Cabin heating is the largest avoidable energy cost in an electric vehicle during cold weather. A heat pump addresses it by changing how the heat is produced rather than how much is used.
Resistive heating is the simple approach
A resistive heater passes current through an element, converting electrical energy into heat almost completely. Every unit of energy drawn becomes roughly one unit of heat.
This is cheap, compact and reliable, and it works immediately from cold, which matters for demisting and for driver comfort in the first minutes.
The problem is that the energy comes from the traction battery, so cabin warmth competes directly with range on the same limited supply.
A heat pump moves heat instead of making it
A heat pump uses a refrigerant circuit to absorb heat from one place and release it in another, which is the same principle an air conditioner uses in reverse.
Because it transports existing heat rather than generating it, the heat delivered to the cabin can exceed the electrical energy consumed by a considerable factor.
The source is usually ambient air, and in many vehicles also waste heat from the motor, inverter and battery pack, which would otherwise be discarded.
Efficiency falls as the outside air gets colder
The advantage depends on there being heat available to move. As ambient temperature drops, there is less to extract and the pump works harder for less return.
Below a certain point the benefit narrows enough that vehicles supplement with resistive heating, which is why most systems retain an element alongside the pump.
This is also why the gain is largest in cool damp conditions rather than in extreme cold, and why reported benefits vary so much between climates.
Integration with battery thermal management multiplies the benefit
Modern systems tie cabin heating, battery conditioning and component cooling into one refrigerant loop managed by a single controller.
Heat rejected by the power electronics can be routed to warm the battery before rapid charging, or to the cabin, depending on which need is greater.
This integration is where much of the recent improvement in cold weather range has come from, rather than from the heat pump considered in isolation.
Cost, weight and complexity are the trade
A heat pump adds compressors, valves, additional heat exchangers and control software, all of which add cost and mass to the vehicle.
Manufacturers have therefore often offered it as an option or restricted it to higher trim levels, which is why two examples of the same model can behave differently in winter.
The additional plumbing also introduces more potential failure points, and diagnosis of a thermal management fault is considerably more involved than replacing a heating element.