Enter your battery specs and load below to find out how long your battery will last. This calculator works for any battery type — LiFePO4, lead-acid, AGM, or lithium-ion — at any voltage.
How the Battery Runtime Formula Works
The formula multiplies your battery's capacity (Ah) by its voltage to get total stored energy in watt-hours. It then applies depth of discharge and efficiency factors before dividing by your load.
Runtime (hours) = (Capacity x Voltage x DoD x Efficiency) / Load
A 100Ah 12V LiFePO4 battery at 80% DoD and 90% efficiency powering a 100W load: (100 x 12 x 0.80 x 0.90) / 100 = 8.64 hours. That same battery on a 50W LED worklight lasts over 17 hours — enough for two full nights of use.
The formula assumes a constant load, which rarely happens in practice. A refrigerator cycles on and off, pulling 150W during the compressor run and near-zero in between. For cycling loads, use the average power draw over a full cycle. Most fridge manufacturers list this as "average consumption" in kWh per day or per year.
Depth of Discharge Changes Everything
Depth of discharge (DoD) is the single biggest variable most people ignore when estimating battery life. A 100Ah lead-acid battery does not give you 100Ah of usable energy. Discharge it past 50% regularly and you will shorten its lifespan from 500+ cycles down to under 200.
LiFePO4 batteries are far more forgiving. Most manufacturers rate them for 80% DoD with 2,000-5,000 cycle lifespans. Some premium cells handle 100% DoD without meaningful degradation.
AGM batteries fall in between — 50-60% DoD is the safe zone. Push an AGM to 80% DoD and you cut its cycle life roughly in half compared to keeping it above 50%.
The practical takeaway: a 200Ah lead-acid bank at 50% DoD gives you the same usable energy as a 100Ah LiFePO4 bank at 100% DoD. Factor in the weight difference (lead-acid is 3-4x heavier per kWh) and the cost-per-cycle math often favours lithium.
Frequently Asked Questions
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Battery runtime depends on real-world conditions that no formula can perfectly predict. Use this calculator as a solid starting point, then add a 20% safety margin for critical applications. Temperature, battery age, and load variability all shift the actual number — so size your system with headroom.
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Written and maintained by Dan Dadovic, Developer & Off-Grid Energy Enthusiast. On the energy side, Dan has hands-on experience with residential solar panel installation, DIY battery bank construction, off-grid power systems, and wind power — all from building and maintaining his own systems..
Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.