Measuring efficiency

In an EV, the gas tank is replaced with a battery. You can consider miles per charge, but it’s difficult to compare efficiencies when a LEAF has a 24 kWh battery and a Tesla has 100 kWh battery. Like in a gas powered car, efficiency is measured per a standardized unit. If you’re comparing EVs, efficiency is often measured in miles per kWh, which is specific to electricity. If you’re comparing across different sorts of vehicles, however, it is common to measure efficiency is MPGe, or miles per gallon equivalent. This uses the standard conversion of 33.7 kWh per gallon of gas, and allows us to understand just how much more efficient an electric car is than a traditional car.

Generally speaking, 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 to power an engine. Modern automobile engines can have hundreds or even thousands of moving parts, providing many cases of friction and heat waste, resulting in a peak thermal efficiency of 35-40%. In addition to engine losses, efficiency is further compromised by drivetrain losses, which typically result in losses of roughly 15% for two-wheel drive vehicles, with higher losses in all-wheel drive vehicles. Taken together, only 15-30% of the energy derived from gasoline is translated to the wheels.

‍

Vehicles using electric motors, on the other hand, are able to convert roughly 80% of electrical energy into mechanical energy. This is primarily contributed to the fact that electric motors typically have fewer than 20 moving parts. Additionally, electric motors generate significantly less heat than gasoline counterparts, meaning more energy is going directly to moving the vehicle. 

What affects efficiency?

Many of the factors affecting EV efficiency are shared with ICE vehicles. 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. Additionally, factors like driving uphill, quick acceleration, and stickier tires can decrease efficiency. However, the critical difference between ICE and EVs in terms of efficiency is that in an EV, the energy consumption and the energy efficiency may not be the same.

Interestingly, researchers from Carnegie Mellon University found that aerodynamics (which determine drag) and powertrain efficiency (how easily the car gets energy to turn the wheels) may be more important for efficiency than battery chemistry in EVs. As mentioned above, electric motors are extremely efficient, having the ability to convert roughly 80% of electrical energy into mechanical energy. The ultra-low drag of the Ioniq 6 is the prevailing explanation for why it is consistently the most efficient EV on the road.

What are the most efficient new EVs?

As of 2021, real-world fuel economy for gasoline vehicles in the US was slightly above 25 MPG. Although this represents a 30%+ improvement over the last two decades, the efficiency of a gasoline vehicle is still dwarfed by electric vehicles. The most efficient EV on the market today, according to the EPA (Environmental Protection Agency), is the 2023-24 Hyundai Ioniq 6, which boasts a phenomenal 140 MPGe combined. This means that this particular vehicle is greater than 5 times more efficient than the average gasoline vehicle. As of April 2023, the sales-weighted average efficiency of new EVs is 3.4 miles/kWh, which converts to roughly 114 MPGe.

What are the most efficient used EVs?

As a baseline, consider that as of 2016, the average fuel efficiency of a new ICE was 25 MPGe. The most efficient EV on the market is a 2020 Tesla Model 3 Standard Range, boasting 141 MPGe combined. The Tesla is almost 6 times more efficient than the average 2016 gas car, without even factoring in lower maintenance or fuel costs. The average efficiency of an EV in 2021 was 99 MPGe - less efficient than the Tesla, but a lot more efficient than a gas powered car. 

How efficient are Electric Trucks and SUVs?

*Estimates for Cybertruck based on memos and projections

Real World Efficiency Research

As part of Recurrent's funding from the National Science Foundation, we sent on-board devices to get realtime driving data from a cohort of Tesla Model 3 and Nissan LEAF participants. One of the data points we collected was efficiency, and we learned a ton.

First, efficiency is not a fixed number, even for the same car. It changes ride to ride based on the factors discussed above. In fact, if you plot efficiency, you get a distribution that shows how likely a car is to have any particular efficiency. You can take statistical values from these distributions to say what value is most likely.

Secondly, we learned that real world efficiency for these two popular models is much closer than the stats above would lead you believe. In the histogram below, you see the frequency that vehicles in each cohort hit various efficiencies. Note that the chart above lists efficiencies as 100 miles/ kWh and this data shows 1 mile/ kWh. You can multiply the efficiency values in the graph by 100 to get them to match.

In this histogram, the most frequently seen efficiency for the Model 3 is only slightly higher than the most frequently achieved efficiency for the LEAF - this means that in practice, the two cars get pretty similar efficiency. The LEAF does show lower efficiencies than the Model 3 does, but it also has rare instances of much higher efficiency values, too.

Consumption vs Efficiency

Efficiency is a relative term, indicating how much energy is used per distance or time interval. Consumption, on the other hand, is an absolute term, describing simply how much energy was used. For an ICE vehicle, the consumption is how many gallons of gasoline were used, and for an EV, how many kWh of electricity were used. In ICE vehicles, consumption and efficiency are very closely related because gasoline is used directly to move the vehicle, whereas generated electricity is used for the auxiliary components (e.g. lights, radio). In an EV, on the other hand, electricity is used for all functions as it’s the only source of energy.

In most EVs, the high voltage battery also powers climate control, lights, and on-board systems. This means that a fraction of your “fuel” is used to support the vehicle and driver, and will not be directly related to miles driven (i.e. fuel efficiency). In an ice vehicle, gasoline is used to move the vehicle, while the 12V battery is used to operate the auxiliaries. As a result, the fuel efficiency is nearly directly proportional to the number of miles driven, with the exception of a small portion of fuel going towards the alternator system. 

Additionally, some electricity is lost while the battery is charging - this is called charging loss, and includes electricity lost to heat, and that is used by your battery management system to regulate the battery’s temperature and rate of charge. You may see when you charge that you add 20 kWh, but only have an additional 18 kWh of drivable range. The 20 kWh is part of the energy efficiency calculation, while you can only consume the 18 kWh that is actually added to the battery. 

Consumption vs. efficiency also captures energy lost to phantom drain, or the electricity required to keep your EV maintained 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. Teslas, on the other hand, have onboard diagnostics and telematics systems that use a fair amount of electricity even when not being used. 

How does the EPA calculate efficiency?

The EPA, or Environmental Protection Agency, 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 miles per gallon (MPG) efficiency value. You may be familiar with the city or highway values, which depict the varying efficiencies that all cars get when driving either on a highway or in city conditions and are combined for the total MPG value. For an ICE car, efficiency is higher on the highway, while for EVs, efficiency is higher with city driving. 

‍

‍

In 2010, the LEAF and Volt hit the mainstream market and the EPA and NREL (National Renewable Energy Laboratory) wanted a way to compare the fuel economy of hybrid vehicles with the fuel economy of traditional, ICE cars - largely to highlight how much more efficient hybrids are. In order to make this comparison, the amount of energy in one gallon of gasoline is converted using the standard values of 33.7 kWh or 121 magajoules per gallon. As an example of how much more efficient the electric motor is, the fuel efficiency for the 2010 Volt showed that in all-electric mode, it gets 93 miles per gallon equivalent, whereas in gas mode, it is only 37 mpg. The combined efficiency for that Volt worked out to 60 MPGe, which is substantially higher than even a high efficiency gasoline car. 

Generally, MPG (as well as MPGe and electric range) are tested according to EPA protocol by the car manufacturer and then reported to the agency. A certain percentage of the reported values are verified by the EPA, but this explains the occasional controversy around manufacturer-stated range values and independently reviewed ranges. 

Did you know that the most efficient EVs tend to be the least expensive? Check out the Best Used Electric Cars under $25K