A heater that cycles on and off all night wastes fuel and wears out faster, and the mistake usually traces back to sizing by van length instead of heat loss. The number that matters is how many watts your van loses through walls, floor, windows, and ventilation gaps at your coldest camping temperature. Get that figure right and the wattage decision takes about a minute.
Below: the heat-demand formula, a quick reference table, the installation limits that trip people up, and settings paths worth knowing before you order.
Work Out Heat Demand Before You Pick Wattage
Heat demand depends on surface area, insulation quality, air leakage, and temperature rise, not van length. The standard formula is Heat demand (W) = A × K × ΔT + 0.359 × V × n × ΔT, where A is surface area in square meters, K is the insulation and sealing coefficient, ΔT is target indoor temperature minus outdoor temperature, V is interior volume in cubic meters, and n is air changes per hour.
The second half is the part most people skip. Ventilation losses are real — every air change per hour drags cold outside air in and forces the heater to reheat it.
Running The Numbers In Practice
Camping at 20°F and wanting 68°F inside gives ΔT of 48°F (about 27°C). A well-insulated mid-size conversion with moderate ventilation needs far fewer watts than the same shell with bare metal walls and a leaky roof vent. Tighten the shell and seal the gaps, and required wattage drops — often enough to move from 5 kW down to 2.5 kW.
Air changes per hour is the cheapest lever you control. A closed roof vent, sealed door gaskets, and insulated window covers cut n substantially, and every point of n you remove subtracts watts.
| Van Type & Condition | Recommended Heater Output | Why This Size |
|---|---|---|
| Insulated T5, T6, Vito, Trafic, Vivaro, Transit Custom | 2.5 kW | Insulation holds heat; larger units short-cycle |
| Insulated mid-size conversion, three-season use | 2.5 kW | Covers worst-case nights without oversizing |
| High-roof, long-wheelbase van, hard winter | 5 kW | Large volume plus deep cold pushes demand up |
| Uninsulated or drafty shell | Insulate first, then size | No heater wattage fixes missing wall insulation |
| Small van, mild climate, insulated | 2 kW or 2.5 kW | Low ΔT keeps demand well under 2.5 kW |
| Any van powered from starter battery | Not recommended | Run from a leisure battery on its own fuse |
| Campervan at 68°F target, 20°F outside | Sized by the formula above | ΔT of 48°F drives the wattage calculation |
To compare actual units rather than calculate, our roundup of tested 12V van heater options covers real models against these sizing bands.
Install It Where The Manual Says, Or The Best Size Still Underperforms
A correctly sized 12V heater only works as rated when installation respects the manual’s airflow and mounting rules. Autoterm’s installation instructions state the unit is designed for a 12V power supply, and carry several constraints worth reading before you drill.
- Air inlet direction: Position the heater so the air inlet doesn’t face oncoming air while moving — prevents snow entry and obstruction, allows drainage.
- Combustion air: Must not enter the vehicle cabin. Route and seal it so exhaust gases and combustion intake stay outside the living space.
- Electrical: Fix circuit components to vehicle components and protect them from overheating or deformation.
Power it from the leisure battery, on its own fuse, never the starter battery — a cold morning with a flat starter battery is the classic outcome of ignoring that rule.
Common Sizing Mistakes That Cost Money
The most expensive error is buying 5 kW for a small insulated van, causing unnecessary cycling, uneven heat, and higher fuel use. Sizing by van length alone produces the same result, because length says nothing about insulation, leakage, or climate.
Ignoring duct and mounting limits is the second trap: exceed the 5 m combined pipe limit or mount off-level, and the heater runs hot, loud, or shuts down on fault. Both look like sizing failures but are installation failures.
FAQs
Is 2 kW enough for a small insulated van?
For a small, well-insulated van in mild to moderate cold, 2 kW covers the load comfortably. The catch is deep winter: drop ΔT into the 40s°F and a 2 kW unit runs near full output all night. If you camp below freezing regularly, 2.5 kW gives headroom without the cycling problems of a 5 kW unit.
Can I run a 12V heater off my starter battery?
No — run it from a leisure battery on its own dedicated fuse. Starter batteries are built for short bursts of high current, not hours of steady draw, and deep discharging damages them. A separate fused circuit also keeps a heater fault from taking down the vehicle’s starting system.
Does a bigger heater warm the van faster?
Marginally, and it’s not worth the trade. An oversized heater reaches temperature quickly, then cycles on and off repeatedly, wearing the glow plug and burner and producing uneven cabin heat. A correctly sized unit runs steadily at partial output — quieter, more fuel-efficient, and easier on components over many seasons.
References & Sources
- Autoterm. “Planar Heaters Installation Instructions.” Supports 12V power supply requirement, combustion air separation, duct limits, service clearance, and mounting angle.
- Webasto. “Webasto Pro 90 ST Installation Manual.” Installation guidance for vented cabin air heaters.
- DEFA. “DEFA WarmUp Kit Generic Installation Instructions.” Vehicle heater installation and electrical protection guidance.