Heaters in ICE vehicles

Have you ever put your hand on the hood of a car after it got off the highway or finished a long drive? It’s hot! This is because traditional gasoline engines are very inefficient - 75%-85% of the energy produced is lost to heat! The nice thing is that in the winter, this waste heat can be directed into the vehicle to heat the cabin and keep the driver and passengers toasty warm. 

For an EV however, there is very little waste heat. Nearly all of the heat needed to warm the driver and the battery must be created. For many vehicles, this is done with resistance heating, in which electricity is passed through an electrical resistive element, the element gets hot, and a blower or fan blows the hot air into the cabin. This is similar to baseboard heating in your home and is the type of heater used in gas cars for decades.

A newer and more efficient type of heater is a heat pump. If we take a look at the data from the Recurrent study, we can see that verified data vehicles with heat pumps show an improvement in winter range of 8-10% near 30F.

Similarly, Michael Kim from EV Charger Reviews saw a difference comparing two Tesla models: a Model 3 with resistance heating and a Model Y with a heat pump. With both operating around 30°F, the Model 3 saw a 26% increase in energy consumption, while the Model Y only had a 8% increase. Remember: an increase in energy consumption means lower range. 

To understand how much of the range reduction is a result of the battery chemistry, let’s look at data from a AAA study. It shows how turning on the heat affects range. 

For the BMW i3, the range decreased 20% in cold temperatures with the HVAC off, but then decreased 50% compared to mild temperatures when the HVAC was on.

In other words, 33 additional miles were lost from heating the vehicle.

For the Chevy Bolt, we see a similar result. With the HVAC off, the range decreased by 10% in cold temperatures, but then decreased 47% with the HVAC on.

In this case, 88 additional miles were lost to heat the vehicle.

What can you do to stay warm in the winter and keep your EV range?

The biggest energy cost with heating a car is getting a cold, frosty vehicle up to a comfy temp. You can reduce the amount of work the HVAC needs to do by preconditioning an EV. Preconditioning essentially allows you to heat your vehicle while it is plugged into the charger so you use energy from the wall, and not your battery, to warm up. Once you’re on the go, try to rely on less energy intensive heaters, such as seat and steering wheel heat. 

Let’s dive into the details of how a heat pump helps save your winter range. As a reminder, these are the two types of heating your car may have. 

1. Resistive heating - Similar to a space heater for your home. Electricity is run through a conductor to produce heat, and that heat is directed into the cabin via a fan. Although nearly 100% of the energy used is converted into heat, it is an energy-intensive process. If you are pulling energy from the battery to turn into heat, it will be a direct hit to your range. One specific type of resistive heater used in cars is a PTC heater, which comes up in the studies below. 
2. Heat pump - This method is very similar to air conditioning, but inverted. Air is drawn from the outside and run through a condenser that contains refrigerant. This solution is compressed, which raises the temperature, allowing for hot air to be pumped into the cabin. This process also uses energy, but is much more efficient. 

Range Effect of Resistive Heaters

The average vehicle needs 4-8 kW of heating and cooling capacity, depending on interior volume, amount of exposed glass, insulation, and ambient temperature. Therefore, using a resistive heater means a direct hit of 4-8kW on the battery, in addition to energy used for the drivetrain. Depending on the difference between the outside temperature and the desired cabin temperature, range losses of up to 50% have been observed. Recurrent’s 2023 winter weather update showed a possible range loss of 45% at only 32F.

But What About Heat Pumps?

Heat pumps are way more efficient until the temperature drops far below freezing. While resistive heating turns each unit of energy into heat, a heat pump can generate 3-4 units of heat for every 1 unit of electricity used. 

Thus, the heat pump is 3-4 times more efficient.

Let’s use some real world data from my Chevrolet Bolt EV for context. If it’s cold enough that I need to use the heater at full capacity, that power demand increases to from 16 kW to 24kW at 65 mph.

Across the entire 65kWh battery, this reduces our effective range from ~264 miles to ~176 miles, a reduction of 33%. 

Even if you figure that you can turn down the heat once the cabin is warm, the range will still drop 20% to 211 miles. 

Let’s pretend that the Bolt EV has a heat pump, with an efficiency 3-4 times that of its resistive heater. The draw for heating + driving would be about 17.3-18.67kW at 65 mph. It leaves us with a range of around 230 miles, only a 10-14% decrease from baseline.

Want to see a list of EVs with heat pumps?

Not Just for Cars

Heat pumps are not just for vehicles, and have become a popular tool to increase energy efficiency in homes, as well. One study found that well below 32°F, heat pump efficiency is still significantly higher than resistive heating systems, especially in climates that generally don’t get below 20F that often.

‍

Energy losses & vehicle range:

Below, we show results from some studies that demonstrate the significant range benefits that heat pumps offer in cold weather. 

Note that a PTC heater stands for positive temperature coefficient heater, the type of resistive heater commonly found in cars. 

The data for this chart is taken from Kang Li et at. "Investigation on the performance and characteristics of a heat pump system for electric vehicles under extreme temperature conditions" and a 2018 NREL study.

A 2022 paper, Zhao C, Li Y, Yang Y, et al. Research on electric vehicle range under cold condition, compared a resistance heater and a heat pump at 20°F against a baseline testing at 77°F. This research found a total range loss of 42.8% with a traditional PTC heater, of which 26.4% was attributed to resistive heating alone. The remainder of range loss came from a deterioration in battery performance and a reduction in drivetrain efficiency. Meanwhile a heat pump showed only a 31% reduction in energy consumption, resulting in a 7.9% increase in range, compared to the resistance PTC heater. The average power demand was 1.6kW with PCT, 1.1kW with the heat pump. 

Similarly, at 14°F, Experimental Investigation on Heating Performance of Newly Designed Air Source Heat Pump System for Electric Vehicles showed an increase in the driving range with a heat pump by 25–31% when compared to PTC resistance heater. 

Finally, an NREL study compared the range loss from baseline with a PTC resistance heater and a heat pump across a range of temperatures. 

For PTC-only heating, the vehicle range loss varies from 28.3% to 53.8%. The heat pump was able to recover 12.8% to 1.1% of the vehicle range, depending on the temperature. This data indicates that the heat pump system provides a very large energy efficiency benefit for mild heating conditions, but becomes ineffective compared to the PTC heater at extremely low temperatures.