When I help with diesel heaters in a boat, I don't just look at the heat output. Power consumption is at least just as important, because the fan, control unit, fuel pump, and start-up sequence must be powered by the battery.
Watts and amperes – how it correlates
A quick estimate is: amperes ≈ watts / voltage. A consumption of 24 W therefore corresponds to roughly 2 A in a 12 V system. That is an estimate; the actual current depends on the real voltage and operation.
A concrete example from AUTOTERM
For the AIR 2D, AUTOTERM specifies a normal power consumption of 10–29 W and for the AIR 4D 10–57 W. This corresponds roughly to approx. 0.8–2.4 A and 0.8–4.8 A at 12 V during operation. The AIR 9D has a significantly larger range, so I always recommend using the data for the specific model rather than guessing based on kW.
Important: start-up and shut-down are not the same as stable operation. The glow plug and other components can cause a higher short-term power demand, so the cable, fuse, and battery must not be sized based on the lowest operating figures.
How to calculate daily consumption
If a heater uses an average of 20 W for 8 hours, the energy consumption is 160 Wh. In a nominal 12 V system, this corresponds roughly to about 13 Ah. Use this only as a calculation example – your heater will modulate up and down, and the battery voltage is not a constant 12.0 V.
What I want you to check
- The specific model's power consumption in the manual.
- How many hours you expect to use the heater.
- Whether the battery also needs to power the refrigerator, pumps, lights, and electronics.
- That the cable and fuse match the heater's requirements – including during start-up.
Calculate the boat's total power consumption →
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Source and verification
The specific AUTOTERM figures have been verified against the manufacturer's current AIR documentation. Always use the manual for your specific model.