This is a follow-up to yesterday’s post regarding the rapid de-carbonization of the electrical grid. First, I’m going to explain why MPGe mis-states the carbon-sparing effects of electric vehicles. And then explain why a grid-connected solar panel array is exactly as “dirty” as the grid it’s connected to.
MPGe: How far can you travel on 33.7 kilowatt-hours of electricity?
Above: Fuel economy for a 2021 Prius Prime (plug-in Prius), from fueleconomy.gov.
Yeah, 33.7, no joke. Just a nice round easy-to-use consumer-friendly number.
First, the “e” in MPGe does not stand for “electricity”. It stands for “equivalent”. If you buy any sort of alternative-fuel vehicle in the U.S. — electric, natural gas, or whatnot — you’ll see the vehicle’s efficiency given in MPGe, or miles per gallon of gasoline-equivalent.
To assure yourself that MPGe has nothing to do with electricity, look up the mileage of the Honda Civic GX (natural gas vehicle). You’ll see that it’s given in MPGe. There’s nothing electrified about that car. That’s just a standard Honda engine, converted to run on natural gas.
They key to understanding MPGe is the circled item on the left of the graphic above. To calculate MPG3e, the EPA:
- Starts with the amount of energy you could get from a gallon of gasoline.
- Converts that amount of energy into whatever fuel the vehicle uses.
- Calculates how far the vehicle can travel on that amount of fuel.
Just to see it in black-and-white, you can look at this reference. Because this is the U.S.A., we of course do all of our energy calculations in British Thermal Units. In any case, if you express the energy in a gallon of gas and a kilowatt-hour in BTUs, then divide, you come up with 33.7, more or less.
In short, the (chemical) energy in a gallon of gasoline is equivalent to the (electrical) energy in 33.7 kilowatt-hours of electricity.
MPGe for an electric vehicle is literally the number of miles the vehicle can travel on 33.7 kilowatt-hours of electricity, under standard EPA test conditions.
Divide MPGe by 33.7 to calculate miles-per-KWH. The Prius Prime above gets about 4 miles per KWH, which, as I explain below, is about as good as it gets in the U.S.A.
Typical MPGe, and the poor electrical mileage of most plug-in hybrids.
If you want to get a feel for typical MPGe for plug-in vehicles, that’s easily done by visiting fueleconomy.gov. Click the link for “find a car”, ask to see alternative fuel vehicles, then selected either fully-electric or partially-electric (plug-in hybrid) vehicles.
Most efficient fully-electric 2021 vehicles
If you look at fully-electric vehicles (EVs), above, you see that many of them get well over 100 MPGe. In fact, the EPA lists 23 different EVs that get 100 MPGe or better. The median of the 40 electric vehicle models listed in the 2021 EPA database is 100.5 MPGe.
Most efficient partially-electric (plug-in hybrid) 2021 vehicles.
But if you look at plugin hybrids electric vehicles (PHEVs), above, you find just five models with MPGe over 100. After that the MPGe rapidly deteriorates. The median of the 52 plug-in hybrids listed in the 2021 EPA database is just 59 MPGe.
To be clear, this isn’t due to to the mix of vehicles. It’s not as if PHEVs are exclusively large cars, and EVs are exclusively small cars. In fact, if you take the EPA mileage database and tabulate by type of vehicle, you find that EVs get much better electrical mileage across-the-board. (You can download the data at this link, but be warned that it takes a while to figure out what’s what.)
I think the answer to “why do plug-in hybrids get such lousy electrical mileage” is that a) they have to carry around a gas engine, and b) most of them weren’t really designed to be electric vehicles in the first place. The electrical components were designed to allow the car to operate as a hybrid. In most cases, they weren’t designed to be a stand-alone battery-fed electrical drive train. And everyone except Toyota and Ford has to design around the patents on the most successful drive train (which is one that Toyota and Ford use.)
In that context, the Prius Prime’s 133 MPGe really stands out. There are only two pure electric vehicles that get better electric mileage than the Prius Prime.
In fact, as I’ll show below, a lot of plug-in hybrids actually produce more C02 per mile, on electricity, than a Prius does, on gasoline. The mere fact of electric propulsion does not necessarily translate into the lowest possible carbon footprint.
MPGe may or may not be a hugely misleading figure.
MPGe may or may not be a misleading figure if you are interested in the actual relative efficiency or relative carbon footprints of electric vehicles, compared to gasoline-powered vehicles. It’s misleading because it ignores the fossil fuels burned to generate that electricity in the first place.
But if you think about it for a bit, the EPA couldn’t issue a number that actually showed the carbon footprint. That’s going to depend on how carbon-intensive the electricity is in your area. If you charge your EV in a state with coal-fired electricity, plausibly, you actually increase your carbon footprint by traveling on electricity rather than gasoline. By contrast, most of the electrical generation of the Pacific Northwest is hydro, and there, you’d be hard-pressed to find any electrical transport that’s inefficient enough to raise your carbon footprint relative to gasoline-fired transport.
The upshot is that if you want to understand the carbon footprint of your electrical transport, you have to know both the MPGe for the vehicle, and the pounds of C02 per KWH for your local electrical grid. You can find that second figure on this US EIA web page. Take the pounds of CO2 per megawatt-hour and divide by 1000. For Virginia, for the most recent year, that yields 0.681 pounds of C02 emitted per KWH of electricity consumed.
Source: U.S. Energy Information Agency, data shown for Virginia.
The only other bit of information you need to know is how many pounds of C02 you generate when you burn a gallon of gasoline. You’ll see slightly varying numbers, because the actual chemical formulation of gasoline varies. Just to pick one, the fueleconomy,gov website says that a gallon of gas generates a nice, round 20 pounds of C02. (I’ve seen numbers ranging from 19 to 21.5).
Now we can do the calculation. A Prius Prime gets 54 MPG on gasoline, and 133 MPGe on electricity. In Virginia, how much lower is your carbon footprint if you run that car on electricity rather than gas? Let’s work that out in terms of pounds of C02 per 100 miles.
- Gas: 100/54 gallons to go 100 miles, x 20 lbs C02/gallon = 37 lbs C02.
- Electric: 100/133 x 33.7 KWH to go 100 miles, x 0.68 lbs C02/KWH = 17 lbs C02
Using the battery in the Prius Prime reduces your carbon footprint by 54%, relative to using the gas engine. At least, that’s what happens here in Virginia. YMMV depending on how carbon-intensive your local electrical generation is. (That 54% reduction is not quite as good as the ratio for my 2005 Prius, but that’s because my gas mileage isn’t as good as the 2021 Prius.)
Note that the ratio of MPG to MPGe modestly exaggerates the true difference in carbon footprint. In Virginia, the MPG/MPGe ratio of (54/133 = ) .40 is lower than the actual ratio of carbon footprints of (17/37 =) .46. Here, the MPG/MPGe ratio makes the electrical side of the car look just slightly too good.
The fact that these two sets of numbers — MPG/MPGe and carbon footprint — are even remotely close to one another is purely a matter of chance. If I redo this calculation for coal-intensive West Virginia, where each KWH generates 1.96 pounds of C02, I find that running the Prius Prime on electricity actually generates 34% more C02 than running it on gasoline.
The upshot is that you can’t look at the EPA sticker showing MPG and MPGe and have any understanding of the relative benefits of electrical versus gasoline transport in a car. You have to understand how carbon-intensive your local grid is in order to get a handle on that.
But there’s one final question. Let’s say I’m in Virginia. How low can the MPGe go before I actually increase my carbon footprint, relative to just driving a gasoline-powered Prius? That’s a reasonably straightforward calculation, just requiring a bit of convoluted algebra.
And the answer is 62 MPGe. In Virginia, today, running any electric vehicle or plug-in hybrid that gets less than 62 MPGe results in more C02 emissions than just driving a standard gasoline-driven Prius.
Are there any? Sure. Look at the MPGe table above. The average large car or standard SUV, configured as a plug-in, gets well under 62 MPGe. And so, there is no free lunch. The mere fact of electrical transport does not automatically make for a lower carbon footprint.
But I have a grid-connected solar array, so my electricity is carbon-free!
Nope. No it’s not. Electricity in a home with a grid-connected solar array is exactly as dirty as the grid it’s connected to.
This is probably the single most widely-misunderstood aspect of home solar power. When you install a grid-connected photovoltaic system, you make the entire grid in your area slightly cleaner. You get full credit for reducing society’s carbon footprint. Your local grid is just that much cleaner for your having installed that grid-connected array.
But you, personally, use the same electrons as everybody else on your grid. And every KWH you use results in creating the average C02 emissions per KWH of your local grid.
Why? Simply put, the KWH you don’t use, from your grid-connected solar array, goes out onto the grid and displaces a KWH that your local power company would otherwise have had to generate. Every KWH you don’t use reduces total C02 emissions by avoiding a KWH generated by your local utility company.
That’s the part that every solar array owner agrees with. By putting their excess electricity onto the grid, they are helping to reduce C02 emissions and global warming. I doubt you’d ever see a solar array owner disagree with that.
But as a matter of arithmetic, every KWH that you use increases total C02 emissions by failing to avoid a KWH generated by your local utility company.
The only way I ever “got” this was to put pencil to paper, and consider what happens when you charge up your plug-in hybrid. Take one KWH of solar-generated electricity and track what happens to total emissions if you use that to charge your PHEV.
If you use that KWH to charge the battery in your car, one way or the other, your local utility has to generate one more KWH of power. That’s either one KWH supplied to your home (if your array does not produce enough power to meet all your electrical needs), or one KWH supplied to others (if you have surplus power and are feeding electricity back into the grid.)
And so, by charging your car battery with that KWH, you cause your local utility to have to generate one more KWH. And so, to emit the C02 associated with that additional generation.
And if you choose not to put that KWH into your car battery, then the same calculation holds true, but in reverse. One way or the other, your local utility has to generate one fewer KWH of electricity. That’s either one fewer KWH supplied to your home (if your array does not cover all your electrical needs), or one fewer supplied to others (if you are putting surplus power on the grid).
The upshot is that the KWH that goes into your car battery, from your grid-connected solar array, has exactly the same carbon footprint as your local grid. It is, in fact, exactly as dirty as the average KWH on your local grid. It results in the same C02 release as the average KWH on your local grid.
Installing a grid-connected solar array has a profound environmental benefit for society. And, the dirtier the grid you are connected to, the larger the environmental impact the home solar array has.
But the owner of that solar array does not consume that benefit personally. One way or the other, the benefit is spread out across the grid the array is connected to, and manifests in terms of KWH that don’t need to be generated with fossil fuel. And the upshot of that is that every KWH consumed by the solar array owner generates exactly as much additional C02 as every other KWH consumed on that grid.
