What Size Battery Does a Diesel Heater Need?

Published October 6, 2026. Built by Jeremy Panasuk.

Direct answer: The US Air Top 2000 STC page lists power consumption of 1.25–2.45 amps (14–29 watts). Eight hours is this page’s example night, not a Webasto runtime. At 12 V, 2.45 A × 8 h is 235.2 Wh, and Ah = (235.2 × 1 × 1.25) ÷ 12 ÷ 0.80 = 30.625 Ah. The low end, 1.25 A × 8 h, is 120 Wh and 15.625 Ah. The same high case at 0.50 depth of discharge is 49.0 Ah. A 120-second start at 7–8 A is 0.2333–0.2667 Ah and is not multiplied by the eight hours.

The formula is this site’s, Ah = (Wh/day × autonomy days × 1.25) ÷ voltage ÷ depth of discharge, with one day because the example is one night. The battery bank calculator is the table for a whole daily load. A heater is one line on that list. The van page does not publish a heater wattage. The currents on this page are the ones to use.

Three current statements, not one average

Webasto’s US product page for the Air Top 2000 STC prints power consumption as 1.25–2.45 amps (14–29 watts), operating voltage as 10.5–16 or 21–32 V, and rated voltage as 12 or 24 V. This page does not “correct” 1.25 A × 12 V = 15 W against the printed 14 W, or 2.45 A × 12 V = 29.4 W against the printed 29 W. Both pairs are quoted. The night math below uses the amp endpoints times 12 V, because that is a 12 V bank.

The Webasto Thermo & Comfort Australia installation guide says the Air Top 2000STC requires 72–85 watts, 6–7 amps during start-up. It says cigar lighter sockets or auxiliary outputs for other electronic accessories are not adequate for the heater’s power supply. It says that if any battery management system is used, contact a Webasto authorised dealer so the shutdown cycle is maintained and carbon build-up is prevented. On the fuse figure, F1 is 20 A for a 12 V heater and 15 A for a 24 V heater, and F2 is 1 A for the multi-controller. A later note in the same guide says “15A fuse for both 12V and 24V heaters.” Those two fuse lines are both printed. This page does not pick one.

That later note sits next to the sentence that carbon build-up is not a warrantable condition. The situations listed there include under-voltage — the system should have at least 12.5–13 V — and low current: 7–8 A for the initial start-up phase for 120 seconds, then 2–3 A once the flame is established and the glow plug goes out. That 12.5–13 V line is the guide’s carbon note. It is not the US page’s 10.5 V operating-voltage floor, and it is not a state-of-charge percent from a battery table.

StatementWhat it printsWhat this page does with it
US product page1.25–2.45 A (14–29 W)The 8-hour example at 12 V
Australia guide, electrical72–85 W, 6–7 A at start-up; F1 20 A at 12 V, 15 A at 24 VQuoted. Not averaged into the night.
Australia guide, carbon note7–8 A for 120 seconds, then 2–3 A; also “15A fuse for both 12V and 24V”120 seconds stay 120 seconds. The second fuse line is not erased.

One example night at the US amp band

2.45 × 12 × 8 = 235.2 Wh. (235.2 × 1.25) ÷ 12 ÷ 0.80 = 30.625 Ah. 1.25 × 12 × 8 = 120 Wh. (120 × 1.25) ÷ 12 ÷ 0.80 = 15.625 Ah. Change only the depth of discharge on the high case: (235.2 × 1.25) ÷ 12 ÷ 0.50 = 49.0 Ah. One 100 Ah battery is larger than 30.625 Ah nameplate. That does not mean the van’s other loads fit in the leftover amp-hours. Add the heater’s watt-hours to the rest of the day, then run the six steps.

The Australia note’s 2–3 A running figure, applied to the same 8-hour example at 12 V, is 2 × 12 × 8 = 192 Wh and 3 × 12 × 8 = 288 Wh. That is not the US page’s 120–235.2 Wh band. This page does not pick a midpoint.

Startup stays on its own clock. 8 A × 120 ÷ 3,600 = 0.2667 Ah. 7 A × 120 ÷ 3,600 = 0.2333 Ah. The electrical section’s 6–7 A start is a third start sentence and is not given a duration in that sentence, so this page does not assign it 120 seconds. Multiplying 8 A by 8 hours would be 64 Ah and would treat a two-minute glow-plug interval as an all-night current. Do not do that.

12 V in the example, 24 V left as printed

The US page rates the heater at 12 or 24 V and prints one amp band beside 14–29 W. 1.25 A and 2.45 A line up with that watt band at 12 V, not at 24 V. This page does not halve the amps for a 24 V heater, because the page did not print a second amp band. The Australia guide’s fuse line does distinguish voltage: F1 is 20 A at 12 V and 15 A at 24 V. That is the fuse callout in the guide. It is not a wire-gauge chart.

A common mistake

Folding the three documents into one average current, then sizing the bank from a watt figure this page does not publish. 2.45 A at 12 V for eight hours is 235.2 Wh before this site’s 1.25. Using a different current as if it were that figure changes the amp-hour result. The second mistake is reading 12.5–13 V on the heater guide as Trojan’s 80% mark or Lifeline’s boost trigger. Those battery documents are not this heater page. A rested-voltage table is how to check a 12V battery’s capacity.

Limits, and when to call a pro

This page does not describe fuel plumbing, exhaust, or mounting. The Australia guide says repairs have to be carried out by authorised Webasto service dealers, and that parts other than genuine Webasto service parts void the warranty. If a battery management system is used, the guide’s instruction is to contact a Webasto authorised dealer about the shutdown cycle. Cold-charge cutoffs for a LiFePO4 bank are on the lithium charger page, and they are battery limits, not heater limits. A cabin that heats with a large electric element is a different load; the cabin page already flags electric heat as the case that stops being a battery problem.

Run the US amp endpoints

The form multiplies a published US amp endpoint by 12 V and by the hours you set. Eight hours is the example. Startup amp-hours are printed beside the result and are not multiplied by those hours.

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Webasto Air Top 2000 STC on Amazon

The whole-day table is the battery bank calculator.

Sources

Compiled with AI assistance from the sources below. Spot an error? Email jpanasuk@gmail.com.