Battery Runtime Calculator
Estimated runtime from battery capacity and load current.
Reviewed by James Whitfield · Updated 24 October 2026 · How we check our maths
Your values
ⓘ Live results
- Estimated runtime
- 5.67 hours
- Estimated runtime
- 340 minutes
- Usable capacity
- 8.5 Ah
Quick answer
Battery runtime in hours is the usable capacity divided by the load current: runtime = Ah × usable fraction ÷ amps. A 100 Ah battery that you only discharge to 50%, powering a 4 A load, lasts about 100 × 0.5 ÷ 4 = 12.5 hours. Use watt-hours divided by watts instead when the load is rated in watts.
What the Battery Runtime calculator does
This calculator estimates how long a battery will keep a device running, from three numbers: the battery's rated capacity in amp-hours (Ah), the average current the device draws in amps, and the percentage of that capacity you can realistically use. It covers anything from an 18650 cell in a flashlight to a 12 V leisure battery running a camping fridge, and it answers the same question as a battery life calculator.
The usable percentage does two jobs. It reflects how deeply the chemistry should be discharged (lead-acid batteries last far longer if kept above about half charge; lithium iron phosphate tolerates 80–90%), and it absorbs losses from heat, voltage sag and any inverter. If your load is in watts, divide by the battery voltage to get amps, or work in watt-hours throughout.
Runtime from a single average current is an estimate. For loads that cycle, such as a fridge compressor, use the average current over a full on/off cycle. To size a battery bank that a solar panel array recharges, work out daily amp-hours first and then size the panels to replace them.
The formula
t = (C × η) ÷ I or, for loads in watts, t = (C × V × η) ÷ P
| Symbol | Meaning | Unit |
|---|---|---|
| t | Estimated runtime | hours |
| C | Rated battery capacity (mAh ÷ 1,000 = Ah) | Ah |
| η | Usable fraction of capacity: depth-of-discharge limit combined with losses | % ÷ 100 |
| I | Average load current | A |
| V | Nominal battery voltage (for the watt version) | V |
| P | Average load power (for the watt version) | W |
This is the simple capacity-over-current method used in battery manufacturers' application notes. It assumes a steady load near the rate the battery was rated at. Lead-acid capacity is usually quoted at the 20-hour rate (C/20); drawing current much faster than that delivers noticeably fewer amp-hours, an effect described by Peukert's law, so lower the usable percentage for heavy loads. Lithium cells are much less affected. Cold weather also reduces available capacity.
Worked examples
Camping fridge on a lead-acid leisure battery
A 12 V, 100 Ah AGM battery runs a compressor fridge averaging 4 A. You want to stop at 50% charge to protect the battery.
Inputs
- Battery capacity100 Ah
- Load current4 A
- Usable efficiency50 %
Result
- Estimated runtime12.5 hours
- Estimated runtime750 minutes
- Usable capacity50 Ah
About 12.5 hours, so a lead-acid battery alone will not get a fridge through a full day and night. A second battery, a lithium battery or daytime solar charging would close the gap.
LiFePO4 battery with a small inverter load
A 100 Ah lithium iron phosphate battery supplies 8 A on average (roughly 100 W at 12.8 V including inverter losses), and you allow 90% depth of discharge.
Inputs
- Battery capacity100 Ah
- Load current8 A
- Usable efficiency90 %
Result
- Estimated runtime11.25 hours
- Estimated runtime675 minutes
- Usable capacity90 Ah
The same nominal 100 Ah delivers 90 usable Ah instead of 50, giving about 11 hours at twice the current of the fridge example. This usable-capacity difference is the main reason lithium batteries replace lead-acid in vans and boats.
Flashlight on a single 18650 cell
A 2,500 mAh (2.5 Ah) lithium-ion cell powers a flashlight drawing 0.5 A on its medium setting.
Inputs
- Battery capacity2.5 Ah
- Load current0.5 A
- Usable efficiency90 %
Result
- Estimated runtime4.5 hours
- Estimated runtime270 minutes
- Usable capacity2.25 Ah
Roughly 4.5 hours. Many flashlights step their output down as the cell voltage falls, so actual time at full brightness is shorter while total runtime can be longer.
Usable capacity by battery chemistry
Pick the usable percentage from the chemistry and how long you want the battery to last, then knock off a further 5–15% if the load runs through an inverter or in cold conditions. Manufacturers' datasheets give the exact recommended depth of discharge.
| Battery type | Typical usable % | Why |
|---|---|---|
| Flooded lead-acid | 50% | Cycle life drops steeply with deeper discharges |
| AGM / gel lead-acid | 50–60% | Same chemistry, slightly more tolerant |
| Lithium iron phosphate (LiFePO4) | 80–90% | Thousands of cycles even at deep discharge |
| Lithium-ion (NMC, 18650/21700 cells) | 80–90% | Device cut-off voltage stops the last few percent |
| NiMH (AA, AAA) | 80–90% | Voltage stays flat until nearly empty |
| Alkaline (AA, AAA) | Varies widely | Capacity falls sharply at high current |
Converting watts, mAh and Wh before you start
- Load in watts: amps = watts ÷ battery voltage (the power form of Ohm's law). A 60 W load on a 12 V battery draws 5 A; through an inverter, divide by about 0.85–0.9 for conversion losses.
- Capacity in mAh: divide by 1,000. A 5,000 mAh phone battery is 5 Ah.
- Capacity in Wh: divide by the nominal voltage to get Ah, or keep everything in watt-hours and divide by watts. Power banks are often rated at the internal 3.6–3.7 V cell voltage, so they deliver fewer mAh at 5 V USB than the label suggests.
- Batteries in parallel add amp-hours at the same voltage; in series they add voltage at the same amp-hours.
Why real runtime comes in shorter
- High discharge rate: pulling a lead-acid battery down in 2–5 hours gives noticeably fewer amp-hours than its 20-hour rating.
- Cold: below about 0 °C (32 °F) lead-acid and lithium cells both deliver less, and many lithium batteries block charging in freezing temperatures.
- Age: batteries lose capacity over their life; a battery at 80% of its original capacity is often treated as worn out.
- Standby loads: controllers, displays and inverters on idle draw current around the clock. Add them to the average current.
For long outages where batteries cannot keep up, compare against a fuel generator with the generator sizing calculator.
How to use the Battery Runtime Calculator
- 1Find the rated capacity on the battery label or datasheet and enter it in Ah (divide mAh by 1,000).
- 2Enter the average current your device draws in amps. If you only know watts, divide by the battery voltage first.
- 3Set the usable percentage: about 50% for lead-acid, 80–90% for lithium, lower for cold weather or inverter loads.
- 4Read the runtime in hours and minutes, and the usable amp-hours it is based on.
Frequently asked questions
How long will a 100Ah battery run a 100W load?
At 12 V a 100 W load draws about 8.3 A. A lithium battery with 90 Ah usable runs it for roughly 11 hours; a lead-acid battery limited to 50 Ah usable manages about 6 hours, less if an inverter is involved.
How do I calculate battery life in hours from mAh?
Divide the capacity in mAh by the device current in mA, then multiply by the usable fraction. A 3,000 mAh battery powering a 150 mA device at 85% usable lasts about 3,000 × 0.85 ÷ 150 = 17 hours.
Does a bigger battery always last proportionally longer?
Close to it at the same current: doubling amp-hours roughly doubles runtime. With lead-acid, a larger bank also lowers the discharge rate relative to its rating, so it can last slightly more than twice as long.
How do I measure the current my device draws?
Put a multimeter set to DC amps in series with the load, or use a DC clamp meter or battery monitor. For cycling loads, average the reading over a full cycle rather than using the peak.
How long does it take to recharge the battery?
Roughly the amp-hours used divided by the charger current, plus 10–20% for charging losses, and longer for lead-acid because the final stage tapers. Replacing 50 Ah with a 10 A charger takes at least 5–6 hours.
References
- Battery University (BU-501 Basics about Discharging and related articles) — Cadex Electronics
- Linden's Handbook of Batteries — McGraw-Hill
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Last reviewed: October 24, 2026
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