An EV’s battery capacity is like the size of its fuel tank. While we measure a fuel tank in gallons, we measure battery capacity in kilowatt hours (kWh). We already explained that a watt-hour is a measurement of energy, so a kilowatt-hour is simply 1,000 of those watt-hours.
As an example let’s take a car that has an efficiency rating of 235 wh/mi. Let's say this car has a 50 kWh battery. That’s a "fuel tank" holding 50,000 watt-hours of power, of which each mile driven uses (on average) 235. If we divide 50,000 units of power by 235 per mile, we get 212 miles. That's approximately the amount of range this vehicle would have available.
While we’re on the subject, what’s a typical battery size? Fully electric cars and crossovers typically have batteries between 50 kWh and 100 kWh, while pickup trucks and SUVs could have batteries as large as 200 kWh. Of course, a larger battery will take longer to charge than a smaller battery, and it will cost you more in electricity to do so.
Some automakers may limit how much of the battery's capacity is available for use. They do this to keep the battery healthy, because the types of batteries currently used in EVs don't like to sit around while fully charged and can be damaged if fully discharged. To prevent issues, manufacturers may keep some buffer space in your battery's capacity, so that when your car says the battery is at 0%, it's really at something more like 10% (but it won't let you use any more). Same is true at the top end, where 100% charge might only be something like 90% of total capacity. When you see that a car has "100kWh battery capacity", the next question is, "is that total capacity, or usable capacity?"
OK we know we just dumped a lot on you. Before we go further, let’s compare some cars and make sure that all of this made sense.
Which of the following cars is more efficient?
250 mile range
65 kWh battery
250 mile range
95 kWh battery
Both cars have the same 250 mile range, but Car B needs a larger battery to reach that distance. We don’t need to know the efficiency rating of either car to know that Car A is more efficient.
Let’s look at another example.
245 wh/mi
75 kWh battery
351 wh/mi
75 kWh battery
Here we can see that battery sizes are equivalent, but Car C is MORE efficient (lower wh/mi is better) so we can determine that Car C will have more range than Car D. We can also estimate that range ourselves:
75,000 Wh divided by 245 wh per mile = 306 miles of range.
Getting the hang of it? Let’s talk about real-world range.
Most EVs will display how much range you have left in your battery. For example, you may look down at your dash and see that you have 50% charge, with 150 miles remaining. The 50% part is accurate, but the miles remaining is just an estimate. It’s your car’s best guess, usually based on your most recent driving habits, about how far you can still drive. As you continue driving, the car will take averages of your efficiency and update your range estimate.
Let’s say you’ve driven 100 miles on mostly flat roads, but are now entering a mountain range, odds are your car will look at your recent driving, see it was all on flat roads, and guess pretty poorly about how much mileage you have remaining because it thinks you’re going to continue on flat roads. In this case, you’ll likely start to notice range dropping faster than actual miles as you climb up hills, and the car realizes something has changed.
In situations like this, there is a way you can approximate your real range remaining, with the knowledge of the type of driving you’re about to do.
For example, as you’re driving in mountainous terrain, you can go into your car’s menu and look at real-time efficiency data. Most cars will make this available to you, and it will allow you to see real-time numbers for wh/mi. Let’s say your real-time mountain-driving efficiency is 450Wh/mi.
If you can see that you have 50% battery remaining, and know that you have a 75 kWh battery pack, you can use your current efficiency to estimate how much real-world range you’d have if the terrain continues to be mountainous.
50% of a 75kWh battery remaining = 37.5 kWh energy.
That’s 37,500 watt-hours, of which you’re using 450 per mile.
37,500 watt-hours divided by 450 miles = 83 miles remaining.
Remember, your car estimated 150 miles remaining, because it was likely using a lower efficiency number based on flat terrain. You can see how knowing how to do this math could help you better plan your drives if you’re in a pinch for range. Most of the time, there's no need to think about this stuff.