EV Batteries & Range
Why You Should Care About Efficiency
Why You Should Care About Efficiency

Why You Should Care About Efficiency

Considering efficiency is even more important in and EV. Here's why.

An EV's efficiency is rated by the the Environmental Protection Agency (EPA) using a simulated driving schedule, and their result is printed on the Fuel Economy and Environment label of every new electrified car. An EV’s advertised range is only an estimate.

Buying a new EV rated by the EPA to have 200 miles of range means that it was found capable of traveling 200 miles on a single battery charge, under these simulated conditions. There are a variety of factors that could decrease (or increase!) the actual range that you get in the real world.

Simply put, your mileage may vary! This uncertainty is a primary cause of “Range Anxiety”, or the fear that you may not have enough range to get to where you’re going.

To avoid range anxiety, it’s important to understand how range works, and how to anticipate the ways it might fluctuate.

Efficiency is key

Range is a product of how efficiently an EV uses its available battery power. Just like gas cars, there are more efficient vehicles and less efficient vehicles.

Why is efficiency so important for EVs? It all comes back to charging. With a gasoline vehicle, the less efficient a vehicle is the more money you're going to spend at the gas pump. With an EV, the less efficient the vehicle is, the more time you'll spend at the plug (and also more money).

Trucks and SUVs are very popular, so clearly many of us are willing to spend more money for a less efficient vehicle, but how many of us are willing to spend more time at a plug when we're trying to get somewhere? That's a very different value proposition.

Let’s talk more about efficiency, including how to measure it and how to compare the efficiency of cars when shopping.


The efficiency of a gasoline car is measured in Miles Per Gallon (MPG), which of course tells us how many miles we can travel on a single gallon of liquid gasoline. Electric cars don't use liquid fuel, so we have to measure their efficiency differently.

In an EV, we measure efficiency with something called watt-hours per mile (wh/mi). Instead of how many miles we can travel with a gallon of fuel, wh/mi tells us how much electrical energy is being used to travel a single mile.

That's all well and good, but what about cars that use some combination of liquid fuel and electricity? And what if we want to compare the efficiency of an electrified car to the efficiency of a gasoline car? That brings us to MPGe, or Miles Per Gallon Equivalent. MPGe is the measure of how many miles a vehicle can travel per 33.7 kWh of energy, which is the amount of equivalent energy contained in a single gallon of gasoline.

This equivalency allows us to understand which cars are more efficient with their energy usage no matter what form of energy they're using. With MPGe, a higher number is better.

What's a good MPGe rating?

At time of writing, the most efficient EVs on sale in North America are rated above 140 MPGe. Meanwhile a larger, heavier vehicle like an all-electric SUV may have a rating less than 80 MPGe. Both are significantly more efficient with their energy than the most efficient non-hybrid gasoline car, which is closer to 40 MPG.

MPGe isn't perfect. While it's a useful tool for comparison shopping, it's less useful for trying to understand your real-world mileage. Which brings us back to…

Wh/mi (or Wh/km)

A watt-hour is equal to the amount of power (watts) used over time (an hour). Together, it's a measure of energy. That energy can be used for a task or stored in a battery.

If you were to turn on a 100-watt lightbulb, and keep it illuminated for one hour, you would have used 100 watt-hours of total energy. The task in this instance was lighting up a room for an hour. If you wanted to run this lightbulb off of a battery, you would need a battery that can hold 100 watt-hours of energy. Now, what if our task was driving a distance instead?

Watt-hours per mile (wh/mi) is a measure of how much electrical energy it takes to travel one mile. For wh/mi, a lower number is better.

What’s a good Wh/mi rating?

With Wh/mi, a lower number is better. At time of writing, the most efficient EVs on sale in North America average less than 250 wh/mi. A larger, heavier vehicle like an all-electric SUV might have a rating over 400 wh/mi.

So now that we know how electrical usage is measured, how much of it do we have available? We'll talk about battery capacity in the next article.

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