--- title: "12V Battery Runtime Calculator for RV, Boat and Camping" description: "How long will a 12V battery last? Calculate runtime for 12V deep-cycle, AGM and LiFePO4 batteries running fridges, lights and fans in an RV, van or boat." canonical: https://batteryruntimecalculator.com/12v-battery-runtime-calculator language: en updated: 2026-11-11 source_revision: 59f9d32 maintainer: Battery Runtime Calculator (hello@batteryruntimecalculator.com) --- # 12V Battery Runtime Calculator Work out how long a 12 V deep-cycle battery will run your fridge, lights, fans and electronics in an RV, camper van, boat or off-grid cabin. Updated November 11, 2026 Quick answer Runtime (h) = Ah × usable % ÷ load in amps. A 12 V 100 Ah LiFePO4 battery runs a 50 W load for about **21 h 36 min**; a 100 Ah AGM battery, used only to 50%, about **12 h**. Estimated runtime 21 h 36 min ≈ 21.6 hours on 1,200 Wh at 50 W Usable energy 1,080 Wh Battery current 4.2 A Power from battery 50 W Discharge rate 0.04C How this was worked out 1. Battery energy: 1,200 Wh rated 2. Power drawn from the battery: 50 W ÷ 100% efficiency = 50 W 3. Rate effect (Peukert 1.05): the battery delivers 100% of its rating at this load 4. Runtime: 1,200 Wh × 100% × 90% usable ÷ 50 W = 21.6 h Runtime at other loads | Load | Runtime | | --- | --- | | 13 W | 3 days 11 h | | 25 W | 43 h 12 min | | 50 W | 21 h 36 min | | 75 W | 14 h 14 min | | 100 W | 10 h 32 min | ## 12V 100Ah battery runtime by load Hours from one 12 V 100 Ah battery with the load wired directly to it (DC, no inverter). LiFePO4 is counted to 90% usable, AGM to 50%. | Load | Watts | LiFePO4 | AGM | | --- | --- | --- | --- | | 1 A | 12 W | 3 days 18 h | 2 days 2 h | | 2 A | 24 W | 45 h | 25 h | | 5 A | 60 W | 18 h | 10 h | | 10 A | 120 W | 8 h 42 min | 4 h 30 min | | 20 A | 240 W | 4 h 12 min | 2 h 2 min | | 50 A | 600 W | 1 h 36 min | 42 min | At 50 A the AGM battery is drained in well under an hour, much less than the simple “100 Ah ÷ 50 A × 50%” would suggest. That is Peukert’s law: lead-acid loses usable capacity as the current rises. ## Working out runtime with amps On a 12 V system it is often easier to think in amps, because battery capacity is already in amp-hours: ``` Runtime (h) = battery Ah × usable % ÷ load (A) ``` Example: a 100 Ah battery, 90% usable, running a 4 A load: 100 × 0.9 ÷ 4 = 22.5 hours. To convert a device rated in watts, divide by 12: a 60 W TV draws 5 A. Running AC appliances through an inverter adds about 10% to the current, so switch the calculator to _AC via inverter_ for those. ## Lithium or AGM for an RV or boat? - **Usable capacity:** a 100 Ah LiFePO4 battery gives about 90 Ah you can use every day; a 100 Ah AGM about 50 Ah if you want it to last. - **Weight:** a 100 Ah LiFePO4 battery typically weighs 11–14 kg against 27–32 kg for a 100 Ah AGM, so it is about a quarter of the weight for the same usable capacity. - **Cycle life:** LiFePO4 typically lasts thousands of cycles; AGM hundreds when used to 50%. - **Cold:** most LiFePO4 batteries must not be charged below 0 °C unless they have built-in heating. AGM charges in the cold but delivers less. To size a bank for a weekend off-grid, use the [battery size calculator](https://batteryruntimecalculator.com/battery-size-calculator). If you recharge from solar panels, the [solar battery calculator](https://batteryruntimecalculator.com/solar-battery-calculator) works in daily energy and days of autonomy. The method behind the numbers is in [how battery runtime is calculated](https://batteryruntimecalculator.com/how-battery-runtime-is-calculated). ## Frequently asked questions ### How long will a 12V battery last? It depends on its amp-hours, its type and the load. A 12 V 100 Ah LiFePO4 battery runs a 50 W DC load for about 21 h 36 min. The same size AGM battery, used to 50% to protect its life, lasts about 12 h. ### How many years does a 12V battery last? That is lifespan rather than runtime on one charge. A 12 V lead-acid deep-cycle battery (AGM, gel or flooded) typically lasts 3–5 years, or a few hundred cycles to 50% discharge, and less if it is often run flat or left discharged. A LiFePO4 battery is usually rated for 2,000–6,000 cycles, which is often 10 years or more. A car starting battery lasts about 3–5 years. ### How many amps can I draw from a 12V 100Ah battery? A 100 Ah battery can supply 5 A for 20 hours (its rating), but not 100 A for one hour, especially lead-acid. For lead-acid, keep the continuous draw under about 20–25 A (0.2–0.25C) for a useful runtime. Most 100 Ah LiFePO4 batteries allow 100 A continuous, limited by their BMS. ### How do I convert amps to watts on a 12V system? Watts = amps × volts. A device drawing 5 A on a 12 V system uses 60 W. The calculator accepts loads in amps: switch the load unit to A (DC). ### How long will a 12V battery run a fridge? A 12 V compressor fridge draws 45–60 W while running but cycles on and off, averaging about 20–30 W. At 25 W, a 100 Ah LiFePO4 battery lasts about 43 h 12 min, or about 24 h from an AGM battery used to 50%. Hot weather raises the average draw. ### What is the difference between a starting battery and a deep-cycle battery? A starting (cranking) battery delivers a huge current for a few seconds to start an engine and is damaged by deep discharges. A deep-cycle battery delivers a steady current for hours and survives repeated discharges. 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