One of the few advantages of using combustion engine technology is the plentiful supply of spare waste heat generated by exploding gas in the engine. For each gallon of gas burned to move the car forward, about 2/3rds of the energy goes to waste heat. This is incredibly wasteful, except in the winter when you can take advantage of that and have all the heat you want for the cabin without having to burn more fuel.
So what are we to do with an electric car? There are a few types of electric heaters that are commonly used. The car already has AC, and if you have ever used a window AC unit, you will know that one side puts out cold air into your house, and the outside of the unit puts out hot air. What would happen if you turned the unit around and put the hot side on the inside and the cold side on the outside? You get a nice heater. This is called a heat pump, and the car can add some additional hardware to make it so it can reverse the AC coils and run in either AC or heat pump mode. These are efficient at cool temperatures as they use heat energy from the air. Average maybe 3 kw of power.
Another type of heater is a resistance heater. Most of you have used an electric oven. The coil in the bottom or top is an example of a resistance heater. You run an electric current through a wire and put off a massive amount of heat. However, this is very expensive electrically, although near 100% efficient (all the energy is going to heat), it takes a lot of current from the battery. Since you don’t really want a hot coil like in your oven in the car for obvious safety issues, it is in a special enclosed unit that heats coolant, and the warm coolant is routed through your usual heat system that would pull engine heat. For an electric car this heater might draw 6 kw or more.
What if you want to heat the air directly? This would be done using a positive temperature coefficient (PTC) heater, where it is a type of resistance heater, but has an element that is much safer and lower temperature than a nichrome wire used in an oven (photo above). It is self limiting current so it doesn’t overheat. These have similar power draws as a normal resistance heater, but are safer to use and last longer than a wire exposed to the air. Your small portable house heaters are usually PTC.
Which type of heater do I want and why? The answer is complex. Heat pumps really only work above 0 to 10 F (-18 to -12 C), with most efficient range being around 20 to 40 F (-7 to 4 C), where they use about 1/4 of the energy as a resistance heater at the same temperatures.
Resistance heaters are the only way to go for a battery only electric vehicle (BEV) below 0 F (-18 C) air temperature, so any BEV will have at minimum a resistance heater, and many will add a heat pump for efficiency for those mildly cold days.
A plug-in hybrid gas electric vehicle (PHEV) has more choices. The designer can choose to use either electric or gas burning for heat generation. Some like Hyundai Ioniq PHEV choose to not have any electric heat and run the gas engine for heat, others choose to add resistance heat.
The Honda Clarity PHEV uses a coolant based electric heater that works like an on demand water heater. If you ask for heat it will heat the coolant and flow it through the heater core in the cabin (just like a gas engine would). Given that this is a coolant system, it is surprisingly quick to heat the air. It switches to using the engine for heat at very low temperatures (less than about 5 F (-15 C) or so, seems related more to battery temperature than air temperature).
The Honda Clarity BEV uses a different system. Since it doesn’t already have the cabin coolant loop heater core that a gas engine would use, it instead has a PTC heater for air-to-air heating at cold temperatures and a heat pump to use at cool temperatures.
So which is best? The answer is it depends. Hyundai chooses no electric heater with their PHEVs to save costs and figures people won’t mind running the engine even in only cool temperatures. For the Clarity PHEV, Honda chooses the coolant based heater since the engine already has a coolant based loop to provide heat, they can share the same heater core. For the BEV, Honda chooses to use the heat pump to maximize range and the PTC heater for colder weather. They all have cost benefit trade offs.
Ideally, any BEV needs to have a heat pump to maximize your range in cool weather. It means your maximum range will drop more slowly at low temperatures. It will still drop off dramatically, but not as quickly. A PHEV with a moderate range (say 20 miles or more) should have some form of electric heater to prolong how cold the car can be driven on EV only.