15 kW vs 35 kW: Sizing Your HVCH for Heavy-Duty Trucks

A few days after the quotation is issued, the customer will usually send an email. They will have the product specifications for the ‘Q Series 15–35 kW Coolant Heaters’ to hand, with two power ratings circled, and will ask: ‘Which one do we actually need?’

Rather than simply providing a model number as an answer, we ask three questions in return: How much does the battery pack weigh? How low does the ambient temperature drop whilst the lorry is parked overnight? How many minutes after the driver arrives must the lorry be ready to depart?

In short, in practice, the deciding factor is often not the lorry itself, but the ‘departure time window’. The 35 kW heater is not merely an ‘upgrade’ of the 15 kW model; they perform the same task; the only difference lies in the time taken to complete it.

First, calculate the total weight of the battery pack.

Take a long-haul tractor unit as an example: its battery pack capacity typically ranges between 300 and 350 kWh. Leaving the heater aside for now, consider the weight of the battery pack—a pack of this size weighs approximately two tonnes.

Next, consider the heat requirement. The specific heat capacity of a battery cell is approximately 1 kJ/(kg·°C). To raise the temperature of a two-tonne (2,000 kg) battery pack by 20°C, a total of 40,000 kJ (i.e. 40 megajoules) of heat is required based on this specific heat capacity—which means that approximately 11 kWh of thermal energy must actually be transferred into the battery cells. As losses occur during heat transfer via the coolant, casing and pipework, 11 kWh represents only the ‘lower limit’ of the heat requirement, not the total requirement.

Finally, divide the total heat by the heating power. If a 15 kW heater is used, it would need to operate at full power for approximately 45 minutes to complete the heating process (the time required would be longer if the aforementioned losses are taken into account); whereas with a 35 kW heater, the same heating process can be completed within 20 minutes.

This simple division usually resolves most enquiries of this nature.

Consider this scenario: a lorry has become completely ‘frozen solid’ in -25°C conditions, the driver has arrived, and the lorry is due to depart in 30 minutes—in such a scenario, a 15 kW heater is clearly inadequate. Consider another scenario: the same lorry is parked indoors, connected to the depot’s mains supply, and has two hours to preheat. The calculation logic is completely reversed: the excess capacity actually becomes the energy consumed to heat the cab. The same lorry, the same battery pack, yet the conclusions are diametrically opposed. This is why we refuse to base our choices solely on the rated parameters on the nameplate.

The heating requirements for a lorry cab are not usually determined by its size; sometimes, they depend instead on the area of the glazing.

Once the interior has warmed up, maintaining the cab temperature in bitterly cold conditions requires only a few kilowatts of power. After all, the cab is a small space. It is the first ten minutes that consume the vast majority of electrical energy: at this stage, the seats are cold, the interior panels have not yet warmed up, and the windscreen must be defrosted before the vehicle can start. The defrosting process requires a high intensity of heat output for a short period, and this is precisely when the battery system demands the highest heating power.

The situation is entirely different for buses; therefore, one should not simply apply data from lorries when selecting a model. For a 12-metre-long city bus, where the doors are opened every few hundred metres, the heating power required may be two to three times that of a lorry cab. We have discussed the planning considerations for such extreme low-temperature operating conditions in detail in our guidance on ‘Preheating electric bus batteries in −30°C conditions’.

Next, we must consider the circuit’s capacity.

On paper, the power rating appears to be adequate, but it is the current that truly puts the hardware to the test. Take a 15 kW heater, for example: at a voltage of 400 V, its current is close to 38 A. This is no longer a simple circuit, and this is merely a smaller model.

The 35 kW model illustrates this difference even more clearly: at 800 V, its current is approximately 44 A, which remains within manageable limits; however, at 400 V, the same unit requires a current of around 88 A—meaning that thicker cables and larger fuses must be used, and one might even be forced to redesign the wiring harness at a later stage of the project, which is clearly a hassle that no one wishes to face. For this very reason, the Q Series’ voltage range spans from 400 V to 1,100 V. On an 870 V platform, high-power models no longer pose spatial layout challenges; in fact, we have been supplying equipment for truck projects utilising 870 V DC systems since 2021.

The situation with mining trucks takes this a step further: no one would install just a single, oversized heater. Our solution is to connect six 35 kW heaters in parallel, delivering a total power output of 210 kW. This is because the actual thermal load is indeed very high, and by using standardised units, maintenance personnel can easily replace any one of them at the roadside—whereas if a bespoke ‘giant’ heater were used, replacing it at the roadside would be far from straightforward.

A case-by-case analysis

For the 15 kW model, it is the ideal choice in mild climates with moderate battery pack capacity, provided the truck returns to the depot every night and can be fully preheated using an external power source (shore power). Regional distribution transport is a typical application scenario, and we are happy to supply lower-power models in such cases; blindly pursuing high power would be pointless.

The 35 kW model, on the other hand, is suitable for the following situations: ambient temperatures consistently below –20°C (cold soak conditions); preparation time before departure of less than half an hour; battery pack capacity exceeding 300 kWh; or where the vehicle must be heated whilst in operation (rather than whilst charging). Mining and construction lorries fully meet these criteria; long-haul transport fleets operating in northern regions and parked outdoors overnight also largely fit this profile.

If the wrong model is selected, the consequences are often severe: the heater is forced to operate at full capacity, the coolant temperature consistently fails to reach the target value, the battery charges slowly, and the driver is trapped in a vicious cycle where the ideal operating conditions can never be achieved. If the selection is misaligned, it may lead to ‘hidden’ wastage of resources: for example, selecting a unit weighing 12 kg when a lighter model would actually suffice; or configuring cables and fuses with specifications that are excessively high, exceeding the load requirements under actual operating conditions.

Therefore, please provide the following four key parameters: battery pack capacity, the expected minimum cold soak temperature (i.e. when the vehicle is stationary in extremely cold conditions), the departure time window, and the cabin heating requirements. Before recommending a specific model, we will present you with a detailed selection calculation process; if 15 kW of power is sufficient to meet your needs, we will recommend a 15 kW model. For a complete list of parameters that OEMs need to clarify prior to selection, please refer to the guide ‘How to Select a High-Voltage PTC Coolant Heater’; for further information on our product range, please visit our ‘High-Voltage Coolant Heater Product Range’ page.

Frequently asked questions on this topic:

Can two smaller heaters be used as a substitute for a single 35 kW heater?

Yes. In large vehicles, it is common practice to use standard heaters in parallel; the solution we provide for mining trucks achieves a total power output of 210 kW precisely in this way. Furthermore, this approach offers a degree of redundancy: even if one unit fails, the vehicle can continue to operate without being forced to shut down.

Does a higher heater power rating drain the battery faster?

It will drain the battery more quickly, but the total energy consumption will not increase. The total energy required to heat a battery pack depends on its mass and the temperature rise. A more powerful heater can complete the heating process in a shorter time, and during this process, less heat is typically lost to the surrounding environment.

If the heating power requirements for the battery pack and the cab differ, how are the heater specifications ultimately determined?

Take the higher of the two values and verify this against the departure time window. In practice, the specifications for heaters in heavy-duty lorries are typically determined by the battery pack’s preheating requirements, whilst the cab heating shares the same circulation circuit.

Are you selecting a heater for a lorry project? Please provide the battery pack capacity, cold soak conditions, departure time window and voltage platform. We will provide you with detailed calculation results and recommended product ranges, accompanied by the relevant selection criteria.