery, only 8 kWh is used. This will significantly impact the rest of your calculations and your synopsis. Please post a correction based on this fact.

As for the Volt’s city fuel efficiency rating of 230mpg – this is based on the EPA’s draft methodology. The same methodology which will be used for all other vehicles of this type.


I responded to Rob with two comments, which you can read here.  One of those comments questions his sincerity about “blaming” the 230 mile per gallon claim on the EPA.  However, he is essentially right about the the charging cycle of the 16 kWh battery only using about half of that.  He has asked me to “Please post a correction based on this fact.”  I have done so, but the final numbers for the vaunted Volt are still underwhelming.

Here is a table comparing miles per gallon, kWh per mile, and pounds of CO2 per mile for the Chevy Volt, the Toyota Prius, and the a couple of ancient Honda Civics.  You can read the details of how I derived the numbers for the Volt, using Rob’s partial capacity charge cycle scheme in the text below.  Note that the prices for the Honda Civics have been adjusted for inflation to 2009 dollars for a fair comparison.

milage chart copy

Now that I have posted a correction, can I expect Rob Peterson to post a retraction of GM’s preposterous 230 mile per gallon claim?  Not Likely.

I  have not yet been able to find an official specification for the number of kilowatt-hours per mile for the Volt.  I am hoping Rob will provide one.  I have found Rob’s description of the charging scheme for lithium-ion batteries to be essentially correct.  That is, the battery is typically charged by the electrical grid to around 90% of total capacity.  Then the car is propelled entirely off of battery power until it reaches about 30% capacity.  This is known as the “charge depletion” mode.  When the battery gets to about 30% of capacity the gasoline powered generator kicks in and maintains the charge at about 30% capacity.  This is known as the “charge sustaining” mode.  

Then, when the battery is plugged into the electrical grid it is recharged with grid energy from about 30% capacity back up to about 90% capacity    That is a range of about 60% of the total capacity.  So, for a 16 kilowatt-hour battery, a complete charge off the electric grid puts about 9.6 kWh (0.6 x 16 kWh) into the battery.  But an extra 10% or so is lost due to transmission line and battery conversion losses.  So the amount of power taken from the electrical grid will be about 10.6 kilowatt-hours. If that charge will propel the car for 40 miles, then that works out to 3.8 miles per kWh (or about 0.27 kilowatts per mile) 

I cannot find the value of about 0.27 kWh per mile anywhere in the specifications for the Volt, but I did find this somewhat cryptic statement at Chevy.com:

“Under the new procedure, the EPA weights plug-in electric vehicles as traveling more city miles than highway miles on only electricity. The EPA procedure would also note 25 kilowatt-hours/100 miles electrical efficiency in the city cycle.

So, lets accept the value of 25 kilowatt-hours/100 miles (0.25 kWh per mile) for the moment.  What is the affect that this will have on the numbers I reported for CO2 emissions?

The number of pounds of CO2 emitted per mile while powering the car with gasoline (known as the “charge sustaining” mode) will remain unchanged.  There are 19.4 pounds of CO2 produced per gallon of gasoline burned, and GM claims 50 miles per gallon in “charge sustaining” mode.  So:

( 19.4 lbs of CO2 / gallon) / (50 miles / gallon) =
0.39 lbs of CO2 per mile

This 0.39 lbs of CO2 per mile for the Volt running on gasoline (charge sustaining mode) is the same as for the Toyota Prius, because it also gets 50 miles per gallon.

Here is the same calculation for my ancient 1988 Honda Civic hatchback that got 47 miles to the gallon:

( 19.4 lbs of CO2 / Gallon) / (47 miles / gallon) =
0.41 lbs of CO2 per mile

And for the 197887 Honda Civic Coupe HF, which got 57 miles per gallon:

( 19.4 lbs of CO2 / Gallon) / (57 miles / gallon) =
0.34 lbs of CO2 per mile

Lets assume now that the Volt uses 0.25 kilowatt-hours per mile (“25 kilowatt-hours/100 miles’) when running off of power provided to the battery by the electric grid (known as the “charge depleting” mode).  On the average the grid yields 1.34 pounds of CO2 per kilowatt-hour. The grid transmission losses and grid to battery conversion losses  add up to about 10%.  So the amount of CO2 yielded per mile will be:

(1.34 lbs of CO2 per grid kWh) x (0.25 kWh per mile)  x 1.1 =
0.37 lbs of CO2 per mile

Almost identical to the CO2 emitted when it is running off of gasoline (0.39 lbs of CO2 per mile).  And it is also nearly identical to the amount of CO2 per mile as the  much cheaperPrius generates while running off of gasoline.

But here it the rub.  If the Volt is driven in an area where the electricity is predominantly generated with coal (by far the most common source or electricity generation in the US), then the CO2 emissions go way up.  That is because Coal emits about 2.1 pounds of CO2 per kilowatt-hour generated for the electric grid.  So again we can asume 10% for the sum of the grid transmission losses and grid to battery conversion losses.  Then the amount of CO2 that the Volt yields per mile driven in a region where coal is the primary source of electricity will be:

(2.1 lbs of CO2 per grid kWh) x (0.25 kWh per mile) x 1.1 =
0.58 lbs of CO2 per mile

If we really concerned about reducing CO2 (I’m not), saving energy (I am), creating American jobs (I am), and saving money (I am), then we should support the production of an American car that is similar to a 1988 Honda Civic.  Why argue the merits of a $40,000 car that few people will ever be able to afford?  A $15000 dollar car that gets as good or better mileage and generates as little or less CO2 would be bought by millions and have a much greater impact.

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  1. Thanks, Tom. I believe it is a valuable exercise to truth check the claims on the CO2 front. However, I think your knock on the Volt is a bit too much. For those of us who aren’t concerned about CO2 emissions, CO2 is a total non issue. And as another commenter pointed out, there are production and transmission costs involved with getting gas to the station as well, we we need to be careful we’re comparing apples with apples.

    In any event, what is interesting for me about the Volt is that it is a true hybrid, in the sense that you can charge the battery at home and run on electricity, or you can run it on gas when needed. Given that my commute is less than 20 miles each way, I could conceivably drive the Volt for years without using gas, other than the occasional times when I take a longer trip or forget to charge at night. Further, given that I have solar, the Volt could mesh quite well.

    The big problem, as I see it, is the price tag, which will probably keep me from buying a Volt, even though I have been following the Volt and waiting for its release for several years. The price definitely needs to come down for most of us to be able to afford it. Nevertheless, there is much to be said for a vehicle that breaks some new ground in terms of its true hybrid option. Hopefully the Volt will be the first of many vehicles along this line.

  2. I’m a little confused on the mpg claim for the Volt. Rob says the Volt gets 40/8 or 5 miles/kwh. A kwh=3412 BTU and gasoline has 124000 BTU/gal then this is 124000/3412=36.34232 kwh/gal. Therefore this comes to 5 mi/kwh x 36.34232 kwh/gal=181.7 mpg. If the gasoline engine is 30%, then the bottom line is more like 54.5 mpg. Where did I go wrong?

  3. Dear Bob Weber,

    The energy content of gasoline is, in fact,about 36 kWh per gallon. However,when the gasoline is burned the vast majority of that energy simply ends up as heat that is transferred into the surrounding air. However, enough of that energy gets into the battery to propel the car 50 miles. Hence, 50 miles per gallon. You could say that the car gets 1.39 miles per kWh of gasoline energy content (50 miles per gallon divided by 36 kWh per gallon). But that is not a very useful metric.

    The specs for the volt claim that it will get 50 miles per gallon when driving in the “charge sustaining” mode. The “charge sustaining” mode is the only time the Volt is using gasoline – so that means 50 miles per gallon – period. If it got more than 50 miles per gallon in the “charge sustaining” mode I am sure that they would make that claim. The reality is that it will probably be less.

    best regards

  4. I think you are missing one thing that will give the volt broad appeal. First of all I think you are correct to look at the energy from the grid needed to the car when you start making comparisons with other vehicles. But the high gas mileage means that the tree huggers can feel an air of superiority when they drive down the street and the rednecks who hate taxes will get an uplift since they won’t be paying road taxes. So the Volt is the perfect vehicle for redneck tree huggers.

  5. I believe Li ion batteries loose ability to fully recharge over time.

    Will this be a problem for the Volt?

  6. Tom- This is an excellent analysis. I have been thinking about how the Chevy Volt mpg ratings will impact their adherence to CAFE standards.

    This is a great PR and strategic move by Government Motors. By ‘helping’ EPA to define mpg rules for hybrids, it is possible, with enough driving range on batteries only (perhaps only 50 – 60 miles), to build a hybrid vehicle that gets an EPA-defined infinite mpg. This is due to EPA not accounting for the energy in the fully charged battery when the test begins.

    The strategic importance of this situation is due to the way the automaker fleet mpg is calculated. Selling a single hybrid that gets an EPA-certified 200 mpg can offset the sale of ten SUV’s that get 20 mpg. If you can give away for free, one hybrid car with an infinite mpg rating, you can sell as many conventional SUV’s as you desire and still meet any new mandated fleet mpg requirement. Its really quite clever, in my opinion. And ironic in light of the fact that GM is owned by the same government that runs the EPA.

    A few years ago, VW’s CEO drove a concept diesel two-seater to work for a few months. This non-hybrid vehicle demonstrated over 200 mpg.

  7. So how do you reconcile the factor of ten difference in miles/KWH?

    This is like what happened when I tried to estimate the CO2 from a nuclear power plant. I found that the ‘concrete production causes CO2′ argument was meaningless, but my numbers were off by a large factor from others that I saw posted. My net result was that building 3000 nuclear power plants would release negligible amounts of CO2, on the order of 1000 MTCE.

    • Dear MikeN,

      Can you clarify your question “So how do you reconcile the factor of ten difference in miles/KWH?” What factor of 10 are you talking about?

      Best Regards,

  8. [...] web site which appear to be OK on the surface (I did not research the numbers to validate them): I was (partially) wrong Climate Sanity Basically claims that: 50mpg = 0.39 lbs CO2/mile EV[1] = 0.37 lbs CO2/mile EV[2] = 0.58 lbs [...]

  9. [...] horsepower. and will go from 0 to 60 in less than 10 seconds.  And it would cost half as much as a Chevy Volt.   Edison2 is headed up by Oliver Kuttner, and according to consumer reports Kuttner says [...]

  10. http://www.georgiapower.com/electricvehicles/
    check the calculator out

  11. I think it is interesting you chose to compare the volt to other cars optimal gas milage quotes. You exclusively quoted the highway CO2 figures from the other vehicles. I would imagine their efficiency would quickly drop if you included city driving. When you look even further the milage methodology has changed considerably in more than 20 years. If you check the numbers you quoted would have dropped dramatically. 57 mpg car you chose was also a much smaller two seater. These cars also did not have the burden of carrying a heavy of legally mandated safety equipment.


    You have also failed to account for the CO2 created from gasoline production.

    • Alex,

      Thanks for the comment.

      I included that two seater Honda coupe HF, but the ’88 Honda Civic Hatchback was not a two seater. I drove it with two child carseats in the back and a third child sitting between them.

      I kept detailed records of the gasoline usage for many years. It got 47 mpg consistently, city or highway. Even in the mountains (I am in Colorado) it still averaged out to 47 mpg, but it would not maintain a high speed on the steepest inclines. Nothing complicated about that “methodology.” It was a great car. In many respects the best I have ever owned.

      You may have a point about the safety equipment though. My ’88 Civic did not have air bags. I would not buy a car these days without air bags, but air bags don’t weigh that much.

      The old ’88 Civic was a simple vehicle, without many weight adding features that are nearly universal on most vehilcle today, such as electric windows, etc. The thing that adds the weight to the Volt is its own battery.

      You have also neglected an important point. I am talking about technology that is now a quarter of a century old. A small fraction of the money spent on developmnt of the Volt would have yeilded incremental improvements to that old technology, boosting mileage and safety.

      As for failing “to account for the CO2 created from gasoline production,” there is similar CO2 overhead with the production of fossil fuels used to generate electricty that charges the Volt’s battery.

      Don’t get me wrong, I do not have some “philosophical” objection the electric vehicles. If an affordable one, without government subsidies, decent range and mileage (miles per kilowatt hour) comes along, I would consider buying it. But no such car has come along. THe Volt was unreasonably hyped and is way too expensive. And my math above is correct.

      I think you should feel free to buy a Volt if you like it, but as a taxpayer I do not want to pay a subsidy</b so other people can feel like environmental saints.

      Best Regards,

      • Bob,

        And why should I have to pay for your children subsidy if you don’t want to pay for my car ?

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