A heat pump in an electric car: when the option pays back

A resistive heater converts electricity into heat, completely: one kilowatt-hour of electricity gives one kilowatt-hour of heat. It cannot do better, because no more heat can appear than the energy put in. Such a heater is cheap, light, maintenance-free and warm immediately — and in an electric car it costs range directly.

A heat pump does not make heat, it moves it. A refrigerant evaporates at low temperature, taking heat from its surroundings; a compressor compresses the vapour, raising its temperature; in the condenser it gives that heat to the cabin. The compressor needs electricity, but the heat moved is greater than the electrical energy used.

The ratio of heat delivered to electricity used is the coefficient of performance. In mild conditions it is well above two, meaning the same heat for less than half the electricity. Technically it is the same machine as the air conditioning, running the other way — which is why the hardware cost is modest and there is an option price all the same.

The simplest layout takes heat from the outside air. Even at zero degrees, air contains energy that can be transferred to a still colder refrigerant; it is the same process as in a domestic heat pump. The evaporator sits at the front of the car where air flows through.

More elaborate systems also take the waste heat of the car's own components: power electronics, motor and battery get warm in use, and that heat is already in the coolant circuit. A system able to switch between several sources works more efficiently, because it picks the warmest available — and in winter it can work the other way and warm the battery when it is too cold.

How good a system is therefore depends less on “heat pump yes or no” than on how many sources it can use. That is rarely in the brochure but can be found in tests and specialist write-ups on the specific model.

A heat pump's coefficient of performance depends on the temperature lift — the difference between source and target temperature. The larger that lift, the more work the compressor has to do and the smaller the gain. At five degrees outside and a comfortable cabin the lift is small and the figure high; at minus fifteen the lift is large and the advantage much smaller.

A second effect follows: icing. Cool the evaporator below freezing and humidity settles on it as frost, impeding heat transfer. The system then has to defrost, which needs energy or a temporary reversal. So in exactly the hours you need it most, it works intermittently.

Which is why practically every car with a heat pump also has an electric booster heater. In hard cold it cuts in, and consumption is then the same as in a car without a heat pump. Anyone who sees the system as insurance against winter will be disappointed; anyone who sees it as a saving on the many mild winter days gets exactly that.

Because heating power is roughly constant while traction power rises with speed, any saving on heating counts most, in percentage terms, where you drive slowly. In town and on short journeys the heater can be a substantial share of total consumption; on the motorway it is a minor item.

The second factor is trip length. Warming a cold cabin takes a lot of power; holding the temperature afterwards takes little. On a ten-minute drive the heating demand is almost entirely warm-up — and that is where the heat pump helps most, because it covers that peak more cheaply.

Which gives the profile where the option is most likely to pay: many short trips, mostly in town, in a climate with many days between zero and ten degrees. That describes a large part of a central European winter — and the daily routine of most commuters.

No amount appears here, because electricity prices differ by country, supplier and tariff. The calculation can be done all the same, in four steps with your own numbers.

The calculation is deliberately rough. It is not meant to be exact to the last unit but to answer whether the result lands in the region of the option price or far below it — and that is usually decided in the first step already.

On many models the heat pump was an extra, on some part of a trim level, on a few standard. It rarely appears in a listing. Three routes lead to the answer: the previous owner's full specification list, the vehicle documents with their option codes, or a query on the chassis number at a franchised dealer. The third is the most reliable and takes minutes.

Some cars show the operating state on the energy flow screen, which helps on a cold test drive but is not proof. What does not help is looking under the bonnet: the components look much like the air conditioning.

Retrofitting one is essentially not possible. It touches the refrigerant circuit, the coolant circuit and the control software; the effort bears no relation to the benefit. If you want one, buy a car that has one.

It does not enlarge the battery. The range loss from cold cells remains, as does the higher driving resistance from dense air and winter tyres. Of the three winter losses it softens exactly one — though the largest one in town.

Nor does it replace preheating on the cable. Heating before you leave while the car is on the wallbox takes the peak demand out of the battery entirely, whether the heat comes from a pump or an element. The two together work best: preheat from the mains, hold the temperature on the road with the pump.

In a mild climate with few frosty days, on a car kept in a garage, and with mostly long journeys, the option does not come back over the time you keep the car. The same holds for a car that hangs on a cable at both ends of the journey and sets off preheated anyway.

But there is a second reason to take it that appears in no consumption calculation: at resale it is an item buyers ask about. Two otherwise identical cars look much the same in a listing but not in a negotiation — and that difference can recover part of the original option price.