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, some of this waste heat can be directed into the vehicle to heat the cabin and keep the driver and passengers toasty warm. 

image by Karin Kirk for Yale Climate Connections with data from fueleconomy.gov

Provided that EVs do not have engines, and have drivetrain efficiency of around 80%, very little waste heat is produced, so cabin warming has to happen another way.

Many groups have shown that the climate control systems (heating and air conditioning; HVAC) are the largest energy consumption system in an EV, with several studies showing that a driver can expect to lose an average of 30-40% driving range with conventional HVAC systems. Recurrent found that in 100+ degree weather, air conditioning can cut range by up to 30%, and heating a car in freezing weather can drop range by more than 40%. Another study showed the range reduction was up to ~16% for cooling and ~50% for heating, depending on conditions and drive cycle. Since range anxiety is a major issue preventing widespread EV adoption, it’s crucial that car makers modernize HVAC systems to be more efficient.

How do EV Heaters Work?

Given that EVs have almost no waste heat that can be shuttled into the cabin, EVs rely entirely on electricity to produce heat, which can be broken into two major categories: resistive heating, and heat pumps. 

  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, while also energy intensive, is much more efficient. 

Measuring and comparing the energy efficiency of heat pumps vs. resistive heaters has been a major topic of interest with EV drivers in the past few years, so we hope to shed some light based on the best available research. 

Range Effect of Resistive Heaters

The no-brainer approach to an EV heat and cooling system is to use the same technology that has been done in ICE cars forever: use a resistive heater and an electric refrigerant compressor for cooling. The heater itself is cheap, simple, and nearly 100% efficient, meaning all electricity is converted into heat. However, while this method is effective, it is not very efficient in terms of overall vehicle energy consumption. 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 of 4-8kW draw 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.

Let’s use some real world data from my Chevrolet Bolt EV for context. On a relatively flat stretch of road at moderate temperatures (no HVAC needed,~80F), the Bolt requires ~16kW of power to maintain 65 mph, which puts the efficiency around 4.06 miles/kWh. If it’s cold enough that I need to use the heater at full capacity, that power demand increases to 24kW at 65 mph, with an efficiency of 2.71 miles/kWh. 

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. 

But What About Heat Pumps?

While a resistive heater is 100% efficient at generating heat from electricity, the process is still relatively inefficient because heat must be converted from another energy source. A heat pump, on the other hand, uses electricity to move heat from one place to another. To do this, heat pumps use a reverse refrigeration cycle to bring in heat from the outside air that has been boosted in temperature by a reverse refrigeration cycle. 

If we want to compare efficiency between a heat pump and a resistive heater, we need to use a concept called the coefficient of performance (COP), which is the ratio of the heat produced and transferred to the cabin vs. the energy required to make and move that heat. And this is where it gets interesting: while the net efficiency of a traditional heat pump is quite low (~50% at ambient temperatures), it can produce heat using less energy. The COP of a heat pump can reach 3 or 4, meaning 3-4 units of heat are transferred for every 1 unit of electricity used. Meanwhile, the resistive heater, which turns 100% of energy into heat, has a COP of 1. The heat pump is 3-4 times more efficient.

Not Just for Cars

Heat pumps are not unique to vehicles, and have emerged as a promising tool in the global transition toward clean and reliable energy, specifically in cold climates. One study found that well below 32°F, heat pump efficiency is still significantly higher than resistive heating systems at an appliance level. The heat pumps investigated in the study demonstrated efficient heating during cold winters where temperatures rarely fall below 14°F. Even in extreme climates, with temperatures near -20°F, heat pump may provide heat at efficiencies up to double that of resistive heating. 

Image from Engtiger47, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

Let’s go back to the Bolt EV example and assume we have a heat pump with a COP of 3 while heating. The draw for heating in this case is reduced to 1.33-2.67kW, implying a total demand of 17.3-18.67kW when traveling at 65 mph with the heat on. This results in an effective vehicle efficiency of 3.5-3.7 miles/kWh. It leaves us with a range of around 230 miles, only a 10-14% decrease from baseline.

In agreement with this data, in early 2022, a UK-based company performed real-world winter range testing of several popular EV models, revealing that “models equipped with a heat pump fell short by an average of 25.4% from their official figures, compared with the 33.6% deficit suffered by those that relied on a regular interior heater.” Vehicles with heat pumps equipped averaged 3.2 miles/kWh, while those with resistive (or PCT-based) heaters averaged 2.9 miles/kWh. 

Emerging Research: vehicle range loss with different heaters

One of the big topics of debate on EV forums (and energy efficiency forums, in general) is if there is a temperature at which heat pumps stop being more efficient than resistive heaters. For instance, a popular comment is that in Canadian and Northern US winters, a heat pump will not produce enough heat, or will not save you any range above a traditional, resistive heater. 

While Recurrent lacks sufficient really cold weather data to confirm or deny these claims, we have aggregated existing data on the efficiency of heat pumps and resistive heaters in different temperatures. Below are summary results from several studies that compare heat pump vs PTC resistive heater in freezing and below-freezing conditions. Note that a PTC heater stands for positive temperature coefficient heater, the type of resistive heater commonly found in cars. 

Below, we show that in two studies, heat pumps have significant range benefits in cold weather as compared to a resistance heater. While the range savings is more pronounced at 32°F, both studies show a range benefit at 14°F.

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, this 2015 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%. When operating the heat pump system, the recovered vehicle range varies from 12.8% to only 1.1%. 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.

One potential way to improve the low-temperature performance would be to use a larger capacity compressor to offset more of the supplemental PTC heating. Additionally, the inclusion of waste heat recovery can provide an additional 1.5% to 2% under all conditions. While this is a moderate benefit when compared with the heat pump at mild temperatures, at colder temperatures, it becomes a valuable contribution. 

Finally, it is important to consider how quickly you need to warm up the cabin and how warm you need to get it. For example, the study above uses a system in which the cabin temperature increases from 28°F to 72°F in 23 minutes. Dramatically different results are seen when you raise the temperature to more modest temperatures (e.g. 60°F), or use a compressor that takes less time to heat the cabin.