Should the megajoule replace the kWh as the unit of energy for electric cars? Listen to me

When people use and charge electric cars, they talk about how powerful a charger is 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 EV electricity. The problem is that the general public confuses them a lot, sometimes harmlessly, sometimes leading to errors. It’s not a big deal, but it could be better.

The similar names add to the confusion people have between energy (which is what kWh measures) and power, which is a rate of energy delivery per unit time. In gasoline cars, our unit of energy was the gallon of fuel. When it came to delivering gas, you might wonder how many gallons/minute a gas pump delivered, but most people never really thought about that.

With electric vehicles, the power is quite important. A 1.4kW wall outlet is different from a 7kW EVSE home “charger” and these are very different from fast chargers ranging from 50kW to 350kW. Slower charging works overnight, while fast charging can get you started in 30 minutes, although almost always you try to do something else (like eat or sleep) while charging so it doesn’t take long. time of your day.

Unlike gasoline, where you buy gallons and the price is very similar across town, when it comes to electric power, you may feel like you’re buying power, but most of what you pay is for the service and the key elements of the service. are the power and the location of the load. You can see electrical energy much like water. It’s a commodity, with a price, but you could pay a penny/gallon on tap or $10/gallon for bottled water at a good restaurant. Same hydration when you’re done but very different price. Similarly, although charging is often priced per unit of energy, the cost ranges from “free” (in a surprising number of places, including for customers of thousands of hotels) to 8-25 cents/kWh at home and 25 – 60 cents at pubic chargers, especially the faster ones.

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

It wouldn’t be so interesting if it weren’t for a happy coincidence. In a typical electric sedan, such as the Tesla Model Y, which is the most popular electric vehicle, one megajoule provides about a mile of range. In fact, many EV drivers like to measure their energy in miles – the car displays the battery charge level in miles, and while charging it shows how many miles you’re adding per hour of charging. Of course, that’s not exact – your car gets over a 1 mile per MJ at lower speeds, and less at 80 mph or when going up hills. But it averages very close to 1 mile per MJ. It’s the 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 there in the places that use miles – although trucks and SUVs approach one km per MJ.

If we started using the 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 rather than 70 kWh. And it would be more difficult to confuse power, in kW, and energy, in MJ, but it’s still not that difficult. A kW for 1000 seconds (~17 minutes) is an MJ, just like a kW for 3600 seconds is a kWh. The MJ and the kWh have the same function, but one is bigger, like the foot and the meter.

With this switch, a car’s efficiency could be in mpMJ (like mpg) or the inverted number of MJ/mile which is similar to watt hours/mile. The EPA ‘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, that number makes them look better than they are not.

To make a change like this, automakers and governments — and power companies — would have to coordinate. Which makes the task more difficult. But it would reduce some confusion and produce a unit the meaning and economy of which, being so similar to miles, the public would easily understand. Even where they use km.

To have fun

Not that we ever use it, but it’s fun to note that one megajoule equals 240 food calories. Calories are, like BTUs, a unit of heat, which is energy. You can’t just turn heat into electricity or motive power, but if you could, 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, even if we’re not pushing a car while we’re 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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