For conventional vehicles, miles per gallon (mpg) is the gold standard for measuring fuel efficiency. This figure tells us how far a car can travel on a single gallon of gasoline and allows us to make apples-to-apples comparisons between gas-powered vehicles based on that criteria. The higher the mpg number, the more efficient the car. Overall, it is a simple and straightforward way to calculate the cost of driving a gas-powered vehicle.

However, when discussing electric vehicles, measuring and comparing the efficiency between different EVs can be more difficult. To do so in a way that applies equally across all EVs and helps determine the operating costs based on electric charging rates, the EPA expresses energy consumption and efficiency in terms of how many kilowatts of energy per hour (kWh) are needed to run a vehicle for 100 miles.

What is a kWh? It is a measurement of electricity equivalent to the amount of energy expended in one hour by one kilowatt. As such, the kWh/100 miles rating aims to define an EV's efficiency in the same way that an mpg rating applies to conventional vehicles.

**Range vs. Efficiency**

The first mistake consumers often make is to confuse an EV's range with its efficiency. In other words, they assume the further an EV can travel on a single charge, the more efficient it is. But that approach does not account for the amount of energy it takes to travel each mile or the battery's overall capacity.

Think about it this way: you would not compare the range of a vehicle with a 10-gallon gas tank to a car with a 20-gallon gas tank to establish efficiency. The car with the larger gas tank may have a longer travel range because it has more fuel to burn, but that does not make it more efficient.

In the same way, an EV that can travel 300 miles on a single charge is not necessarily more efficient than one that can travel 200 miles.

**What is MPGe for Electric Cars?**

To compare the relative energy consumption of electric cars with vehicles that run on fuel, the EPA provides a miles-per-gallon equivalent measurement, called "MPGe."

This unit of measure establishes that one gallon of gasoline equals 33.7 kWh of electricity. Based on this calculation, if an EV can travel 92 miles on 33.7 kWh, it receives a 92 MPGe rating from the EPA. For a conventional gas-powered vehicle to be as efficient, it would have to travel 92 miles on a single gallon of fuel.

Although MPGe directly compares energy consumption between EVs and conventional vehicles, it does not address the energy cost to operate an EV. So, for example, it may tell us that an EV rated at 92 MPGe is far more efficient than, say, a gas-powered vehicle that would give 32 miles per gallon, but what does 92 MPGe equate to in terms of the cost of electricity? This is when we turn to the EV-specific metric, kWh/100 miles.

**What is the Kilowatt Hour Per 100 Miles Rating (kWh/100 Miles)?**

For comparing EV operating costs, the kWh/100 miles rating is more relevant than MPGe. Although both ratings correlate to efficiency, the kWh/100 miles rating levels the playing field by standardizing range to an exact figure (100 miles) and then communicates how much electricity is required to travel that distance. This lets an EV shopper know how good a vehicle is at turning its electrical energy into miles of range.

The EPA provides the kWh/100 miles rating for every EV sold, allowing a shopper to directly compare two or more EVs. This rating is on the vehicle's window sticker, alongside the MPGe rating. But unlike MPGe, which is better when it is higher, the kWh/100 miles figure is better when it is lower. The lower number means it takes less energy to travel 100 miles. The MPGe and kWh/100 miles ratings have an inverse relationship, in that the higher the MPGe figure, the lower the kWh/100 miles rating.

**How to Calculate Electric Car Charging Cost**

Once a kWh/100 miles rating is known, a shopper can quickly calculate the cost to operate the vehicle by multiplying this figure by the price of electricity per kWh.

For example, a 2021 Tesla Model 3 with a standard-range battery has a rating of 24 kWh/100 miles, while a Ford Mustang Mach-E with a standard-range battery has a rating of 34 kWh/100 miles.

If the national average price of residential electricity costs 13.29 cents per kWh*, then the cost of charging the Tesla would be 24 kWh times 13.29 cents, which equals $3.19. The Ford, at 34 kWh, costs $4.52. That means driving the Model 3 a distance of 100 miles costs $1.33 less than driving a Mustang Mach-E the same distance.

Furthermore, in this scenario, the annual cost of electricity to drive a Tesla Model 3 for 12,000 miles would be a thrifty $382.80 ($3.19 times 12,000 divided by 100). The Ford Mustang Mach-E would run you $542.40 for the year. That's an annual cost difference of $159.60.

However, keep in mind that the cost of electricity can vary significantly based on the time of day and where you are charging a vehicle.

**Driving Style**

Another factor that will affect energy consumption and cost is driving style. Like mpg or MPGe, the EPA's kWh/100 miles rating is contingent on ideal conditions within a lab. Real-world situations such as carrying passengers or cargo, using climate control, or driving aggressively can increase the amount of electricity consumed. These factors will raise an EV's kWh/100 miles rating and decrease its efficiency and increase the cost to drive it.

**Summary**

Because vehicle charging infrastructure is still in short supply, many EV shoppers prioritize range over efficiency. But as infrastructure becomes more abundant and battery technology continues to improve, range will become less of an issue, and the focus will shift to efficiency and operating cost.

Since the kWh/100 miles rating easily reconciles energy consumption with electricity rates to determine efficiency, this metric will become as important to EV owners as mpg is to those who drive gas-powered cars. This is especially true if you install your own home charging station and plan to recharge an EV at a consistent time of day.

**Source: U.S. Energy Information Administration (March 2021)*