An electric car’s EPA range and efficiency ratings are rarely the same numbers drivers see in the real world. The EPA figures are determined in carefully controlled settings that do not reflect individual driving styles, terrains, climates or battery degradation.
All of those things change as soon as a carbuyer drives off the dealership lot.
The time of year also has an impact on EV range and efficiency. Cold weather range loss that accompanies winter is something we’ve talked about and done a lot of research. While these effects are short-term and don’t lead to any long term battery degradation, they can be a surprise to drivers who are expecting close to the full EPA-rated range, year round.
This article explores the impacts of summer and winter on EV driving to explain why your EV is about to get way more efficient. We combined learnings from the Recurrent community of over 100M recorded miles with our recent Enhanced Analytics Program, funded by the National Science Foundation. Using that program’s real-world efficiency data from a cohort of 50 Tesla Model 3s and 49 Nissan LEAFs with onboard tracking devices, we can now hone in on the temperature effects on efficiency.
This information gives us even more reason to be excited for summer. But, before talking about summer, we need to understand how winter impacts EVs.
Let’s Start With Winter: Cold Weather Effects On Electric Cars
Why does cold weather reduce your range? It all comes down to efficiency, measured in miles per kilowatt-hour, which is closely related to range (miles).
In the cold, EVs get a few hits to their efficiency:
- Chemical reactions in the battery that power an electric car happen more slowly when it’s cold.
- Warming the battery (see #1) requires energy. If you don’t preheat (a.k.a precondition) the car, some of the available energy in the battery has to be used to warm up the battery itself.
- Using climate control for the passengers takes a surprising amount of energy. In an internal combustion car, the engine creates a lot of waste heat that can be used to warm the passenger. EV motors are less wasteful, meaning that electricity from the battery has to be devoted to climate control.
All of these things are normal, and seasoned EV drivers know to expect range to drop in the cold. However, estimating your winter range used to be an exercise in scouring forums for anecdotes. This new data brings us closer to robust temperature models for Model 3 and LEAF and predictions that can inform drivers.
Summer Brings Efficiency (and Range)
The data from this study charts how efficiency increases with temperature between 5 degrees and 85 degrees Fahrenheit.
This first graph is combined efficiency data for the 99 vehicles in our first cohort – 50 Tesla Model 3 cars and 49 Nissan LEAF cars. Around 70 degrees Fahrenheit, the efficiency gains begin to level off. Once it gets above 85 degrees Fahrenheit, air conditioner use and ambient temperature is expected to decrease efficiency again.
The data variance, or spread, around the mean increases with higher temperatures. This means that there is less certainty in the exact efficiency values at higher temperatures. We suspect this is due to the data we collected, which was mainly in the cooler months from November to March, but it can also be due to differing climate control preferences at hot temperatures. As our dataset grows, these charts will be updated and improved. We expect to see the variance around the higher temperatures narrow down.
Breaking down the data by vehicle model, we see that both Tesla Model 3s and Nissan LEAFs follow a similar pattern in their efficiency values with temperature.
In this more granular data, you can see clear inflection points where LEAF efficiency declines above 70 degrees, but Model 3 efficiency continues to climb. This is interesting because early LEAFs (labeled “Old LEAF”) lacked active thermal management and were known for excessive battery degradation in hot climates. This data suggests that whatever systems Nissan built to increase battery endurance in hot climates reduces efficiency more than Tesla’s system, even while preserving the long-term health of the battery.
LEAF model years after 2017 have the most steady efficiency gain between 25 and 65 degrees Fahrenheit, which is likely where their thermal management system is being switched off. Model 3 and pre-2017 LEAFs behave quite similarly in this region.
Understanding Our Data and Methodology
We provide monthly and on-demand reports for EV drivers, shoppers, and sellers to help them adequately evaluate the range and battery condition of a used EV. For these reports, we use over-the-air data streams from the vehicle battery management systems, which can be biased by manufacturer-specific differences.
Last year, we were awarded a grant from the National Science Foundation to compare different methods and frequencies of over-the-air data collection for some of the most common EVs on the road. We started with two models: Tesla Model 3s and Nissan LEAFs. Deploying on-board telematics devices in test vehicles alongside our standard hardware-free approach allows us to calibrate and see what drivers actually experience in terms of range and efficiency on a more granular basis. More details on our Enhanced Analytics Program here.
How We’re Measuring EV Efficiency
The EV-specific measure of efficiency that we used in this analysis is miles per kilowatt-hour. Kilowatt-hours (kWh) are the unit of size for batteries, so mi/kWh is roughly equivalent to miles per gallon. We used mi/kWh because it represents how we calculated efficiency: total energy used while the car was in motion, divided by miles driven. This means we are not calculating phantom drain but we do capture the effects of climate control.
Below, see the frequency of our data points grouped by temperature.
While seasoned EV owners may not be shocked to learn that their efficiency decreases in the winter, it’s important that new drivers know what to expect to feel satisfied with their electric cars. This new dataset will help us model seasonal efficiency changes more accurately and tease out the temperature effects on range.