How Long Will a Battery Run an RV Furnace?
Last updated: September 3, 2026
Ask this on any RV forum and you'll get answers from "2–3 hours" to "8–9 days". They're almost all correct — for unspoken assumptions. This page states the variables, gives the real draw numbers, and works the examples. If you want your own setup computed, the battery runtime calculator does the same math with your inputs.
Step 1 — what the furnace actually draws
An RV furnace is a propane burner with an electric fan. The battery powers the fan, igniter, and control board. Manufacturer spec plates rate the draw modestly; measured running draws are usually a bit higher:
| Furnace model class | Spec-plate rating | Typical measured running |
|---|---|---|
| Atwood/Dometic Hydro Flame 8525 (25k BTU) | 1.8 A rated | ≈ 2–3 A running |
| Atwood/Dometic Hydro Flame 8535 (35k BTU) | 2.9 A rated | ≈ 3–4 A running |
| Suburban SF-30/35 class | ≈ 2.9–3.4 A rated | ≈ 3–5 A running |
| Large furnaces / older blowers | up to ≈ 7–11 A | check your spec plate |
Sources: Atwood/Dometic Hydro Flame service documentation (8535-IV series; rated 1.8–2.9 A by model) and common owner measurements (3–4 A running, briefly higher at blower start). Check your own furnace's plate — larger or older blowers can exceed these.
Step 2 — the variable everyone skips: duty cycle
The fan only runs while the thermostat calls for heat. Duty cycle depends on the gap between outside temperature and your setpoint, insulation, and how much heat you're leaking to the underneath:
| Conditions | Typical burner duty cycle | Meaning for a 4 A fan |
|---|---|---|
| Mild night (≈10 °C / 50 °F) | 15–25% | ≈ 0.6–1.0 A average |
| Cool night (≈0 °C / 32 °F) | 30–50% | ≈ 1.2–2.0 A average |
| Hard cold (−10 °C / 14 °F and below) | 60–80% | ≈ 2.4–3.2 A average |
These ranges are planning rules of thumb assembled from owner-reported cycling behaviour — not manufacturer data, because manufacturers don't publish duty cycles. The honest way to get yours: run one night and read your battery monitor.
Step 3 — the arithmetic
runtime ≈ usable Ah ÷ average amps, where usable Ah is 50% of rated for AGM/flooded (≈90% for lithium),
nudged down further for lead-acid at high draws (Peukert). For example:
| Battery bank | Scenario | Avg draw | Runtime |
|---|---|---|---|
| 100 Ah AGM (50% usable) | 40% (cool night) | 1.6 A | ≈ 30 h |
| 100 Ah AGM + fridge 1.6 A avg + lights 0.5 A avg | 40% furnace | 3.7 A | ≈ 14 h |
| 100 Ah LiFePO4, same mixed loads | 40% furnace | 3.7 A | ≈ 24 h |
| 200 Ah AGM, mixed loads as above | 40% | 3.7 A | ≈ 29 h |
| 100 Ah lithium, hard cold snap | 70% furnace (4 A fan) | 2.8 A + loads | ≈ 13–14 h |
Step 4 — the five things that actually move the answer
- Duty cycle — weather and thermostat setting. Doubling it halves runtime. This is the whole reason forum answers disagree.
- Everything else on the battery — the fridge, lights, detectors, and water pump share the bank. A gas-mode absorption fridge (~1.4 A when cycling on DC mode) can rival the furnace fan.
- Chemistry — lithium's ~90% usable capacity and lack of Peukert penalty roughly double the real runtime of the same "100 Ah" label. See lead-acid vs lithium.
- Temperature — lead-acid loses 20–30% of capacity near freezing, precisely when you need the furnace. Lithium keeps more capacity but must not be charged below freezing without a low-temp-charging BMS.
- Battery age and charge state — an "80% charged" 100 Ah battery is an 80 Ah battery before any other losses.
Practical verdicts
- One 100 Ah AGM, furnace + fridge + lights, cool night: plan for ~14 hours. That's one cold night, not a weekend.
- Two 6 V golf-cart batteries (≈220 Ah, 50% usable): a comfortable weekend at 40% duty.
- 100 Ah lithium, same loads: ≈24 h, and it recharges from solar/alternator far faster thanks to high charge acceptance.
- Multi-day hard freeze: no reasonable lead-acid bank carries a furnace alone — you're recharging daily or running propane heat management (see propane math).