Should the Megajoule replace the kWh as the unit of electric car energy? listen to me

When people use and charge electric cars, they will talk about the power of a charger and how many kWh they put or can put into a battery. They also talk about miles of range and their battery’s “State of Charge” percentage and a few other things.

kilowatts (kW) and kilowatt-hours (kWh) are the standard units used by people who work with electricity. The problem is that the general public confuses them a lot, sometimes harmlessly, sometimes leading to mistakes. Not a big problem, but could be better.

The similar names add to the confusion people have between energy (which is what the kWh measures) and power, which is a rate of energy delivery per unit of time. In gasoline cars, our unit of energy was the gallon of fuel. When it comes to gas delivery, you could ask about the number of gallons/minute a gas pump delivers, but most people never give it much thought.

With EVs, power is very important. A 1.4 kW wall plug is different from a 7 kW EVSE home “charger” and very different from fast chargers ranging from 50 kW to 350 kW. Slower charging works overnight, while fast charging can get you up and running in 30 minutes, although you’re almost always trying to do something else (like eat or sleep) while charging so it doesn’t take time from your day.

Unlike gasoline, where you buy gallons and the price is similar across town, when it comes to electricity, you may feel like you’re buying energy, but most of what you’re paying for is for the service, and the basics of the service are the power and location of the charge. You can see electricity a bit like water. It’s a commodity, with a price, but you might pay a penny/gallon from the tap or $10/gallon for bottled water at a nice restaurant. Same hydration when you’re done, but a very different price. Likewise, although the charge often varies per unit of energy, the cost ranges from “free” (in a surprising number of places, including guests in thousands of hotels) to 8-25 cents/kWh at home to 25-60 cents at pubic chargers , especially the fast ones.

While people confuse the kilowatt and the kilowatt-hour, it turns out that outside of electricity, the rest of science and engineering measures energy with a different unit, the joule. The joule is the watt-hour, as opposed to the watt-hour or kilowatt-hour. To be strict, the watt is actually defined as “one joule per second”, and calling the joule a watt-second (or 3.6 million joules per kilowatt-hour) has it backwards. For EVs, the unit of interest is the megajoule or MJ. And one kWh is 3.6 MJ.

This would not be so interesting if not for a happy coincidence. In a typical sedan EV, like the Tesla Model Y which is the most popular EV, one megajoule happens to provide about a mile of range. In fact, many EV drivers like to actually measure their energy in miles—the car shows how full the battery is in miles, and when charging it tells you how many miles you’re adding per hour of charge. Of course, it’s not accurate — your car does more than 1 mile per MJ at lower speeds and less at 80 mph or on hills. But on average it is very close to 1 mile per MJ. These are cars — trucks and SUVs aren’t as efficient, and the Ford F150 lightning only gets 0.6 miles per MJ (mpMJ) And, of course, that coincidence isn’t here in places that use mileage — although trucks and SUVs are approaching one kilometer per MJ.

If we started using MJ, we would start talking about the price of electricity in cents per MJ — which for these sedans would be the price per mile. You would think your battery has 250 MJ instead of 70 kWh. And it would be harder to get confused between power, in kW, and energy, in MJ, though not that hard. A kW for 1000 seconds (~17 minutes) is MJ, as a kW for 3600 seconds is kWh. MJ and kWh are identical in function, but one is larger, like the foot and meter.

With this switch, a car’s performance could be in mpMJ (like mpg) or the inverse of MJ/mile which is similar to watt-hours/mile. The EPA’s “mpge” where they use a “gallon equivalent” of 33.7 kWh per gallon (or 121 MJ) is quite misleading, and while EVs are very efficient, this number makes them look better than they are.

To make such a change, automakers and governments — and energy companies — will have to coordinate. Which makes it harder. But it would reduce some confusion and produce a unit that, being so similar to miles, the public would easily understand the meaning and economy. Even where they use kM.

For fun

Not that we’d ever use it, but it’s fun to note that one megajoule equals 240 calories of food. Calories are, like BTUs, a unit of heat, which is energy. You can’t just convert heat into electricity or engine power, but if you can, it’s fun to see that MJ is similar to the energy in a glass of juice or many common small snacks. Humans are pretty efficient — we only need about 100 calories of food to walk a mile, although we’re not pushing a car while doing it. Scooters and e-bikes are actually more efficient at moving people than our muscles. On the other hand, there are 121 MJ of heat energy in a gallon of gasoline, or about 29,000 calories.

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