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Researchers find Nissan LEAF creates less CO2 than Toyota Prius hybrid in west US and Texas, but more in N. Midwest

1 July 2015

Regionally specific lifecycle CO2 emissions per mile traveled for plug-in hybrid electric vehicles (PHEVs) and battery electric vehicles (BEVs) in the US can vary widely based on grid emission factors (i.e., the “carbon footprint” of electricity production and use), according to a new study by researchers at Carnegie Mellon University. Under some conditions, the battery electric Nissan LEAF can produce higher emissions than a Toyota Prius hybrid. The paper is published in the ACS journal Environmental Science & Technology.

The team characterized the vehicle emissions across the United States under alternative assumptions for regional electricity emission factors, regional boundaries, and charging schemes. Among the findings were that:

  1. delayed charging (i.e., starting at midnight) leads to higher emissions in most cases due largely to increased coal in the marginal generation mix at night;

  2. the Chevrolet Volt has higher expected life cycle emissions than the Toyota Prius hybrid electric vehicle (the most efficient US gasoline vehicle) across the US in nearly all scenarios (a breakout of the findings re: the Volt is available in the Supplemental Information for the paper);

  3. the Nissan LEAF BEV has lower life cycle emissions than the Prius in the western US and in Texas, but the Prius has lower emissions in the northern Midwest regardless of assumed charging scheme and marginal emissions estimation method; and

  4. in other regions the lowest emitting vehicle depends on charge timing and emission factor estimation assumptions.

Probability that the Nissan LEAF is lower CO2 emitting than the Toyota Prius Hybrid by region and charging scheme. Green indicates that the Nissan Leaf is lower emitting than the gasoline vehicle (Toyota Prius Hybrid or sales-weighted ICEV), while red means that the opposite holds. Credit: ACS, Tamayao et al. Click to enlarge.

Most studies indicate that the key factor when comparing PEVs and gasoline vehicles is the magnitude of emissions associated with electricity production. However, many of these studies rely on a single electricity production emission factor estimate or conduct sensitivity analyses on grid emission factors over a range of power plant types. A more detailed assessment is needed to estimate regionally specific emissions from those power plants that respond to PEV charging. … In particular, variation in grid emission factors and regional boundaries are key drivers of differences in the estimates of regional PEV benefits.

… to properly assess the CO2 emissions implications of adding new PEV charging demand in a particular region, one should estimate and use marginal consumption emission factors. … In this work, we assess regional variation in electric and conventional vehicle CO2 emissions under a range of assumptions for regional boundaries of analysis, electricity emission factors, and charging patterns.

—Tamayao et al.

For the study, the researchers used the Nissan LEAF and Chevrolet Volt as representative of battery-electric and plug-in hybrid electric vehicles, respectively. They compared these vehicles with the Toyota Prius HEV. They also assumed lifetime vehicle miles traveled ranging from 100k to 150k miles with a best estimate of 125k miles used for the base case analysis.

Given substantial regional differences in PEV GHG emissions implications, differential regional policy may be warranted, but current differences in state subsidies do not align particularly well with regions where PEVs provide the largest GHG emissions benefits. For example, the state with the largest state subsidies ($7500) for BEVs is West Virginia, which is under the RFC region, where the Nissan LEAF and the Chevy Volt are likely higher emitting than the Toyota Prius. Under the Clean Power Plan Proposal, West Virginia is expected to bring down its carbon rate to about 730 kg/MWh, but that level is not yet low enough for PEVs to be lower emitting than the Prius, and the effect of average emissions reductions on marginal emissions has not yet been characterized. The second highest state subsidies ($6000) are in Colorado, part of the WECC region where the LEAF is likely lower emitting than the Prius and the Volt may be higher or lower. The third highest state subsidies ($5000) are in Georgia, where the comparison of the Leaf and Prius is inconclusive and the Volt is higher emitting. Of course, GHG benefits must be balanced against other goals, including reduction of air pollution and oil dependency as well as economic factors.

—Tamayao et al.

The researchers offer policymakers a number of recommendations based on the study’s findings:

  1. Be wary of regional claims about electric vehicle air emissions implications based only on regional electricity generation mix, since the emissions associated with new PEV charging in a region can differ substantially from the average generation mix in that region.

  2. Consider federal and regional strategies for promoting electric vehicle adoption most strongly in the regions where they can do the most good. (Earlier post.) When considering GHG reductions alone, this would mean the western US and Texas (where there is high confidence that GHG emissions of the Nissan LEAF are lower than the best gasoline vehicles) and in Florida and New England (where the LEAF also likely has lower GHG emissions). However, other factors beyond the scope of this analysis, such as air quality implications, should be considered as well.

  3. Continue to reduce the emissions intensity of the electricity grid. When electricity generation is sufficiently clean, electric vehicles have lower GHG emissions than the most efficient gasoline vehicles.

  4. Avoid treating PEVs as though they are all the same. While the Nissan LEAF has lower GHG emissions than the gasoline Toyota Prius in several regions, the Chevy Volt has higher GHG emissions than the Toyota Prius across much of the US. Policies that target outcomes (e.g., GHG emissions reduction) rather than specific technologies are generally preferred.

  5. Incentivizing nighttime charging should be avoided: while night charging can be preferred by grid operators and can lower costs, in most regions nighttime charging increases GHG emissions, and nighttime charging can also increase health costs in some regions due primarily to increased air pollution from coal-fired power plants.


  • Mili-Ann M. Tamayao, Jeremy J. Michalek, Chris Hendrickson, and Inês M. L. Azevedo (2015) “Regional Variability and Uncertainty of Electric Vehicle Life Cycle CO2 Emissions across the United States” Environmental Science & Technology doi: 10.1021/acs.est.5b00815

July 1, 2015 in Electric (Battery), Emissions, Hybrids, Lifecycle analysis, Policy | Permalink | Comments (11)


Many USA's electricity generating facilities will have to clean up their act? One of he best way to manage it may be with a progressive carbon tax of $10/tonne/year for the next 10 to 15 years or so?

This is all very sensible stuff.
Co2/KwH is variable from country - country and time to time.
This is all fairly predictable except for wind where the Co2 rating will change based on local wind energy from hour to hour.
Nontheless, it is predictable in a 48 hour timeframe which might help people time their charging strategy if they have a "2 day battery".
(in Ireland, eirgrid publish wind energy predictions
which could help people time charging.

Thus, say you had a leaf and could charge it in 4 hours, you could at least decide to charge it for the first 4 or the last 4 hours of the night time period.

The problem (as stated in the article) is that the generators want to sell excess electricity at night, which is baseload and often coal, while for lowest CO2, other times of day might be a better charge time.

What you need is a "low co2" charge plan which gives the lowest rates when the CO2 (and demand) are lowest, and which can be moved around with the vehicle (thus works at home (in the evenings) and at work (in the afternoons)[ when you might have a lot of solar on the grid].

But basically the observations are correct: CO2 intensity varies from time to time and from place to place and charge plans and models need to reflect this.

One sure way to lower CO2 and pollution would be to accelerate the replacement of CPPs with REs and/or NPPs where environmentally and economically acceptable.

Interesting to see the status as it is today, but no more than that. (Actually, I didn't read the article... Too long)

Who here does not believe fully that the CO2/kWh is going to come down in the future? I mean, it has to! All over Europe ambitious targets have been set. Denmark has already met its 2020 goal in the first half of 2015 due to strong wind, and more wind power is slated to be installed before 2020.

As car buyer you are doing your part of the job by going electrical as much as possible, and then the power utilities have to do their job. Only by electrifying our vehicles can we realistically make serious cuts in CO2 emissions.

Just at note about coal power: Coal fired power plants need not make any pollution, and not even CO2 emission. I should know, because I work in the emissions control dept. of a engineering consulting company that does utility size power plants. And my specialty is carbon capture. Now, if the world never embraces carbon capture and storage/sequestration/EOR, then fine by me; as long as we do something else that works to reduce all kind of emissions from our industries, whether power or otherwise.

Elon Musk seems to have cracked the equation with dedicated EVs and solar - a combination that works very well in most of the world. Add some Gen IV nuclear (thorium, please) as base load, and we're home free!

They left out the most powerful CO2 reduction scheme. E85 fueled hybrid. The math would take a dive to hybrid technology overall. Also, consider the Cummings ultra low carbon project and results for E85 engine upon medium duty van. The engine could have been improved more with intro of cooled EGR gas and increased design strength, yet achieved benchmark in present design state. A modification of off the shelf diesel with spark ignition. Regular low cost pressure fuel injection o.k.. Engine was half the weight and half the size of typical diesel and still produced superior diesel torque and superior Hp similar to gas engine. A new transmission required to utilize new torque curve. The engine beat gasoline MPG in all field tests except low Hp. If the engine had cooled EGR, this may not be the case. Upon production and use of cellulosic ultra low carbon ethanol fuel and optimized ethanol engine, well, consumers would continue to enjoy typical refueling practices, no expensive infrastructure burden, similar driving range, and lower cost fuel. The ethanol technology is on a steep curve of carbon reduction that would provide more environmental benefits upon typical lower cost vehicle sale. I doubt if ever the grid could catch up with this solution. The vehicle is easily recycled, a plus. Consider the impact of ultra low cost car, extremely high MPG that would become popular with those whom would make the switch from used car to new car i.e. Elio's 84 MPG costing $6,800 utilizing the most common low cost parts upon automotive technology? What if this car ran on cellulosic E85?

Well, I am in the "Upper Midwest" (N. IL / S. WI region). Sometimes the power is from coal, sometimes from wind or even nuclear. It's predominantly coal, although the facilities are fairly modern and a high grade of fuel is used.

Nonetheless, I have decided to make a very adamant and public statement about my choice to drive the Leaf over the past 20 months or so.

Sometimes, when I'm in a raucous mood, I select "D" rather than "Eco" and raise the radio volume up really high (usually with some Scorpions or Man o' War blasting). Targeting SUVs, I pass them at high speed and cut into the lane right in front of them, all the while POURING electrons out of the battery and into the inverter and motor. I call it "Lepton Rolling", and it's driving the Suburban and Escalade moms NUTS. I'll post youtube videos soon.

@Herman - LOL!

I think the main takeaway is that we need to accelerate decarbonization of the grid. With solar and storage pricing continuing to drop, it could start to happen very quickly. Just look at the recent news of solar costs in Austin, TX - less than 4c/kWh.

Yes...decarbonizing the grid in USA, China and India (and many other countries) + accelerated introduction of BEVs and FCEVs would:

1) progressively put the 80+% of the Oil & 95% of the coal industries out of business.
2) reduce GHG and pollution by up to -80% in the 3 major polluting countries.
3) create new industries to replace ++ all jobs cut in Oil and Coal industries.
4) capture the CO2 from NGPPs and use it to create useful chemicals.

Either Prius or Leaf reduce oil consumption and that is what matters to me. They both work. If I had to choose between just these two models it wouldn't be on CO2 considerations. It would be range needed, whether or not I could home charge, etc....

Regarding: "... in most regions nighttime charging increases GHG emissions, and nighttime charging can also increase health costs in some regions due primarily to increased air pollution from coal-fired power plants."

While it might seem that answering the question, "Where did these 2 AM electrons come from that charged my Nissan Leaf?" would be important, that assumes that a coal power plant is (figuratively) throwing another shovelful of coal on the fire when charging starts. I'd understood that the great majority of US coal plants are operated in a continuous baseload mode, and don't attempt to be load-following plants. Especially since the US (shamefully) has no nationwide carbon tax, it makes economic sense for the utility to run these plants continuously at near full capacity, and the design itself of the typical coal plant doesn't facilitate load-following.

Isn't a much more relevant question:
1) "If 1000 car buyers in my area chose a Leaf over an HEV Prius, what would the difference in CO2 emissions be?

Since (as I understand it) the great majority of the increased-load generation is provided by peaking natural gas plants or renewables (or mid-merit gas plants) then late-night charging would not seem to meaningfully contribute to greater CO2.

Two further questions might be:

2) How will the emissions balance likely change in future?
In Q1 of 2015, 75% of new US generation capacity was in renewables. (Will this trend continue?)

3) Did the study consider the substantial emissions costs in refining gasoline? I confess I did not pay to buy the original study. I know they did mention that other pollutants are at play besides CO2, but I don't know if refinery and distribution CO2 costs were included in their CO2 comparison.

Of course, the Leaf/Prius dichotomy was necessary simplification. A real-world new-car buyer who might be persuaded to buy a Leaf because of the battery EV credits might well have chosen, absent those credits, something other than a HEV Prius, and that probability-spread would have included dirtier vehicles. Maybe an absolutely rational credits policy would have rewarded both Leaf and Prius buyers to some extent, but isn't the real policy question "Did my state get a reasonable environmental bang for my taxpayer bucks?" rather than "Does a Leaf always result in lower carbon than any other choice?"

As to the late-night charging question, I'm only a layman. Are my assertions and logic reasonable?


Load following plants.

New generation mix:

Unfortunately, the way that this story is playing out in the media is that Electric cars may not be that clean to drive after all... The emphasis is on the wrong syLAble.
What is really the root of this story is that if you have Coal generated electricity, you pollute so much that even electric cars can't save the old technology. Instead the stories should stop trying to spin the story to say the opposite of what the data says:
Coal generated electricity is what needs to be cleaned up or phased out instead of implying that electric cars are not really as clean as we thought...

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