New study: Nighttime EV charging may increase carbon footprint (Updated)

Nissan Leaf Plugged In Charging

The question of the “long tailpipe” is a fraught one: does an EV deliver lower carbon emissions than a legacy vehicle if the electricity comes from dirty coal? The latest study to address the issue comes from Carnegie Mellon University, and it includes a couple of surprising new findings.

Regional Variability and Uncertainty of Electric Vehicle Life Cycle CO2 Emissions across the United States” compares the lifecycle carbon emissions of the (pure electric) Nissan LEAF to those of the (plug-in hybrid) Chevrolet Volt and Toyota Prius Plug-in.

The main conclusions here agree with what earlier studies (including one from the Union of Concerned Scientists and another from the University of Minnesota) have found: The carbon content of grid power varies tremendously – some regions rely heavily on coal, others have a high proportion of cleaner sources such as hydropower – but in most areas of the US, electric powertrains are cleaner on a well-to-wheels basis than the fossil-fueled kind.

“In some markets, the Prius Plug-in with its gasoline engine generator can be more carbon-efficient than the fully electric Nissan LEAF. For the most part, however, the LEAF is more efficient in most regions of the United States.”

However, here’s something that EV advocates may find disturbing: Delayed charging, which allows EV drivers to shift consumption away from peak-use hours, often translates to higher emissions, because the energy mix at night tends to include a higher proportion of coal.

Another interesting situation (but one that’s not likely to change): Differences in state subsidies do not align well with regional difference in carbon efficiency. For example, the state with that once had the largest state subsidies ($7,500) for BEVs is coal-happy West Virginia, where the carbon advantages of EVs are comparatively small.


[Updated: This article was updated July 14, 2015 – 12:00pm EST to reflect that the West Virginia state subsidy for Alternative Fueled Vehicles does not include vehicles purchased after April 15, 2013 that are powered by “electricity,” only those powered by “compressed natural gas, liquefied natural gas, or liquefied petroleum gas”.]

Source: Environmental Science & Technology via Torque News
Image: Kārlis Dambrāns (CC BY 2.0)

  • Shockwave Motors

    Yet another “research study” seems to assume that gasoline magically appears at the local station. (In other words, it does not seem take into account the energy needed to get the crude oil out of the ground, refine it, and then transport it to the corner gas station). For the sake of discussions, lets assume that the electricity comes from the dirtiest coal fired plant. Our Defiant EV3 Electric Roadster (like all electric cars) is very efficient and our roadster can travel about 40 to 45 miles on about six kilowatt-hours of electricity. It just so happens to take approximately 6 to 8 kilowatt-hours of electricity to refine one gallon of gasoline. (Not including getting the crude oil out of the ground and any transport costs.) A typical gas powered vehicle will go about 25 miles on that one gallon of gas. From that point alone, it is easy to see an electric car is about twice as efficient, much more economical, and pollutes significantly less when compared to a typical gasoline powered car.

  • Jay Donnaway

    As opposed to the base load coal plants that were going to waste overnight an/or squandering power on old sodium street lighting? This study depends on marginal overnight power sources, which assumes a net increase in overnight demand from EV charging. LED exterior lighting plus EV charging can mean no net increase in overnight usage for quite some time into the adoption curve. Let them show that baseload plants are pouring on the coal to make up for increased overnight EV charging loads, rather than piling assumptions on top of assumptions!

  • jstack6

    FACT Power companies offer low Off Peak nights rates because they have excess and can’t turn down fossil fuel power plants or store energy. They can’t even turn down hydro plants or Nuclear. In our are they told me they shunt 70-90 Megawatts each night.

    So if you charge at night Off Peak as 80% of EV drivers do you use the excess waste energy. You also don’t overload the GRID transmission lines or transformers. We have 2 large transformers blow during the day time peak loads. It took weeks to replace and get back on line.

  • Richard Sachen

    Does this put the lie to Clean Coal? If coal can be clean, then the EV using has electricity generated by it has to be clean too…
    Clean Coal not withstanding, if you live west of the Mississippi, you can pretty much ignore these articles. The Western grid uses so much hydroelectric, renewable energy, and natural gas, and so little coal that you can charge an EV any time and come out ahead of any hybrid or gas burner in terms of emissions.
    If you live east of the Mississippi, then just add some solar panels at home or work to balance the electricity used and you’ll clean the air vs gas too. Some 30-40% of EV drivers are already adding solar when they get their EV.

  • tomsax

    We need to stop burning coal. We need to do it globally and we need to do it soon. We need to do that independent of EV adoption.

    As we do that, EVs get cleaner. It’s going to take many years to transition the global vehicle fleet to electricity. We can’t afford to wait to start that transition until the grid is 100% renewable.

    We need to make both transitions in parallel and as quickly as possible.

  • David Sharp

    Power generation aside. Who would want to sit in a room of cars with the engines running vs a room filled with EVs switched on. The answer of which cars are the safest for our future will be obvious.