What Size Diesel Heater for Caravan Use: 2kW vs 4kW
For most insulated vans above freezing, a 2kW is the right answer to what size diesel heater for caravan use. Canvas walls or sub-zero nights can need 4kW. Autoterm's sizing maths and our diesel heaters show where yours lands.
That's the short version. If your caravan is insulated and your nights mostly stay above freezing, you can stop here and look at the 2kW. If you've got canvas, a pop-top or alpine trips planned, the numbers below are worth five minutes of your time.
How to Size a Diesel Heater for a Caravan
Match your diesel heater to the space you're heating and how chilly your nights get. How cold you feel on the night is a poor guide.
Picture 2am at a frosty camp. Your diesel heater has been flat out since tea, the fan's roaring and the caravan still feels chilly. You lie there wondering if you bought the wrong one.
Five minutes with a tape measure before you buy saves you that night.
Autoterm's Watts per Cubic Metre Method
Autoterm explains its own method in a short video on choosing between a 2kW and 4kW heater. It starts with the volume you're heating in cubic metres, then adjusts for climate and insulation.
Above 0°C, Autoterm allows 100 watts per cubic metre for an insulated compartment and 150 watts for an uninsulated one. Below 0°C, it says every compartment needs at least 220 watts per cubic metre. That's 2.2 times the insulated figure.
Its worked example is a mid-size camper like a VW T6 or Renault Master, at 10 to 17 cubic metres. Allow for furniture and about 8 to 10 cubic metres is left. At 220 watts, 8 cubic metres needs 1.76kW, so Autoterm picks its 2D.
Measuring Your Caravan's Heated Space
Measure the open living space inside your caravan, not the outside. Multiply length by width by ceiling height in metres, then take off what your bed and cupboards fill.
Say your open space is 5 metres long, 2.2 metres wide and 1.9 metres high. That's 20.9 cubic metres before you allow for furniture.
Insulation, Canvas and Pop-Tops
Treat canvas and a raised pop-top as having no insulation. That moves Autoterm's figure from 100 to 150 watts per cubic metre above 0°C, so the same diesel heater covers a third less space.
Owners on Caravaners Forum see the same thing. A Goldstream camper owner worried a 2kW wouldn't cope with canvas walls and went 5kW. Another member, whose 21 to 26 foot van does nicely on a 2kW, still advised the bigger unit for all that canvas.
Camping Below Zero
Below freezing, Autoterm's 220-watt figure applies to insulated and uninsulated compartments alike. On paper, that more than halves what your heater covers.
Owners of insulated caravans often report needing less. A Jayco pop-top owner on the same thread runs a 2kW Webasto in a 17 to 18 foot van, and says that at -10°C outside it ran less than half the time.
How Much Space a 2kW and 4kW Diesel Heater Heats
Run Autoterm's figures backwards and you get the most space each heater covers at full output.
| Conditions | 2kW (Autoterm 2D) | 4kW (Autoterm 4D) |
|---|---|---|
| Insulated, above 0°C | 20 m³ | 40 m³ |
| Uninsulated or canvas, above 0°C | 13.3 m³ | 26.7 m³ |
| Any compartment, below 0°C | 9.1 m³ | 18.2 m³ |
Back to that 20.9 cubic metre example. On Autoterm's numbers, it's just past the 2D's limit above freezing, and past even the 4D's limit below zero.
That's why owner reports matter as much as the maths. Treat the table as your starting point, then weigh up how well your caravan really holds heat.
Diesel Heater Size for a 22ft Caravan
For an insulated 22 foot caravan that mostly sees nights above freezing, owners on Caravaners Forum mostly say a 2kW does the job.
One member runs a 2kW Eberspächer in a 7.2 metre (24 foot) van and says it's "any amount big enough", slowing to almost inaudible in a white frost. Another member, with a 21 foot 6 inch van, chose 5kW and said they'd be hesitant putting a 2kW in one that size.
So a 2D suits most insulated 22 footers. If yours has canvas, a pop-top or regular sub-zero trips, measure it and check the table first.
2kW vs 4kW Diesel Heater for a Caravan
The 4D gives you twice the heat output and about twice the airflow, for $449.95 more. At full output, it also uses about twice the fuel and 12V draw.
Autoterm Air 2D and Air 4D Specifications
These figures come from Autoterm's spec tables for the Air 2D and Air 4D, so you're comparing the maker's numbers, checked on 13 September 2026. Ducting sizes are from Autoterm's manual, and prices are ours on the same day.
| Spec | Autoterm 2D | Autoterm 4D |
|---|---|---|
| Heat output | 0.8-2kW | 1-4kW |
| Hot air flow | 34-86 m³/h | 70-168 m³/h |
| Fuel use | 0.10-0.24 L/h | 0.12-0.51 L/h |
| 12V draw | 10-29W | 10-57W |
| Max altitude, by controller | 1,600-4,200 m | 1,600-4,200 m |
| Heater weight | 2.9kg | 5.5kg |
| Minimum ducting | 60 mm | 90 mm |
| Price | $1,449 | $1,898.95 |
High Altitude Operation and Controller Limits
Autoterm's product pages say both heaters carry a high altitude sensor that lets them heat up to 4,200 metres. Its FAQ gives lower figures: 1,600 metres for air heaters, or 2,500 metres with a high-altitude control unit.
Geoscience Australia lists Mount Kosciuszko at 2,228 metres, above the FAQ's standard figure but below the other two. If you camp in the high country, ask which controller your kit comes with before you buy.
Webasto, Eberspächer and Other Brands
We don't sell Webasto, Eberspächer, Truma Combi or the cheap Chinese kits.
If your caravan already has a working heater from another brand, repairing or replacing it like for like may be the easier job. Autoterm's manual says a swap from another brand means checking the exhaust pipe diameter and fitting a new fuel pump, fuel pipes and wiring.
On the cheap kits, an ExplorOz member warned that many "2kW" units advertised on eBay are really 5kW.
When a 2kW Diesel Heater Is Enough
A 2kW diesel heater like the 2D suits you if your caravan is insulated, your nights mostly stay above freezing and your open space is under about 20 cubic metres.
It's $449.95 cheaper, and its 10-29W draw is easier on your battery overnight. If that's your setup, don't let anyone talk you into the bigger heater.
Smallest Diesel Heater for a Van
The 2D is the smallest model in Autoterm's series, which also includes the 4D, 8D and 9D. At 2.9kg, it's the easier one to fit into your camper trailer or small van.
When a 4kW Diesel Heater Makes Sense
Go for a 4kW diesel heater like the 4D when your caravan sits past the 2D's numbers. That means canvas campers, raised pop-tops, big open areas over about 20 cubic metres, or regular trips below freezing.
Owners who went 2kW in those conditions describe the heater working flat out. In the comments on Autoterm's sizing video, one owner said a 2D took 3 hours to warm their van from 2°C to 19°C, and that running flat out all the time made the fans too noisy. They didn't say where they camp.
On Caravaners Forum, a member who previously had a 2kW Dometic said it ran flat out most of the time and struggled to hold temperature. Their 4kW Webasto idles nicely once it's warm.
Not sure which side of the line you're on? Ring the store on 03 7056 6732 with your caravan's size and where you camp, and we'll help you pick.
Oversizing, Cycling and Carbon Build-Up
Bigger isn't free. In its sizing video, Autoterm says heaters should work on high output, because low output leads to carbon build-up that shortens heater life.
The 4D turns down to 1kW, against 0.8kW for the 2D. In a small, well-insulated caravan, even that low setting can give you more heat than you need. Autoterm's controller modes switch combustion off once your caravan passes the set temperature, then start again as the temperature drops.
One Caravaners Forum member warned, in both threads, that an oversized heater cycles on and off, and that starting is when it uses the most electricity and makes the most noise. On the canvas thread, another member knew of owners who oversized and ended up with on-off noise and extra electricity use.
On ExplorOz, a different member said a 5kW run at its minimum in a typical caravan soots up and stops working.
Autoterm's manual also asks you to run your heater for 20 minutes at full output once a month, even in summer, to keep the fuel pump healthy.
Size it right and your heater ticks over quietly while you sleep.
Running a Diesel Heater Overnight in a Caravan
A diesel heater gets its heat from diesel. Your 12V battery runs the fan, fuel pump, glow plug and controls, which is why your overnight numbers are small.
Fuel Use per Hour and per Night
The 2D uses 0.10 to 0.24 litres of diesel an hour, and the 4D uses 0.12 to 0.51 litres. Over a 10-hour night, that's 1 to 2.4 litres for the 2D and 1.2 to 5.1 litres for the 4D.
That's why owners run diesel heaters for long stretches. The owner of a 23 foot caravan on Caravaners Forum ran theirs about 17 hours a day for 5 days at Bingara, and called it excellent, fuel use included.
12V Current Draw at Start-Up and Running
Once it's running, the 2D draws 10 to 29 watts, about 0.8 to 2.4 amps at 12V. The 4D draws 10 to 57 watts, or about 0.8 to 4.8 amps.
Start-up is different. Autoterm's manual describes a self-test of under a minute and a glow plug phase of up to 120 seconds. Then the heater runs at 100% for up to 15 minutes before it settles to your setting.
Autoterm doesn't publish a start-up current figure, so don't plan your battery around the running figure alone. The 12V models also switch themselves off if your battery voltage drops below 10V, so a tired battery ends your night early.
Parked for a long stretch? Your heater still draws 30 to 40 milliamps when it's off. Over 30 days that's 21.6 to 28.8 amp-hours, so Autoterm says to disconnect it for long-term storage.
How Long a 100Ah Battery Runs a Diesel Heater
On paper, a 100Ah battery runs the 2D for about 41 hours flat out and the 4D for about 21 hours, with nothing else connected. On the lowest setting, both stretch to about 120 hours.
Real life is shorter. Your fridge and lights share that battery, and every start costs extra. How much of the 100Ah you can use also depends on your battery type, so check your battery maker's figure.
If your house battery is already stretched, a 100Ah lithium battery pack gives your heater its own supply.
Portable Power Station Size for a Diesel Heater
A power station can run your heater from a 12V DC outlet that handles its draw. Work from the nightly figures: 100 to 290 watt-hours for the 2D and 100 to 570 watt-hours for the 4D over 10 hours, plus start-ups and the rest of your gear.
Browse our portable power stations or use the power station finder to match one to your full load list.
Exhaust, Carbon Monoxide and Installation
Product Safety Australia warns that carbon monoxide is toxic, colourless and odourless. It says the main cause of accidental poisoning is burning fuel where there's little or no ventilation, and your heater burns fuel.
Autoterm's installation manual says exhaust gases must be carried outside the vehicle and kept out of the interior. The intake and exhaust should sit at least 20 cm apart.
The exhaust pipe can reach 500°C, and the manual says the vehicle must carry a fire extinguisher. It also says installation must be done by certified specialists from organisations Autoterm approves, because a poor install is a fire hazard.
We don't install heaters, so follow the manual and use a qualified installer. Autoterm's product pages say the warranty extends to 3 years or 2,000 hours when a certified Autoterm service registers the installation.
A Caravaners Forum member said they'd seen installs where heated air blows straight back into the intake. Get yours fitted properly and you can sleep warm without worrying about the pipework.
Choosing Between the Autoterm 2D and 4D
An insulated caravan that mostly sees nights above freezing gets the Autoterm 2D. Canvas, a raised pop-top, a big open space or regular sub-zero camps point to the Autoterm 4D.
Still torn? Ring the store on 03 7056 6732 or text 0476 330 590, and we'll work it out with you. Both sit in our Autoterm range.
Related reading: choosing a battery or an all-in-one unit, sizing a unit to run your fridge, how long a battery unit runs an electric heater and using a generator for your caravan.
