Transportation

Should The Megajoule Replace The kWh As Our Unit Of Electric Car Energy? Hear Me Out


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

kilowatts (kW) and kilowatt-hours (kWh) are the standard units used by people working with EV electricity. The problem is that the general public gets them confused a lot, sometimes harmlessly, sometimes leading to mistakes. It’s not a huge problem, but it could be better.

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

With EVs, power is quite important. A 1.4kW wall plug is different from a 7kW home EVSE “charger” and those are very different from fast chargers ranging from 50kW to 350kW. Slower charging works overnight, while fast charging can get you going in 30 minutes, though almost always you try to be doing something else (like eat or sleep) while charging so that it takes no time from your day.

Unlike gasoline, where you buy gallons and the price is very similar all over town, when it comes to electrical energy, you may feel you are buying energy, 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 charging. You can view electrical energy 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 very different price. Likewise, though charging is often priced per unit of energy, the cost ranges from “free” (in a surprising number of places, including to guests at thousands of hotels) to 8-25 cents/kWh at home to 25 to 60 cents at pubic chargers, especially fast ones.

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While people confuse the kilowatt and 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-second, in contrast to the watt-hour or kilowatt-hour. To be strict, the watt is actually defined as “a joule per second” and calling the joule a watt-second (or calling 3.6 million joules a kilowatt-hour) has it backwards. For EVs, the interesting unit is the megajoule, or MJ. And one kWh is 3.6 MJ.

This would not be that interesting if not for a happy coincidence. In a typical EV sedan, such as the Tesla Model Y which is the most popular EV, one megajoule happens to deliver about one mile of range. In fact, many EV drivers like to actually measure their energy in miles — the car displays how full the battery is in miles, and while charging it reports how many miles you are adding per hour of charging. Of course, it’s not exact — your car gets more than one 1 mile per MJ at lower speeds, and less at 80mph or climbing hills. But it averages to very close to 1 mile per MJ. That’s cars — trucks and SUVs are not as efficient, and the Ford F150 lightning only gets 0.6 miles per MJ (mpMJ) And, of course, this coincidence isn’t here in places that use kilometers — though the trucks and SUVs get close to one km per MJ.

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

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With this switch, efficiency of a car could be in mpMJ (like mpg) or the reversed number 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 a change like this, automakers and governments — and power companies — would have to coordinate. Which makes it harder to do. But it would cut back on some confusion and produce a unit that, being so similar to miles, the public would readily understand the meaning and economics of. Even where they use kM.

For amusement

Not that we would ever use it, but it’s fun to note that a megajoule is equal to 240 food Calories. Calories are, like BTUs, a unit of heat, which is energy. You can’t just turn heat into electricity or motor power, but if you could, it’s fun to see that the MJ is similar to the energy in a glass of juice or many common small snacks. Humans are pretty efficient — we only need around 100 Calories of food to go a mile on foot, though we’re not pushing a car while we do it. Scooters and eBikes 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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