How long will a 100Ah battery last at 12V?

A 100Ah 12V battery stores 1,200Wh of total energy. At 80% depth of discharge and 90% efficiency, that gives you 864Wh of usable energy. Divide by your load: a 100W device runs for about 8.6 hours, a 50W device for about 17.3 hours. For medical devices like CPAP machines, use our CPAP battery runtime calculator with brand-specific power draws. The actual runtime varies with temperature, battery age, and whether your load is constant or variable.

Does battery voltage affect runtime?

Voltage affects total stored energy, not runtime directly. A 100Ah battery at 12V stores 1,200Wh, while the same 100Ah at 24V stores 2,400Wh — double the energy. When comparing batteries, always look at watt-hours (Ah x V), not amp-hours alone. A 50Ah 24V battery has the same energy as a 100Ah 12V battery.

Why is my actual battery runtime shorter than calculated?

Several factors reduce real-world runtime below the theoretical number. Cold temperatures reduce battery capacity by 10-20% in mild cold (40°F/5°C). At extreme cold (0°F/-18°C), lead-acid batteries can lose 30-50% of rated capacity. Inverter efficiency losses add up, especially at low loads where inverters are least efficient. Battery age matters — a lead-acid battery at 500 cycles may only deliver 70-80% of its rated capacity. And high discharge rates reduce effective capacity through the Peukert effect , particularly in lead-acid batteries.

Battery Runtime Calculator — Free

4 min read

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.

Battery runtime statistics for 100Ah LiFePO4 at 50W with depth of discharge and inverter efficiency.

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. Our battery runtime guide walks through more worked examples across different battery types and load scenarios.

Battery runtime vs load chart for a 100Ah 12V LiFePO4 from 38 hours at 25W to under 5 hours at 200W. — Runtime varies dramatically by load — a 25W LED light runs 8× longer than a 200W power tool on the same battery.

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. Our depth of discharge guide breaks down the chemistry-by-chemistry DoD limits and their lifespan impact.

Worked Examples

Running a CPAP Machine Through the Night

Context

You camp with a 100Ah LiFePO4 battery and a CPAP machine that draws 30W on average. You need 8 hours of runtime and want to keep 20% reserve.

Calculation

Usable energy = 100 Ah x 12 V x 0.80 DoD x 0.90 efficiency = 864 Wh

Runtime = 864 / 30 = 28.8 hours

Interpretation

28.8 hours gives you more than three full nights on a single charge. Even on the highest pressure setting (around 60W), you get 14+ hours.

Takeaway

A 100Ah LiFePO4 is more than enough for CPAP use. To figure out how long your solar panels need to recharge it, run the numbers through our solar battery charge time calculator.

Powering an RV Through a 48-Hour Outage

Context

A storm knocks out power. Your RV has two 200Ah 12V batteries in parallel (400Ah total). Your essential loads — fridge, lights, phone charging — draw 150W combined. You use lead-acid batteries limited to 50% DoD.

Calculation

Usable energy = 400 Ah x 12 V x 0.50 x 0.88 = 2,112 Wh

Runtime = 2,112 / 150 = 14.1 hours

Interpretation

14 hours falls far short of 48 hours. You would need to reduce loads or add battery capacity to survive a two-day outage.

Takeaway

For extended outages, calculate exactly how much battery capacity you need with our battery size for inverter calculator — it works backwards from your target runtime.

Frequently Asked Questions

Glossary

Depth of Discharge

The percentage of a battery's total capacity that you actually use before recharging. Lead-acid batteries should not exceed 50% DoD to preserve lifespan. LiFePO4 batteries can safely discharge to 80-100% DoD.

System Efficiency

The percentage of stored energy that reaches your device after losses in wiring, the inverter, and the battery's own internal resistance. Typical values: 85-95% depending on whether you use DC directly or convert to AC.

Usable Energy

The actual watt-hours available from a battery after accounting for depth of discharge and system efficiency. A 100Ah 12V battery has 1,200 Wh total but only 864 Wh usable at 80% DoD and 90% efficiency.

Planning a solar system? Use our <a href="/solar/solar-panel-output-calculator">solar panel output calculator</a> to see how much energy your panels produce daily.

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. For application-specific calculators with tailored presets, see the marine battery runtime, laptop battery runtime, and e-bike range calculators.

Last updated: March 23, 2026

Written and maintained by Dan Dadovic, Commercial Director at Ezoic Inc. & PhD Candidate in Information Sciences. He works professionally as Commercial Director at Ezoic Inc., leading revenue strategy across digital publishing.

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.