Estimate battery life, runtime, or required capacity from mAh, Wh, voltage, watts, milliamps, reserve headroom, and duty-cycle loads.
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Battery run-time and sizing planner Estimate how long a battery will power a device, reverse the calculation to size a pack, and account for voltage, efficiency, reserve headroom, and duty-cycle loads.
What do you want to estimate?
Battery type shortcuts
Load profile
Load input method
Common load presets
Use nominal voltage, not charger voltage Enter the battery pack's nominal voltage for mAh to Wh conversion. USB output voltage or wall-charger voltage will overstate the available energy.
Estimated battery life
5 hr 40 min
At 5 W, this battery should deliver about 5.66 hours after conversion, health, and reserve factors.
Battery life calculator for run time, watt-hours, and required capacity
Use this battery life calculator to estimate run time from mAh, Wh, voltage, and power draw, or reverse the maths to size the battery you need for a target run time.
How the battery life calculation works
Battery labels often mix two different ideas: charge and energy. Milliamp-hours measure stored charge, while watt-hours measure usable energy. Run time depends on energy and load, so the calculator first converts mAh to Wh with the battery's nominal voltage whenever the starting capacity is charge-based.
Once energy and load are in compatible units, run time is just usable watt-hours divided by watts drawn. The usable-capacity factor matters because most packs do not deliver 100% of their rated energy under real conditions. Temperature, ageing, discharge rate, converter losses, and cutoff voltage all reduce the amount you can actually use.
Capacity (Wh) = Capacity (mAh) × Voltage (V) / 1000
Converts charge-based capacity into energy so it can be compared directly with a load expressed in watts.
Run time (hours) = Effective capacity (Wh) / Power draw (W)
Effective capacity is the rated energy multiplied by the usable-capacity factor after real-world losses.
Run-time mode versus required-capacity mode
If you already know the battery size, run-time mode estimates how long that battery should last at the chosen load. This is the common battery runtime calculator workflow people use for power banks, camera rigs, sensor nodes, and battery-backed portable devices.
If you already know how long the device must run, required-capacity mode works in reverse. It multiplies the load by the target hours, then adjusts upward for losses so you can estimate the rated Wh and mAh you need to buy or build into the pack.
Use run-time mode when the battery is fixed and the question is: how long will it last?
Use required-capacity mode when the run-time target is fixed and the question is: how big must the battery be?
The same voltage and load assumptions should be used in both modes for the comparison to stay coherent.
Continuous loads, duty cycles, and reserve headroom
Many battery runtime calculators assume one steady load, but real devices often switch between high-power and low-power states. A data logger may wake for a few seconds to measure and transmit, then spend the rest of the cycle in sleep mode. A door sensor, tracker, remote monitor, or microcontroller project can look wasteful if you use only the active current, and dangerously optimistic if you ignore the sleep current that continues between bursts.
Duty-cycle mode turns that pattern into an average current draw by weighting the active current and sleep current by their share of the cycle. That average current is then converted into watts with the same nominal voltage used for the battery capacity calculation. This makes the calculator more useful for IoT battery life estimates, low-power embedded projects, and standby-heavy devices than a continuous-load-only worksheet.
Reserve headroom is a separate planning allowance. It represents the part of the rated pack you intentionally leave unused to protect cycle life, avoid deep discharge, or keep a safety margin for cold weather, ageing, and measurement error. A lead-acid battery bank may need a much larger reserve than a lithium pack, while a critical remote sensor may need extra headroom even if the chemistry could technically discharge deeper.
Use continuous load when a device draws roughly the same watts or milliamps throughout the run.
Use duty cycle when a device alternates between active and sleep states.
Use reserve headroom when you want the estimate to leave part of the rated capacity unused.
Use a lower conversion and health factor when cold temperature, battery age, high discharge rate, or inverter losses are expected.
What changes real battery run time
Temperature significantly affects battery performance. Lithium cells generally lose available capacity in cold conditions and age faster under sustained heat. That is why a pack that should last eight hours at room temperature can finish much earlier outdoors in winter or inside an enclosed hot device.
Discharge rate matters too. A battery tested gently in the lab can deliver less usable energy when the load is high, pulsed, or converted through regulators and inverters. Older batteries also store less energy than new ones. The usable-capacity factor is there to keep planning realistic instead of assuming the marketing number is always recoverable in full.
Phones and low-power sensors often draw 1-5 W.
Small laptops and tablets often draw 20-45 W in mixed use.
Drones and portable power tools can draw 100 W or much more, which shortens run time quickly.
Inverter and regulator losses should be reflected in the usable-capacity factor if they are part of the real system.
Worked example: 10,000 mAh at 3.7 V powering a 5 W load
A 10,000 mAh battery at 3.7 V stores 37 Wh of rated energy. If you assume 85% of that is recoverable in practice, the usable energy is 31.45 Wh. A 5 W device divides into that energy to give roughly 6.29 hours of run time.
If you reverse the problem and want the same 5 W load to run for 10 hours, you need 50 Wh of usable energy. At 85% usable capacity, the rated pack size must be about 58.82 Wh, which is roughly 15,898 mAh at 3.7 V. That is the kind of planning the required-capacity mode is designed for.
Battery University — What is C-rate? — Overview of how discharge rate affects delivered capacity and why the same battery behaves differently under heavier loads.
Adafruit — All about batteries — Accessible engineering guide to battery chemistry, voltage, capacity, and practical run-time planning.
Frequently asked questions
What is the difference between mAh and Wh?
mAh measures electrical charge, while Wh measures energy. Two batteries can share the same mAh rating but store different energy if their voltages differ. That is why a battery life calculator uses Wh for run-time maths and only converts back to mAh when it needs to express the answer in charge-based terms.
Why is my actual battery life shorter than calculated?
Rated capacity is measured under controlled conditions. Real systems lose energy to regulator inefficiency, heat, cell ageing, cold weather, heavy discharge, and the cutoff voltage where the device stops before every last watt-hour is extracted. That is exactly why the calculator includes a usable-capacity factor instead of assuming the label rating is fully available every time.
What voltage should I use for a Li-ion battery?
Use the battery pack's nominal voltage, not the charger output voltage or the USB delivery voltage on the device side. Single-cell lithium-ion and lithium-polymer packs are commonly 3.6 V or 3.7 V nominal. Series packs multiply that number by the number of cells, so a 3S pack is around 11.1 V nominal.
Can I use this battery life calculator for power banks, laptops, drones, or inverter systems?
Yes, as long as you use coherent assumptions. For power banks and phones, use the battery pack's nominal voltage and the real device load. For laptops and drones, the same rule applies, but the load is usually much higher. For inverter systems, fold inverter losses into the usable-capacity factor or the effective load, because the DC battery does not deliver all of its energy to the AC side unchanged.
How do I calculate battery life for a device with a duty cycle?
Calculate the average current first. Multiply active current by the fraction of time the device is awake, multiply sleep current by the fraction of time it is asleep, then add the two results. The calculator's duty-cycle mode performs that weighting from active current, sleep current, active seconds, and cycle length before estimating run time.
What reserve percentage should I leave unused?
The reserve depends on chemistry, operating conditions, and how conservative the plan needs to be. Lithium packs often tolerate deeper discharge than lead-acid packs, but leaving some reserve can still protect cycle life and cover cold weather, ageing, or measurement uncertainty. For critical systems, use a larger reserve and validate the estimate against the battery manufacturer's discharge guidance.
Is Wh or mAh better for comparing batteries?
Wh is usually better for comparing batteries across different voltages because watt-hours measure energy directly. mAh is useful only when the voltage basis is known. A 10,000 mAh pack at 3.7 V and a 10,000 mAh pack at 12 V do not store the same amount of energy, so the calculator keeps voltage visible whenever it converts mAh to Wh.
How do I size a battery for a target run time?
Switch to required-capacity mode, enter the target hours, voltage, load, conversion and health factor, and reserve percentage. The calculator multiplies the load by the target hours to find usable energy, then scales that number up to the rated Wh and mAh needed after losses and reserve headroom.
Does this calculator include Peukert's law or a full discharge curve?
No. It uses nominal voltage, average load, and a derating factor rather than a chemistry-specific discharge simulation. That is appropriate for planning and comparison, but high-drain lead-acid systems, safety-critical equipment, and products near their current limit should be checked against manufacturer discharge curves or measured with a battery analyzer.
