A Flaw in Most Well-to-Wheel Studies of EVs and PHEVs

A post on 2GreenEnergy inspired me to talk about a flaw in Well to Wheel studies of vehicles that has been bothering me at a low level for years. Such studies attempt to quantify the emissions of vehicles based on the entire life-cycle of the fuel they use. For plug-in vehicles, this requires understanding the source of the electricity they use.

Put simply, every study I’ve read uses the average electricity generation mix. But all good economists think at the margins: The electricity going into EVs will be marginal generation: that which is built (or runs for additional hours) to meet the new source of demand. Since few new coal plants are being built in the US, and those that are here do not have much extra capacity, the marginal new electricity that will be used to power EVs will be mostly Gas, Wind, and Solar, since these dominate the mix of new generation being built, and among existing plants, only Gas has the ability to increase existing capacity factors substantially.

Since all these sources of electricity are cleaner than the average mix, studies that focus on the average utility mix understate the emissions reduction benefits of EVs. The answer may be different in China, where they are still building coal (as well as nuclear and renewable) generation at a breakneck pace.


  1. Arthur said

    I agree that considering marginal generation is useful and more accurate when comparing vehicle technologies, in addition to considering time of day – potentially solar and gas during the day, and wind and gas during the night ?

    I believe the aversion to using marginal electricity comes from accusations of double-counting where renewables could allegedly both “green” the grid and allow for zero-emission transportation.

  2. Tom said

    It could be that double-counting is the reason. We’d need to use marginal accounting for everything to avoid double-counting.

    My best guess as to why we don’t use marginal accounting for electricity emissions is that it’s just too difficult to model.

  3. bchase said

    Interesting points. Perhaps a stronger driver will be that of charging cars in a smart grid environment. I am on a realtime pricing program in Illinois and am now aware of the high volatility of electric wholesale prices. If we tie charging at off peak hours, the effect will be optima load leveling while providing better use for night time wind generation.

    The chance for peak load leveling from auto based energy storage is also very interesting. This could reduce the requirements for spinning reserve which is presently just wasted energy.

  4. You are right, carbon dioxide emissions has become a deadly threat in today’s world. From a personal point of view I believe wholesale multi crystalline solar module manufacturers and suppliers are really working hard to make this earth a better place for the upcoming generations. I’m going to share your post on Twitter. Thanks once again.

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