Battery Runtime Calculator

Last updated: September 3, 2026

Quick answer A typical RV furnace fan drawing 4 A at a 40% duty cycle uses about 1.6 A average — a 100 Ah AGM battery (50% usable) runs 30+ hours of that cycling alone. Add real loads below and the calculator does the same math for your exact setup.

Loads (defaults are typical values — change anything)

Estimated runtime
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How this calculator works

Every load is converted to average 12-volt amps. DC loads contribute amps × duty % directly. AC loads running through an inverter contribute watts ÷ (12 V × 0.88 inverter efficiency) × duty %. The average draw I is the sum of all loads.

Usable capacity. Your bank's rated amp-hours are first reduced by the chemistry's usable depth of discharge (50% for flooded/AGM, ~90% for LiFePO4). For lead-acid, a Peukert correction then scales capacity by (C ÷ (I·20))k−1, where C is rated Ah, I is your average draw, and k is 1.2 for flooded / 1.1 for AGM. Lithium gets no correction — its capacity barely changes with discharge rate.

Runtime = usable Ah ÷ average amps. The result is hours of your mixed load profile at room temperature. Cold weather cuts lead-acid capacity noticeably (plan on −20–30% near freezing); lithium loses less but charging below freezing is restricted. Battery age reduces every chemistry's capacity.

Worked example

A 100 Ah AGM battery (usable 50 Ah), furnace fan at 4 A with a 40% duty cycle, 12 V fridge at 4 A with a 40% duty, and LED lights at 2 A with 25% duty. Average draw = 1.6 + 1.6 + 0.5 = 3.7 A. Peukert factor at 3.7 A ≈ 1.07, so usable capacity ≈ 53.5 Ah → ≈ 14–15 hours. Switch to a 100 Ah lithium and the same loads run ≈ 24 hours. That chemistry gap — not magic — is why identical-sounding forum answers differ.

Assumptions & sources

  • Depth-of-discharge rules (50% lead-acid, ~90% lithium with BMS cutoff): standard battery-industry planning guidance; see Battery University and your battery manufacturer's manual.
  • Inverter efficiency 88%: typical small pure-sine inverters run 85–92%; check your unit's spec sheet.
  • Default furnace fan draw: Atwood/Dometic Hydro Flame spec plates rate 1.8–2.9 A by model (Atwood 8535-IV service manual); measured running draws are commonly 3–4 A. Larger or older blowers can draw more — check your furnace's plate. See our furnace runtime guide for per-model numbers.
  • Peukert exponents (flooded ≈ 1.2, AGM ≈ 1.1): typical published values; vary by battery.

Frequently asked questions

How accurate is this runtime estimate?
It is a planning estimate. The two biggest real-world factors it cannot see are temperature (a lead-acid battery can lose 20–30% of usable capacity near freezing) and battery age. Treat the result as a middle-of-the-road estimate, and check your appliance nameplates for exact draws.
Why do I need to enter a duty cycle?
Most RV loads do not run continuously. A furnace fan only draws current while the burner is calling for heat — often 30–50% of the time on a cold night. A fridge compressor cycles similarly. Ignoring duty cycle is the main reason forum estimates for the same question range from hours to days.
Why can I only use half of my lead-acid battery?
Flooded and AGM lead-acid batteries age quickly when routinely discharged deeply; the widely used planning rule is to count on about 50% of rated amp-hours. Lithium (LiFePO4) batteries can use roughly 90% before the battery management system cuts off. See our lead-acid vs lithium guide.
What is Peukert and why does it matter here?
Lead-acid batteries deliver fewer total amp-hours at high discharge rates than their 20-hour rating suggests. This calculator applies the Peukert correction to lead-acid banks at your actual average draw; lithium is essentially unaffected, which is one reason a "100 Ah" lithium runs loads longer than a "100 Ah" lead-acid.

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