What does MPGe mean for EV?

Originally published on EVANNEX.

By Blane Erwin, Current Automotive

Electric vehicles offer large cost savings over traditional gasoline-burning vehicles. That’s simply because electric vehicles use electricity as fuel instead of gasoline. Electricity is much cheaper than gasoline, making it much cheaper to recharge a car than to refill a car.

A lesser-known fact contributing to the low cost of driving an electric vehicle is their efficient use of energy. That is, how much energy makes it to the wheels versus being lost as wasted heat.

Tesla Model S & Model X. Photo by Zach Shahan | CleanTechnica

Most internal combustion engines operate at less than 40% thermal efficiency. That means that less than 40% of the energy in each gallon of gasoline is actually used to turn the wheels while the rest is wasted in the form of heat. Meanwhile, electric motors can be over 90% efficient. Nearly all of the electricity onboard is going to the wheels, with very little being lost as heat.

There’s an easy way to illustrate what this means. One gallon of gasoline contains approximately 33.7 kWh of energy. The 100 kWh battery pack in a Tesla Model S 100D contains the energy equivalent of less than 3 gallons of gasoline.

While a BMW 530i (24 mpg city/34 highway) could only go 72–102 miles with that same amount energy stored in gasoline, the Tesla Model S 100D will travel over 300 miles – because so much of that energy is being used to drive the car instead of being wasted as heat.

MPGe (Miles Per Gallon Equivalent)

MPGe is the official metric that the EPA uses to measure the efficiency of alternative-fuel (including electric) vehicles. This makes it the most common number you’ll come across when researching an electric vehicle.

Just like regular MPG shows how far a car will travel on one gallon of gas, MPGe shows how far a vehicle will drive on 33.7 kWh of electricity — the energy equivalent of one gallon of gasoline.

Where Can I Find This Information?

The EPA maintains a website, https://www.fueleconomy.gov, where you can look up fuel efficiency and range numbers for any vehicle sold in the United States. You can even bring up multiple cars at one time for easy comparison — making it easy to get an idea just how much you can save by switching to electric.

For Example

The aforementioned Tesla Model S 100D is rated at 102 MPGe, and numbers can fluctuate from lower to higher, like the Jaguar I-Pace’s 76 MPGe, or the Hyundai Ioniq EV’s 136 MPGe.

These efficiency differences can be expected because the I-Pace is a large and powerful all-wheel-drive crossover while the Hyundai Ioniq EV is an economical front-wheel drive car.

Keep in mind that a very efficient vehicle with a high MPGe rating does not necessarily mean that the vehicle will have a higher range — that depends on the battery size.

The I-Pace has a mighty 90 kWh battery pack compared to the Ioniq EV’s modest 28 kWh pack. No surprise — the Jaguar will go 234 miles on a full charge while the Hyundai will go 124 miles.

But an older Tesla Model S 90D, also sporting a 90 kWh battery, will run 294 miles on a full charge — further than the Jag. Why? Because the Model S is rated at 95 MPGe, meaning it uses its electricity more efficiently than the 76 MPGe I-Pace.

When looking for an electric vehicle, it’s important to consider both MPGe and range. A more efficient vehicle will cost you less to recharge, but you need to make sure you’ve got sufficient range as well.

Source: Current Automotive. Note: Current Automotive is the first-ever U.S. car retailer focused exclusively on used electric cars launched by two former Tesla employees.

If you’ve dipped your toe into the electric car conversation, either as a curious geek or a potential EV convert, you’ve probably focused heavily on an electric car’s range. That’s fair; for many, range really is the name of the game. No one wants to pay thousands of dollars for a car that can barely make it to a local coffee shop and back. Yet we ignore the other part of that battle: efficiency. It’s a little hard to parse out how to judge an EV’s efficiency, and while the Environmental Protection Agency — which assesses vehicle fuel economy in the U.S. — has the mpge metric, it can be quite confusing to understand how that works exactly.

Never fear, we'll guide you through the significance of mpge. You know what? Maybe we can pitch a more comprehensible solution, too.

In short, the EPA created a metric written as MPGe (Miles Per Gallon Equivalent, which we write as "mpge" per the AP Stylebook) to measure the efficiency of a car that runs on something other than gasoline or diesel. But, it is a metric that measures said efficiency in comparison to a purely fuel-powered vehicle.

“MPGe conveys the energy consumption of a non-gasoline vehicle in terms of how many miles the vehicle can go using a quantity of fuel with the same energy content as a gallon of gasoline,” an EPA representative told me via email.

It is said that one gallon of gasoline has the energy equivalent of 33.7 kWh of electricity. So, if an EV uses 33.7 kWh to drive 100 miles, its mpg equivalent would be 100 mpg. This can be extrapolated out to other fuels, albeit with a bit of complicated math (sorry, I was an art major for a reason). Basically, you’d need to find out how much fuel is needed to equivocate the potential energy of a gallon of gas, then divide that by how far you can travel on a said metric — thus, you have mpge. This mathematical proportion is useful for not only electric cars, but also PHEVs, compressed natural gas vehicles (CNG), and hydrogen. In other words, anything that doesn’t use gasoline or diesel.

By now you can probably already realize why mpge isn’t as helpful as a straight-mpg number is. 

Miles-per-gallon is an easy metric to understand for most American drivers. It’s a simple metric, that says, per one gallon of fuel, your car should be able to travel this amount of distance. It makes calculating vehicle range easy; for example, if your car achieves 33 mpg in mixed city and highway driving, multiply that over the vehicle’s fuel tank (say, 9.6 gallons), which means you should have a cruising range of about 317 miles. 

But mpge doesn’t always scale that way, especially for electric vehicles. Take, for example, the Ford Mustang Mach-E. In long-range, RWD spec, the EPA says the car will use 33 kWh to travel 100 miles or 100 mpge. When extrapolated out to the vehicle’s 88 kWh battery, the Mach-E should be rated for 260 miles or so. Yet, the car claims 305 miles of range? If you use the same formula, you’ll get similar results with varying deltas between this mpge calculated range, versus what the actual EPA-rated range is.

No, a car company would never lie to you about something like emissions or fuel economy! Yes, that was sarcasm.

The numbers are off because mpge also includes charging losses. It includes the total amount of power the car consumed, including charging losses from the AC/DC converter, onboard charger, and more. Charging losses also include vehicle functions, like keeping the battery at the correct temperature or powering all the complicated computers that make the car function. In short, it sometimes takes a bit more energy to get the same amount of energy, for your battery pack. That means the Mach-E might have taken closer to 100 kWh to put 88 kWh worth of energy in its battery pack. It's akin to calculating a gas car’s fuel consumption while including the fact that you spilled two gallons of gas while refueling.

If you’d like to dig into that even more, you’ll find a handful of scientific papers cited as sources from the EPA’s site linked above.

Actual energy consumption and efficiency when on the move is different than the mpge calculated, including the wasted energy from charging.

Batteries do not have zero cost to the environment. If a cheap car with a small battery can go the same distance as a luxo-barge with a huge battery, that could pay big dividends for both the environment and your pocketbook. If one design is better at minimizing charging losses compared to the other, that means we’re conserving energy, which is good for all of us in the long run. Smaller power bill, less waste.

Personally, I propose we ignore mpge altogether. It’s too hard to make sense of mpge in context. It offers sky-high numbers that look impressive on paper, but mean little to the consumer; who cares if your car gets 100 miles per gallon if you don’t use gas? We don’t measure electricity in gallons, and the relationship to the potential energy of a gallon of gas has little utility to an average driver. Instead, I propose that we emphasize miles per kilowatt-hour in addition to stressing the car’s overall range.

Miles per kWh is simple; it’s easy to parse out on the move, and it scales without needing a huge pen and paper and remembering how much energy a gallon of gas is worth. For example, you know your car is averaging say, 3.3 miles per kilowatt-hour; and there’s about 25 kWh of energy left in the battery pack. Simple math, (3.3 multiplied by 25), shows that you have about 82.5 miles left of range if you keep maintaining that average. See how much easier that is than whatever the heck mpge is supposed to be?

Electric cars can be confusing and intimidating if you’re used to gas, but I promise, they don’t have to be. I hope this helps you be a little more informed when shopping for an EV.

Correction: Monday, April 18, 2022, 10:55 a.m. ET: A previous version of this story incorrectly equated "kilowatt" and "kilowatt-hour." It has been corrected.