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

When people use their electric vehicles to charge, they talk about the wattage of the charger and the kWh it can or can put into the battery. They also talk about range and battery “state of charge” percentages, as well as a few other things.

Kilowatts (kW) and kilowatt hours (kWh) are standard units used by people working with EV electricity. The problem is that the general public confuses them, sometimes harmlessly and sometimes leading to mistakes.

Similar names confuse people between energy (measured in kWh) and power (rate of energy delivered per unit of time). In gasoline cars, the unit of energy was the gallon of fuel. When it comes to gas delivery, you can ask how many gallons a gas pump delivers per minute, but most people haven’t given it much thought.

For EVs, power is very important. A 1.4kW wall outlet is very different from a 7kW home EVSE “charger” and a fast charger in the 50kW to 350kW range. You can charge in minutes, but most of the time you try to do something else (like eat or sleep) while charging so it doesn’t take much of your day.

Unlike gasoline where you buy a gallon and prices are very similar all over town when it comes to electrical energy it may feel like you are buying energy but most of what you pay is service and service critical element. power and charging station. Electrical energy looks 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. Prices vary greatly. Similarly, charging is often priced per unit of energy, but its cost ranges from “free” (in a surprising number of places, including thousands of hotel guests), to 8 at home. ~25 cents/kWh to 25-60 cents at home. Pubic chargers, especially fast ones.

People confuse kilowatts with kilowatt-hours, but it turns out that non-electrical science and engineering measure energy in a different unit, the joule. Joules are Watt-seconds, as opposed to Watt-hours or Kilowatt-hours. Strictly speaking, a watt is actually defined as “1 joule per second”, the opposite of calling a joule a watt-second (or 3.6 million joules a kilowatt-hour). For EV, the unit of interest is the megajoule (MJ). And 1kWh is 3.6MJ.

This wouldn’t be all that interesting if it weren’t for the happy coincidence. A typical EV sedan, such as the most popular EV, the Tesla Model Y, achieves a range of approximately 1 mile at 1 megajoule. In fact, many EV drivers actually prefer to measure energy in miles. The car shows how many miles the battery has left, and while charging, he reports how many miles have been added in an hour of charging. Of course this is not accurate. Your car will go over 1 mile per MJ at low speeds and less at 80 mph or uphill. But it averages very close to 1 mile per MJ. It’s a car — trucks and SUVs aren’t as efficient, and the Ford F150 Lightning only gets 0.6 miles per MJ (mpMJ). 1 km per MJ.

If you start using MJ, you will start talking about your electricity bill in cents per MJ. This is the price per mile for these sedans. You can think of the battery as 250 MJ instead of 70 kWh. Also, it’s hard to confuse power in kW with energy in MJ, but it’s still not that hard. kW for 1000 seconds (~17 minutes) is MJ and kW for 3600 seconds is kWh. MJ and kWh are functionally the same, but one is bigger, like feet and meters.

With this switch, a car’s efficiency can be expressed in mpMJ (such as mpg) or the reciprocal of MJ/mile, which is similar to Watt-hour/mile. EPA’s “mpge” which uses a “gallon equivalent” of 33.7 kWh (or 121 MJ) per gallon is pretty misleading, EVs are very efficient, but this number looks better than it actually is. Become.

Automakers, governments and power companies must work together to make this change happen. This makes it difficult to run. However, it would reduce confusion and produce a unit that is so similar to the mile that ordinary people can easily understand the meaning and economics of the mile.Even where they use kM.

for amusement

Never use, but note that 1 megajoule equals 240 food calories. A calorie, like BTU, is a unit of heat that is energy. You can’t turn heat into electricity or motor power, but if you can, it’s nice to know that MJ is similar to the energy in a glass of juice or many common snacks. Humans are very efficient. You only need about 100 calories of food to walk a mile, but you don’t need to push a car while traveling. In fact, scooters and e-bikes can move people more efficiently than muscles. On the other hand, 1 gallon of gasoline has 121 MJ of thermal energy, or about 29,000 calories.