Cold weather reduces electric car range substantially, and cabin heating is a large part of the reason. A heat pump changes the arithmetic by heating the cabin in a fundamentally different way.
Why heating costs an electric car so much
A combustion engine wastes most of its fuel energy as heat, and cabin heating simply diverts some of that waste through a matrix. The heating is effectively free because the heat already exists.
An electric drivetrain is far more efficient and produces comparatively little waste heat, so there is nothing convenient to divert.
Heat must therefore be generated from stored energy, and every unit spent warming the cabin is a unit not available for driving.
How resistance heating works
The simplest approach passes current through an electrical element, which converts electricity into heat almost entirely.
That sounds efficient, and in one sense it is, since virtually all the electricity becomes heat. But one unit of electricity produces only one unit of heat.
On a cold morning the demand can be comparable to the power needed to drive at moderate speed, which is why range falls sharply in winter.
What a heat pump does differently
A heat pump does not create heat. It moves heat from outside the car to inside it, using a refrigeration cycle run in reverse.
The electricity is spent compressing a refrigerant rather than generating warmth, so a single unit of electricity can deliver several units of heat into the cabin.
This is the same principle as a domestic refrigerator, which makes its interior cold by moving heat out to the room behind it.
Why performance falls as it gets colder
A heat pump extracts heat from outside air, and the colder that air is, the less heat there is to extract per unit of work.
Efficiency therefore declines as the temperature drops, and at extreme cold the system approaches the performance of a resistance heater.
Most cars retain a resistance element for those conditions and for rapid warm-up, switching between the two according to what is more effective at the time.
How the rest of the car is used as a heat source
Newer systems draw waste heat from the battery, motor and power electronics rather than from outside air alone, since those components generate some heat in normal operation.
The same circuit can be reversed to cool the battery in summer or to warm it before rapid charging, which improves charging speed in cold conditions.
Integrating cabin comfort and battery thermal management into a single system is where most of the recent gain in winter range has come from.