Measuring Efficiency In Electric Cars

Unlearning MPG in the Age of Electric Cars

Efficiency metrics have been in the car owner lexicon for decades. We use terms like range and miles per gallon (MPG) to measure how far you can drive on a given fuel. Although energy efficiency works similarly for gas-powered and battery-powered cars, some of the metrics have evolved, and so must the people behind the steering wheel.

Understanding Fuel Efficiency

The Environmental Protection Agency (EPA) is responsible for measuring and reporting the efficiency of all consumer vehicles on the road in the US. It uses five road tests that simulate different driving conditions and then combines them to get a single efficiency value.

For internal combustion engine (ICE) cars, the fuel is gasoline, and the efficiency metric is either miles per tank or miles per gallon. But in 2010, two plug-in electric hybrids — the Nissan LEAF and Chevy Volt — hit the mainstream market and the EPA and NREL (National Renewable Energy Laboratory) wanted a way to compare the fuel economy of electric vehicles with the fuel economy of traditional ICE cars. In order to make this comparison, the amount of energy in one gallon of gasoline is converted to electricity using the standard values of 33.7 kilowatt (kWh) per gallon. The unit for this is MPGe, or miles per gallon equivalent, which gives us a way to talk about how much more efficient non-gas burning cars really are.

Overall, electric cars will always be more efficient than gas powered cars because less electricity is lost to heat and friction than when you rely on combustion. An EV can use around 60% of electricity consumed to move the car while a combustion engine uses less than 20% of gas energy to turn the wheels (NRDC.org).

Electric Car Energy Drains

The factors of efficiency in an EV are similar to an ICE vehicle. More powerful engines on both types of cars use more energy and decrease efficiency, so the performance models of many EVs are less efficient than their base model counterparts. Things like driving uphill, quick acceleration or hard braking, and the tire choices on your car also can decrease efficiency.

One difference that throws a lot of new EV drivers is what sort of driving is most efficient. In a conventional gas car, highway driving is more efficient since acceleration uses a lot more energy than going at a steady speed. However, in an EV, city driving is a lot more energy efficient. Electric cars use regenerative braking to recapture the energy that is normally lost during braking, using it to recharge the high voltage battery. This means that every time you slow down, the EV battery is charging up.

Another difference that EV drivers might notice is that not all of the electricity that you put into your car — the consumption — actually makes it to the battery. If you consume 20 kWh of electricity while charging, you may only get an additional 18 kWh of driveable range. Some electricity is lost to charging itself — this is called charging loss, and includes electricity lost to heat, and energy used by your battery management system to regulate the battery’s temperature and rate of charge. Auxiliary systems such as air conditioning or heat may also draw energy from the battery when the car is running.

The final puzzle in EV efficiency is phantom drain (or vampire drain). This is the electricity used by your EV when not in use. For some vehicles, such as Chevy Bolt, there is very little phantom drain and what you see available when you turn your car off is what you see when you turn it back on. For cars like Teslas, however, the onboard diagnostics and telematics systems use a significant amount of electricity even when not being used.

Most Efficient Electric Cars

Finally, what is a good efficiency, and how do you know if you’re getting the best you can? As a baseline, consider that as of 2020, the average fuel efficiency of a new ICE was 25.7 MPGe, while the average for an EV is 99 MPGe. The most efficient EV on the market is a 2020 Tesla Model 3 Standard Range, boasing 141 MPGe combined, as rated by the EPA. The Jaguar I-PACE is one of the least efficient models out there at only 76 MPGe.

Year-Make-Model-Trim MPGe

2020 Tesla Model 3 SR 141
2019 Hyundai Ioniq Electric 136
2015 BMW i3 124
2020 Hyundai Kona Electric 120
2017 Chevy Bolt 119
Pre 2016 LEAF 114
2020 Kia Nero 112
2016 Tesla Model S AWD 70D 101
2020 Jaguar I-PACE 76

One of the exciting projects that my company, Recurrent, is working on is measuring actual EV efficiency with cars on the road. We are investing grant money from the National Science Foundation to test real-world miles per kWh for Nissan LEAFs and Tesla Model 3 with more cars to follow in 2022. If you’re interested in participating or learning more, check out our Enhanced Analytics Program that is now accepting new EV drivers.

–Written by Liz Najman, Recurrent

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