--- title: "Battery Runtime Calculator: How Long Will a Battery Last?" description: "Free battery runtime calculator. Enter Ah or Wh, voltage, battery type and load in watts to see how long a battery lasts, with Peukert and inverter loss." canonical: https://batteryruntimecalculator.com/ language: en updated: 2026-11-10 source_revision: 59f9d32 maintainer: Battery Runtime Calculator (hello@batteryruntimecalculator.com) --- # Battery Runtime Calculator Find out how long a battery will run your appliances. Updated November 10, 2026 Quick answer Battery runtime (hours) = capacity (Wh) × usable share × inverter efficiency ÷ load (W). For example, a 12 V 100 Ah LiFePO4 battery (1,200 Wh) runs a 100 W appliance through an inverter for about **9 h 26 min**. Estimated runtime 9 h 26 min ≈ 9.43 hours on 1,200 Wh at 100 W Usable energy 1,080 Wh Battery current 9.3 A Power from battery 111 W Discharge rate 0.09C How this was worked out 1. Battery energy: 1,200 Wh rated 2. Power drawn from the battery: 100 W ÷ 90% efficiency = 111 W 3. Rate effect (Peukert 1.05): the battery delivers 97% of its rating at this load 4. Runtime: 1,200 Wh × 97% × 90% usable ÷ 111 W = 9.43 h Runtime at other loads | Load | Runtime | | --- | --- | | 25 W | 38 h 53 min | | 50 W | 19 h 26 min | | 100 W | 9 h 26 min | | 150 W | 6 h 9 min | | 200 W | 4 h 33 min | ## How to use the battery runtime calculator 1. **Enter the capacity** from the battery label, in Ah, mAh, Wh or kWh. For a bank of identical batteries, enter one battery and set the number of batteries and how they are wired. 2. **Set the voltage and battery type.** The type fills in a sensible usable capacity and Peukert exponent, which you can change under Advanced settings. 3. **Enter the load in watts**: the total of everything running at once. Tap a preset for a typical appliance, or switch to amps for a DC load. 4. **Choose AC or DC.** Appliances plugged into an inverter or power station lose about 10% to conversion; 12 V devices wired straight to the battery do not. The result updates as you type. Use _Copy link_ to save or share the exact setup. ## The battery runtime formula For most batteries and everyday loads, this simple version is accurate to within a few percent: ``` Runtime (h) = Ah × V × usable % × efficiency ÷ load (W) ``` Worked through for the default example, a 12 V 100 Ah LiFePO4 battery running a 100 W load through a 90% efficient inverter: - Energy: 100 Ah × 12 V = 1,200 Wh - Usable energy at 90%: 1,080 Wh - Power taken from the battery: 100 W ÷ 0.90 = 111 W - Runtime: 1,080 Wh ÷ 111 W ≈ 9.7 hours, trimmed to 9.4 hours once Peukert’s small correction for lithium is applied Wire the same appliance directly to the battery (DC) and it runs about 10.5 hours, because no energy is lost in an inverter. The full method, including Peukert’s law for lead-acid batteries, is on [how battery runtime is calculated](https://batteryruntimecalculator.com/how-battery-runtime-is-calculated). ## How long will a 12V 100Ah battery last? Runtime of a single 12 V 100 Ah battery at common loads, through a 90% efficient inverter. Lithium is used to 90%, AGM lead-acid to 50%. | Load | LiFePO4 (90% usable) | AGM (50% usable) | | --- | --- | --- | | 50 W | 19 h 26 min | 10 h 48 min | | 100 W | 9 h 26 min | 4 h 55 min | | 200 W | 4 h 33 min | 2 h 13 min | | 300 W | 2 h 58 min | 1 h 24 min | | 500 W | 1 h 44 min | 46 min | | 1000 W | 50 min | 21 min | The gap widens at higher loads: at 1,000 W the AGM battery is discharging at close to 1C, where Peukert’s law takes a large bite out of its capacity. For loads that big, lithium or a bigger bank is the practical choice. To go the other way and find the capacity you need for a set number of hours, use the [battery size calculator](https://batteryruntimecalculator.com/battery-size-calculator). ## What changes battery runtime - **Battery chemistry.** LiFePO4 delivers most of its rated capacity at almost any load. Lead-acid gives less at high currents and should not be drained past about 50% for daily use. - **Inverter losses.** Converting 12 V DC to 120 V or 230 V AC typically costs 8–15%, and an inverter draws power even with nothing plugged in. - **The real load.** Fridges, pumps and air conditioners cycle on and off, so their average draw is well below their running watts. Use the average for runtime and the peak to size the inverter. - **Temperature.** Cold batteries deliver less. Lead-acid can lose 20% or more near freezing, and most lithium batteries cannot be charged below 0 °C. - **Age.** Every battery loses capacity over its life. A battery at 80% health runs 20% shorter; set this under Advanced settings. Working on something specific? There are tuned versions of this calculator for [UPS backup](https://batteryruntimecalculator.com/ups-runtime-calculator), [home inverters](https://batteryruntimecalculator.com/inverter-battery-backup-calculator), [portable power stations](https://batteryruntimecalculator.com/power-station-runtime-calculator), [12 V RV and boat batteries](https://batteryruntimecalculator.com/12v-battery-runtime-calculator) and [small devices rated in mAh](https://batteryruntimecalculator.com/battery-life-calculator). ## Frequently asked questions ### How do I calculate how long a battery will last? Multiply the battery’s amp-hours by its voltage to get watt-hours, multiply by the share you can safely use (about 90% for lithium, 50% for lead-acid), and divide by the load in watts. If the load runs through an inverter, divide by its efficiency too (about 90%). The result is the runtime in hours. ### How long will a 100Ah battery last? A 12 V 100 Ah lithium (LiFePO4) battery holds 1,200 Wh. Running a 100 W appliance through an inverter it lasts about 9.4 hours, and a 500 W load about 1.7 hours. A 12 V 100 Ah lead-acid battery used to 50% lasts roughly half as long. ### Do I use amp-hours or watt-hours? Either works. Amp-hours only describe energy once you know the voltage, so the calculator converts Ah to Wh (Ah × V) first. Watt-hours let you compare batteries of different voltages directly, which is why power stations are rated in Wh. ### Why is real runtime shorter than the calculation? The usual reasons are a load that draws more than its label average, inverter idle draw, cold temperatures, an older battery that has lost capacity, and, for lead-acid, high currents that reduce usable capacity (Peukert’s law). Set battery health and efficiency in Advanced settings to account for these. ### What is depth of discharge and why does it matter? Depth of discharge is how much of the battery you use before recharging. Lead-acid batteries last far more cycles if you stop at 50%, while LiFePO4 handles 80–100% routinely. The calculator only counts the usable share, so a 100 Ah lead-acid battery counts as about 50 Ah of usable capacity. ### Does the battery voltage change the runtime? Not for the same energy. A 24 V 100 Ah battery holds twice the energy of a 12 V 100 Ah battery, so it runs twice as long, but a 24 V 50 Ah battery holds the same 1,200 Wh as a 12 V 100 Ah one and runs the same load for the same time. ## More battery calculators - [Battery life (mAh)mAh and mA to hours, with sleep current and duty cycle.](https://batteryruntimecalculator.com/battery-life-calculator) - [Battery sizeHours you need in, battery Ah, Wh and count out.](https://batteryruntimecalculator.com/battery-size-calculator) - [Solar battery bankDaily kWh and days of autonomy to a battery bank size.](https://batteryruntimecalculator.com/solar-battery-calculator) - [UPS runtimeBackup minutes for a computer, router or NAS on a UPS.](https://batteryruntimecalculator.com/ups-runtime-calculator) - [Inverter backupHome inverter backup hours for 150 Ah to 220 Ah batteries.](https://batteryruntimecalculator.com/inverter-battery-backup-calculator) - [Power station runtimeWh rating to runtime for portable power stations.](https://batteryruntimecalculator.com/power-station-runtime-calculator) - [12V battery runtime12V deep-cycle runtime for RVs, vans and boats.](https://batteryruntimecalculator.com/12v-battery-runtime-calculator) - [Ah to Wh converterAmp hours, watt hours and kWh at any voltage.](https://batteryruntimecalculator.com/ah-to-wh-calculator) - [mAh to Wh convertermAh to Wh for power banks, with the 100 Wh flight limit.](https://batteryruntimecalculator.com/mah-to-wh-calculator) - [Charge timeHours to charge from any state of charge, by charger amps.](https://batteryruntimecalculator.com/battery-charge-time-calculator) - [How runtime is calculatedThe formulas behind every calculator, with worked examples.](https://batteryruntimecalculator.com/how-battery-runtime-is-calculated)