by John DeCicco
Comparing apples-to-apples based on the gasoline vs. electric Mini Cooper
In recent months, sky-high mile-per-gallon ratings have been bandied about for some upcoming electric vehicles. Skeptics point out that swapping oil-based gasoline for coal-based electricity just moves pollution around, without reducing it much if at all.
For example, General Motors recently claimed a 230 mpg rating for Chevy Volt plug-in hybrid, prompting Nissan to twitter that their Leaf EV would rate 367 mpg. The blogosphere is cluttered with off-the-cuff retorts such as the Environmental Economics experts, who asked, “What is the EPA mppc (miles per pound of coal) for the Volt when running off the electric engine?”
Indeed, half of US electricity still comes from coal. That’s an improvement from decades past, when coal’s share of power generation was near 60 percent. Some states such as California, a long-time booster of cars that can run on electricity and other alt fuels, have cleaner power than average. Nonetheless, per unit of energy delivered to a car’s tank or battery, even California electricity entails more greenhouse gas emissions than petroleum fuel. On a national average, BTU-per-BTU basis, electricity is 1.7 times as planet-polluting as gasoline.
But that’s only half the story. As any EV buff will righteously—and rightly—assert, electric drive is far more efficient than an internal combustion engine.
Doing the Math
Perhaps the best apple-to-apples comparison is provided by the Mini Cooper electric and gasoline stable mates. For the Mini E, the official EPA ratings used on the car’s window sticker are 33 kilowatt hours (kWh) per 100 miles in the city and 36 on the highway, for a combined average of 34.4 kWh per 100 miles. (Note that, unlike mileage numbers, these are consumption numbers, with a lower number corresponding to higher efficiency. So like many electric-drive products, the Mini E is more efficient in the city.)
Under average US conditions, replacing a gasoline mile with an electric mile cuts global warming pollution in half.
Now, on a straight-up, energy-equivalency basis, a gallon of gasoline in the tank is equal to 33.7 kWh of electricity. That’s the number EPA uses in a footnote on an EV’s window sticker and it can be derived by anyone from a set of basic energy conversion factors. Energy-equivalency is the most straightforward way to “do the math” scientifically speaking. Of course, that’s before getting into the economic questions, which matter for pocketbook reasons but which we’ll leave for another day.
Divide 33.7 kWh per gallon by 34.4 kWh per 100 miles and, voilá, you get 98 miles per gallon, the gasoline energy-equivalent rating of the Mini E.
How does that compare to the gasoline-powered Mini Cooper Hardtop? The base model makes for the best comparison with the Mini E, or at least as close as you can get without worrying about where the back seat went. The basic Mini Cooper Hardtop rates 28 mpg city and 37 highway, for a combined average of 32 mpg. Well, 98 is roughly 3.1 times 32, or in other words, the Mini E is 3.1 times more energy-efficient than its closest gasoline counterpart.
CO2 Per Mile
Thus, the electric car’s efficiency gain more than makes up for the higher per-unit greenhouse gas emissions of electricity. Running the numbers for CO2-equivalent per distance driven, the Mini Cooper Hardtop using gasoline results in 357 grams of CO2 per mile. The Mini E results in 196 grams per mile, or 45 percent less. That’s pretty close to the rule-of-thumb I’ve long used, that under average US conditions, replacing a gasoline mile with an electric mile cuts global warming pollution in half.
California electricity is about 22 percent cleaner than the national average, so running that Mini E in the Golden State would result in a 57 percent cut CO2 per mile. And what if you charge your Mini E by mainlining coal-by-wire right into your battery pack? Using emissions factor for coal power from ACEEE’s Green Book methodology, the electric version is still a tiny bit cleaner than gasoline, 355 grams of CO2 per mile.
That’s probably a statistical tie, and no doubt there’s room for endless discussion about how much better or worse you can do depending on when and where you charge. One could also get into whether your gasoline comes from Canadian tar sands or Venezuelan heavy crude, instead of “average” crude oil (whatever that is). So while you might not be that much worse a climate culprit if for some strange reason you plug your car into coal, if you’re going to go through that much trouble, you should look for nuclear power—oops, did I mean to say solar panels?—instead.
Contributed by John DeCicco, senior lecturer at the University of Michigan School of Natural Resources and Environment.