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Where did my Electric Range go?

Anyone who has owned an electric car during winter in a cold climate has asked the question: Where did all my electric range go? I can help answer that question with nerdy graphs.

Background

If you have driven combustion engine vehicles and switch to an electric vehicle and drive in a cold climate, you might be in for a little shock. The one benefit of a gasoline car is they are very efficient at producing waste heat energy, making for essentially free cabin heating. An EV has almost no waste heat so it has to create it using a heater (Don’t worry, even heating a cabin an EV is far more efficient than a gas car overall).

There are two main types of electric heater, a resistance heater or a heat pump. A resistance heater runs electrical current through a wire or element that gets hot (like your oven). All the heat energy comes from the electrical current. A heat pump is essentially an AC unit run in reverse, heat energy is pulled from the outside air and released into the interior. Since the heat both comes from electricity and the outside air, the electrical efficiency is higher.

A typical car AC or Heat Pump will draw between 1 and 3 kW, and a resistance heater between 3 and 6 kW, maybe up to 8 kW depending on car. Usually it won’t draw max power, but somewhere in between.

The final form of heater I will mention is the seat heater. It is also a resistance heater, but since it is in contact with your body, it is very low power. The peak might be 200 watts per seat, but the average is more like 100 watts or less, about the same as a bright incandescent light bulb.

Graphs

So how much does the heater hurt your range? In the Model 3, there is a prominent display that shows energy usage as Wh/mile, so I will do the same for the heater and AC estimates. Figure 1 shows how many Wh/mile range for the various heating methods.

I estimated power draw from previous research. The Wh/mile figure was generated by calculating the average power draw over an hour divided by the number of miles driven. For example, driving 30 mph takes 1 hour to drive 30 miles, if the heater draws 6 kW, that is 6,000 Wh/30 miles = 200 Wh/mile.

Figure 1: Consumption in Wh/Mile of various HVAC components

The motor power was calculated from ecomodder (permalink with parameters used). It can be seen that at low speeds the climate can draw way more power than the motor, but at higher speeds the motor quickly becomes dominate.

The next step is to combine the motor draw with the different climate settings to create a set of curves, this is shown in Figure 2.

Figure 2: Combined consumption with motor added to different heat settings individually.

If only the driver’s seat heater is enabled, turning it off will have nearly no effect on range. If all 5 seat heaters are running on max, it will become noticeable. The peak power for each seat is around 200 watts, so all 5 at max would be about 1 kW, so the same as AC/Heat Pump at 1 kW.

Finally, what impact do the various climate options have on the range? See Figure 3 for the Wh/mile converted to range for the Model 3 Long Range. The capacity estimate for the Model 3 was 73 kWh. Note: This is an estimate, not based on experimental data. I will do some tests to confirm, but behavior will be similar (likely slightly different numbers).

Figure 3: Range for 73 kWh usable battery capacity.

It is important to note that there is a very large negative impact on range, especially at lower speeds. In a large battery capacity car like the Model 3 Long Range, this effect will be less noticeable as it is not common to drive 200 miles around town on a given day at 25 mph as it would take 8 hours, but even at higher speeds the effect is very noticeable.

The impact is less noticeable at higher speeds. At 80 mph, the heater on max could take the car from 259 miles range down to 205 miles range, a drop of 54 miles, or about 21%. The heater isn’t usually on max, but even at 3 kW it would drop it 30 miles, or about 12%.

Conclusion/Comments

So how much would a heat pump help in the case of the Model 3? The answer is: a lot. The estimated power draw for the car from Ecomodder feels a bit low to me. However, using a heat pump could easily add 50 miles to your range or more, especially in climates with temperatures in the 20 F to 50 F range, where the largest gains will be seen.

I think it is time to push Tesla to use, or at least offer, a heat pump. It is leading to these relatively low performance vs EPA ratings, especially in these winter tests (even in mild winter temperatures of 40 F or so). Few EVs use resistance heat exclusively, they almost all employ heat pumps plus an auxiliary resistance heater if the temperature gets lower than the design points of the heat pump.

I have known for a long time resistance heaters are inefficient way to heat an EV interior, but I haven’t bothered to put a number to how much until now. It was a bit surprising to me that it could impact highway range at speed as much as it does.

One thought on “Where did my Electric Range go? Leave a comment

  1. I totally agree that you Tesla should focus more on cold weather driving. A heat pump would help a lot. I think they chose a resistance heater because it’s faster to heat up, but if the heater switched to a heat pump during long highway drives, then it would be far more efficient.

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