If you are going to drive an electric vehicle (EV), you’re going to have to learn to speak the language of electricity.
Some people think that’s asking too much—that even though you understand units like inches and feet, meters and kilometers, cups and gallons, milliliters and liters, kilobytes and megabytes and gigabytes, you’ll never learn the basic units of electricity. And even though you already have appliances in your house, like hair dryers and microwave ovens, that have the wattage printed right on the front, there are some people who think electricity is just too complicated.
So when it comes to EVs, manufacturers and charging station operators try to dumb it down for you. Instead of telling you the actual power rating of a charging station, they’ll tell you how many miles of range you can get per minute or hour of charging. Instead of telling you the size of your car’s battery pack, they’ll tell you how far it goes on a charge.
The problem is that EVs don’t all go the same distance on a given amount of electricity, and the distance you can go will also depend on the weather, your driving habits, how much you’re using the air conditioning or heater, and other factors. While one EV might go 2.5 miles on a kilowatt-hour, another might go twice that distance. So even though you may learn exactly how far your EV can go on a kilowatt-hour of electricity, that number could be totally different for your neighbor’s EV. Which means that there is no correct and consistent way to say how many miles of range a given charger can deliver per hour of charging time.
It’s better to just learn the units. It’s not hard. Trust me—you can do this! Soon, your children will be learning it in elementary school.
So let’s dive in.
Learning the units
A good way to understand electricity is through water metaphors.
A kilowatt, denoted kW, is a rate of energy flow. It’s like the gallons per minute that a water hose or pump can deliver.
A kilowatt-hour, denoted kWh, is a quantity of electricity, like a gallon. A bigger battery pack with a higher number of kWh will hold more electricity, just as a bigger bucket will hold more gallons of water.
Now let’s put them together.
If you run a 1 kilowatt generator (or EV charging station) for 1 hour, it will deliver 1 kilowatt-hour of electricity. (1 kilowatt multiplied by 1 hour equals 1 kilowatt-hour.)
And that’s it! You’re done! You have learned the electricity units.
OK, let’s test your knowledge now.
If you plug in your EV to a 50 kW charging station, and it runs at full power for one hour, how much energy would it pump into your car’s battery? That’s right: 50 kWh, because 50 kW multiplied by 1 hour equals 50 kWh.
The rate of power that EVs actually get while charging changes depending on how full the battery is. Starting from a low state of charge, a battery will charge at the maximum rate that the vehicle’s charge controller will allow, and then as the battery fills up, it will slow down the rate of charging, until it’s down to a trickle when the battery is almost full. So it’s unlikely that you would get a full 50 kW of power continuously for an hour as in this example. But that’s a technical nuance that you’ll understand as you become accustomed to charging your vehicle.
EV charging stations
Now let’s talk about the different levels of charging stations.
If you just plug an EV straight into a standard U.S. wall outlet (Level 1), without using a charging station, you’ll get about a 1 kW rate of charge.
If you get a home charging station (Level 2) to charge up your car faster, it will probably have a power rating somewhere between 7 kW and 19 kW.
If you use an older public fast-charging station (Level 3 or DCFC), it might deliver 50 kW.
And if you use a state-of-the-art public fast-charging station, it might deliver 150 kW or more.
As you can see, it’s important to understand how fast a charging station is, because the power it delivers might be 150 times faster than just plugging your car into a wall socket.
Now let’s talk about battery sizes.
The 2019 Nissan LEAF, for example, is available with two different battery sizes: 40 kWh and 62 kWh. Suppose you had a 40 kWh model, and you bought a 7 kW home charging station for it. If you started charging it with a completely empty battery (which you would likely never do, but bear with me) and charged it at full speed until it was full, how long would it take to get a full charge? That’s right: about 6 hours (40 kWh divided by 7 kW equals 5.7 hours).
Now how long would it take if you did the same thing, only you did it using a 50 kW fast charger? That’s right: under an hour (40 kWh divided by 50 kW equals 0.8 hours, or 48 minutes).
Now that you know how to understand charging stations and cars, the only thing left to learn is how to understand your car’s range.
If you had a Tesla Model X, it might go around 2.5 miles on a kilowatt-hour. Suppose you had one with a 100 kWh battery pack. How far could it go on a charge, in theory, if you could use the entire battery? (Again, you probably wouldn’t and couldn’t thanks to certain details I won’t bore you with now.) The answer is 250 miles: 100 kWh multiplied by 2.5 miles per kWh.
Now let’s take the Nissan LEAF with the 40 kWh battery pack again. If you drive it very efficiently under favorable conditions, it can probably go 5 miles on a kilowatt-hour. So what’s its range, in theory? Roughly 200 miles: 40 kWh multiplied by 5 m/kWh.
Thus, the Nissan LEAF with the 40 kWh battery pack can go almost as far on a charge as a Tesla Model X with a battery pack that’s almost twice as large!
Now, let’s try one last test of your new knowledge, and figure out how much range you can get per minute of charging.
Suppose you charged up the Tesla Model X with its 100 kWh battery pack at a 150 kW fast charger at the maximum rate. How many miles of range could you get per minute of charging? Well, 150 kW multiplied by 1 hour is 150 kWh, divided by 60 minutes in an hour, equals 2.5 kilowatt-hours delivered per minute of charging. Multiply that by 2.5 miles of range per kilowatt-hour, and you get 6.25 miles of range per minute of charging.
Now let’s try the same example with the Nissan LEAF. The charger is the same, so you still get 2.5 kilowatt-hours delivered per minute of charging. But the LEAF can go 5 miles on a kilowatt-hour. So 2.5 kWh multiplied by 5 m/kWh equals 12.5 miles of range per minute of charging—double the distance that the Model X could get for charging the same number of minutes. (Intuitively, this makes sense, since the Model X, at 2.5 m/kWh, gets roughly half the fuel efficiency of a LEAF, at 5 m/kWh.)
Now you see why it’s nonsensical to explain the power rating of an EV charger in terms of miles per minute of charging: It depends on the vehicle.
And hopefully you now understand how to speak the language of EVs. Even if it’s unfamiliar at first, try a few of these simple calculations— sing only multiplication and division — and pretty soon you’ll be able to estimate your charging time and your range on a charge like a pro.
Chris Nelder is a manager with the Rocky Mountain Institute’s mobility practice where he leads the EV-Grid Integration initiative. ©2019 Rocky Mountain Institute. Published with permission. Originally posted on RMI Outlet.
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