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A recent opinion piece, Why the EPA shouldn’t favor school bus electrification, makes the following claim (link in original):

At last check, the average carbon intensity of the U.S. electric grid was approximately 137 grams of CO2 equivalent per megajoule (gCO2eq/MJ). In comparison, conventional propane has a carbon intensity of 79 gCO2eq/MJ [...]

The linked source from the quote includes this image:

Carbon Intensity Values of Certified Pathways, EER-Adjusted

None of the fuels in the graph reach 137, and most of them cover a range of values, so it's not clear where the value of 137 for the grid came from.

Does the U.S. grid actually have a higher carbon intensity than propane?

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    Doesn't that graphic show that electricity has a range of values and a large chunk of it is lower then propane?
    – Joe W
    Commented May 22 at 16:38
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    @MichaelW. Coal isn't used as a direct vehicle fuel, so it won't show up at this metric at all. Just like wind, nuclear, hydro, etc. These values are take into account conversion, storage and transportation costs (which are significant for electricity) and powertrain efficiency (which is good for electricity).
    – user71659
    Commented May 22 at 17:29
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    The values are not absolute values. "The graph below shows the ranges of CI for each type of fuel compared to the two baseline fuels (gasoline and diesel)"
    – Schwern
    Commented May 22 at 21:30
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    The op-ed in question was written by the Propane Education and Research Council, a pro-propane lobbyist organization.
    – gparyani
    Commented May 24 at 17:21
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    Answers on this question are currently having a bias problem; however the source being questioned has the opposite bias problem. Answer should not be what percent of energy on the grid is renewable; but what percent of the next immediate incremental addition of power to the grid would be; because that's what buying the next vehicle by fuel type will be.
    – Joshua
    Commented May 24 at 21:15

3 Answers 3

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tl;dr A full analysis of whether it makes sense to electrify your vehicle fleet depends on fuel cost and carbon intensity in your area and over the lifetime of the vehicle.

It depends on where you're getting your electricity.

Nobody gets "average" electricity. Carbon intensity of electricity generation in the US varies wildly from 0 to 250 g/MJ. An electric anything in Washington will be much greener than in West Virginia.

enter image description here

(100 lbs/MWh = 12.6g/MJ)

If we accept the 79g/MJ (627 lb/MWh) number for propane, then we can see that as of 2020 electricity is equivalent or better in all of the green states. And it will only get better...

Electrification gives you options, and is future proof.

As the US electricity grid continues to get cleaner, electric vehicles (and appliances) will also get cleaner.

enter image description here

New vehicles purchased today will be on the road for decades. Over their lifetime electric vehicles will continue to benefit as electricity gets cleaner, whereas fossil fueled vehicles will remain the same. When considering whether to electrify a fleet of vehicles, one must look not just at the state of things now, but also the state of things in the future. And clearly electricity is getting cleaner.

Electricity can be generated from many sources making it economically elastic. Whereas a fossil fueled vehicle must use that one fossil fuel. Changes in the market and production will impact the cost of running a fossil fueled vehicle to a much greater degree than an electric one.

What about renewable propane?

It is still scaling up. Energy.gov says the US produces 4.5 million gallons per year. Sounds like a lot until you realize the US produces 78 million gallons of conventional propane per day. The future of renewable propane is uncertain, whereas the problems of the US electrical grid are conventional and being solved.

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    @MatthieuM. it is important to note that the question OP links to an article about school buses, not cars. Looking at the top results from a quick search, a current day, presumably ICE, school bus in the US has a life expectancy of anywhere between twelve and twenty years. A school district will consider the TCO of a bus, including maintenance and repairs. As was already noted, electric engines are much simpler to maintain than an ICE, and hopefully whoever orders those busses bakes battery replacement into the contract. TLDR: busses have somewhat different rules than cars.
    – jaskij
    Commented May 23 at 19:22
  • Your 2nd argument ("Electrification gives you options...") is what I was very often thinking about, but couldn't express. Indeed, the proper comparison of carbon emissions should include a forecast.
    – int 21h Glory to Ukraine
    Commented May 24 at 14:57
  • Obviously no one gets "average" electricity, which the article acknowledges: "The cleanliness of the grid varies by state, so it’s important to understand how electricity is being produced regionally." You answer fails to address the claim that propane is better than the grid on average.
    – LShaver
    Commented May 24 at 16:04
  • @LShaver: Still wrong. See comment on question.
    – Joshua
    Commented May 24 at 21:19
  • @LShaver Which "average" are we talking about? Mean, median, or mode? Do we divide by state or by MJ produced? (ie. do we weigh West Virginia the same as California?) And what is notable about being better than "average"?
    – Schwern
    Commented May 28 at 20:25
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TLDR; the article in question is misleading and ignores important specifics which tilt the balance in favour of electricity. The claim about the carbon intensity of propane vs electricity from the grid is true, but because electric motors are far more efficient than propane motors, the carbon intensity of an electric school bus is much lower/better than the carbon intensity of a propane school bus.

The article on UtilityDive is based on a graphic by the "Propane Education and Research Council" which according to Wikipedia is a group advocating for the use of propane.

The Propane Education and Research Council (PERC) is a nonprofit that provides leading propane safety and training programs and invests in research and development of new propane-powered technologies. PERC is operated and funded by the propane industry.

(from Wikipedia, highlight mine)

While the article tries hard to imply that electric school buses are worse for the environment than propane school buses, it does not actually ever state it.

graph by propane advocacy group showing propane is best

This conflict of interest is enough to justify some skepticism and looking at the sources and calculations a bit more in detail. The graph's units give away important information: It's a comparison of CO2 emissions in relation to the primary energy content, ignoring any specifics about the relevant vehicles. This is confirmed by looking up the values from another source, where they roughly match.

The comparison on UtilityDive by PERC is ignoring that electric motors have much higher efficiencies than combustion motors!

Combustion motors have efficiencies like this: efficiency vs rpm for propane and gasoline motors

To refute the claim, take 35% efficiency for propane, which is better than the data actually indicates.

For electric vehicles, let's take the values from here. electric vehicle losses and efficiency overview

80% is a reasonable (pessimistic/bad for EV) number based on this data.

Then the carbon emissions per MJ of energy at the wheel is 130 gCO2eq/MJ / 0.80 = 163 gCO2eq/MJ for electric vehicles and for propane vehicles 79 gCO2eq/MJ / 0.35 = 225 gCO2eq/MJ. So an electric vehicle is more than 30% better than a propane vehicle when it comes to carbon intensity.

Hypothetical widespread renewable propane use could improve the figure for propane - but improvements in renewable electricity production could also massively improve the figure for electric vehicles. So based on energy that is currently available at scale, without speculating about the future, the implied claim about the school buses is wrong.

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    What if the propane was used to power and electric generator instead of a car? I would also say that 95% of my driving is <2000rpm. One the rare occasion I am on the freeway I only reach 3000+ when I am passing and then only for a minute or two. So it seems for normal driving the difference aren't that great. I am not saying we should have propane cars, just that they aren't terrible at normal rpm.
    – cybernard
    Commented May 23 at 14:51
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    @cybernard But this analysis used a flat 35% efficiency - a little above the typical best (i.e., at most efficient RPM) for gasoline and propane. So your driving experience is already taken into account. I also suspect the reason why the peak efficiency of ICE is where it is in terms of RPM is directly related to the normal speed used when driving. Much as large generators that run at a different speed are in turn optimized for fuel efficiency based on the target speed.
    – manassehkatz-Moving 2 Codidact
    Commented May 23 at 15:19
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    @cybernard Electricity generation based on combustion is more efficient than combustion-propelled cars, it's 40% or maybe 50% at most. So the current average electricity mix in the US is already less carbon intensive/better than if it was generated exclusively from propane.
    – Nobody
    Commented May 23 at 19:22
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    @cybernard Efficiency of electric generation using a fossil fuel is dependent on the power cycle used. Typical power plants use boilers to produce superheated steam to drive a turbine. Those hit the 40% range. A combined cycle where the fuel is burned in a gas turbine, and the exhaust gases go through a boiler which sends steam to a steam turbine can hit 50 or 60%. A small internal combustion driven generator probably isn't much, if any, better than a car.
    – Mark
    Commented May 24 at 0:03
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    I'd suspect that the carbon densities of those fuel types is the carbon emissions at combustion, and ignores all other costs.
    – Yakk
    Commented May 24 at 14:13
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The graph is complicated and badly explained; some background information can be found in a PDF by the same agency.

The graph indicates how much CO2 emission is saved by using an alternative fuel instead of gasoline. Negative numbers appear to indicate savings. It is unclear though how hundreds of grams can be saved if a gasoline motor emits only 82 grams.

What appears to be taken into account is the motor efficiency regarding the energy fed into it (electric motors use almost all of the electric energy fed into them, while combustion engines use only around 30% — the rest is wasted as heat in the combustion, which is why conventional motors cannot run without elaborate cooling).

What does not appear to be taken into account are the manufacturing emissions and car mass; famously, producing the batteries releases so much CO2 that a significant part of the lifetime savings are annihilated. Of course, those production numbers depend on the footprint of the energy used for it.

Given the complex subject matter and dynamically changing condititons, I would generally contend that simple statements (from either camp) are almost certainly wrong or misleading.

That said:

  1. The average carbon footprint of electricity in the U.S. is a quickly changing number. The proportion sourced from renewables grows quickly; a Wikipedia chart shows that it has more than doubled in the past 20 years and has now surpassed coal. All public data underestimates the part of renewables because it is missing the last two years or so of this dynamic development.
  2. The switch to an electricity infrastructure is a mid- to long-term process. It must be started even while fossil electricity production is still being phased out. The two developments happen in parallel. In other words: In order to compare the carbon footprint of the two bus systems, one has to estimate the overall footprint over the buses' lifetimes which may well be 20 years. At the 50% mark, in around 10-12 years (if we account for acquisition time) the electricity production will have a smaller carbon footprint already, and the electric buses will be a part of the transition away from burning stuff. Don't get me wrong — I'm not generally opposed to fire. Honestly, it was a great idea when people came up with it, about 2 million years ago.
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  • -1. The quoted section of the article literally says "conventional propane." The article goes on to provide the number for renewable propane, but that's not what the question is about.
    – LShaver
    Commented May 24 at 15:50
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    @LShaver Oh my, good catch. I was sloppy. (In my defense, the renewable was highlighted in the graph, and stands out in its advantage.) The carbon footprint advantage of conventional propane is much less though which makes my other points more valid.
    – Peter - Reinstate Monica
    Commented May 24 at 16:18
  • Virtually all estimates show that battery-electric vehicles surpass ICE vehicles within 2 years when mfg emissions are taken into account. Given that most cars last well past 10 years, I'm not sure how you can claim that "producing the batteries releases so much CO2 that a significant part of the lifetime savings are annihilated". Here is NYT's article, for instance: nytimes.com/2022/10/19/business/…
    – Lawnmower Man
    Commented May 26 at 6:12
  • @LawnmowerMan Two years out of 10 or 15 is still a significant part. And then, yes, what I have in mind may well be 5 years old, and back then the number were much worse. The article also states that in 2% of the counties, electric cars produced more CO2 than gasoline cars; in a much larger number, it will be within, say, 2/3. Given how difficult such life cycle studies are and how many places there are where electricity is really dirty, I stand by my statement. It is well possible that the situation is much better if we apply my own argument about cleaner future electricity though.
    – Peter - Reinstate Monica
    Commented May 26 at 8:18

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