Camper Heater Size Calculator: How Many kW Do You Need?
Enter your rig’s interior volume, the coldest outside temperature, your target inside temperature and your insulation level. The calculator multiplies volume by the temperature gap and a loss factor, then adds 50 % for warm-up. A large, averagely insulated van kept at 68 °F with 23 °F outside needs about 0.9 kW.
This calculator estimates how much heating power your camper van or RV needs. Enter the interior volume, the coldest outside temperature you expect, the inside temperature you want and your insulation level. It returns the heat loss to cover in watts and a recommended heater output in kW, with a tip on which heater size fits.
The inputs are metric. Use °C (°F − 32, then × 5 ÷ 9) and cubic meters (cubic feet ÷ 35.3), or pick a preset.
Heat loss to cover: 0 W
Recommended output (with warm-up margin): 0 kW
Formula: W = volume × (inside − outside) × loss per m³ per degree. Recommended = W × 1.5. It is an estimate: thermal bridges, windows and wind change the result.
How does the calculation work?
Three steps: temperature gap, heat loss, then a warm-up margin.
- Temperature gap = inside − outside temperature, in °C.
- Heat loss (W) = volume (m³) × temperature gap × loss factor. The factor, which includes air leakage, is 1.5 for good insulation (closed-cell foam, double-glazed windows), 2.5 for average (factory or partly insulated) and 4 for poor or none.
- Recommended output (kW) = heat loss × 1.5 ÷ 1,000. The extra 50 % lets the heater warm a cold rig in reasonable time.
Example, the default values: a 10 m³ high-roof van, −5 °C (23 °F) outside, 20 °C (68 °F) inside and average insulation. Heat loss: 10 × 25 × 2.5 = 625 W. Recommended: 625 × 1.5 = about 0.94 kW, or roughly 3,200 BTU/h (W ÷ 0.293).
How do I read the result?
Treat the recommended kW as the output your heater should reach comfortably, not the maximum on the box.
| Recommended output | What the calculator says | What to look at |
|---|---|---|
| Up to 2.2 kW | A 2 kW diesel heater is enough, or a 5 kW unit running on low | The best diesel heaters in the 2 kW class |
| 2.2 to 5 kW | Look at 4-5 kW diesel heaters; a 2 kW unit would run flat out | 5 kW diesel heaters, or propane heaters used with ventilation |
| Over 5 kW | Two heaters, or improve the insulation first | The best van insulation, then re-run the calculator |
On shore power, compare the result with what your hookup can supply before buying an electric space heater; the RV power calculator does that check. To see what your chosen diesel heater will burn, try the diesel heater fuel calculator.
Be wary of heater listings with inflated output. A liter of diesel holds about 10 kWh, so a heater burning 0.2 L/h can deliver at most about 2 kW. Our guide on what size heater you need explains how to spot the difference.
Frequently asked questions
How does the heater size calculator work?
It multiplies interior volume (m³) by the gap between inside and outside temperature (°C) and by a heat-loss factor for your insulation: 1.5 for good, 2.5 for average and 4 for poor. That gives watts of heat loss; it then multiplies by 1.5 for a warm-up margin.
How do I convert my RV’s size into cubic meters?
Multiply inside length × width × height in feet to get cubic feet, then divide by 35.3. The presets give typical values: 6 m³ for a small van, 10 m³ for a high-roof Sprinter-size van, 18 m³ for a mid-size motorhome and 25 m³ for a Class C or travel trailer.
Is a 2 kW diesel heater enough for a van?
Often, yes. With the default values, a 10 m³ van with average insulation needs about 0.9 kW at 23 °F (−5 °C) outside. The calculator suggests 2 kW whenever the recommended output is 2.2 kW or less.
How do I convert the result to BTU/h?
Divide watts by 0.293, or multiply kW by about 3,412. For example, 0.94 kW is about 3,200 BTU/h.
Why is my result only an estimate?
The loss factors are averages. Thermal bridges, single-glazed windows, wind and how often you open the door all change the real figure, so round up when in doubt.