Quick start
- Choose Load current (mA) when you know the average current drawn from the battery. Enter battery mAh, average load mA, and the usable-capacity percentage.
- Choose Power draw (W) when you know average device watts. Enter battery mAh, nominal battery voltage, average watts, and efficiency.
- For a device that sleeps and wakes, calculate the time-weighted average current or average watts before using the calculator.
Best uses
Start here if one of these sounds like your job. The examples below show which inputs matter most.
- Estimate how long a power bank may run a tablet, light, router, or camera.
- Estimate a sensor or small project runtime from battery mAh and average current in mA.
- Convert mAh and volts into watt-hours.
- Add realistic loss instead of assuming 100% battery use.
What this calculator is solving
This guide explains the two common battery-runtime methods: divide usable milliamp-hours by average milliamps, or convert battery capacity to watt-hours and divide usable energy by average watts.
Match each input label on the calculator to either battery capacity in mAh plus average load current in mA, or battery capacity in mAh plus nominal voltage and average device watts; both modes also accept a usable-capacity or efficiency percentage.
The formula in plain language
In plain language: Current method: usable mAh = mAh * usable capacity percent / 100, then runtime hours = usable mAh / average load current in mA. Power method: watt-hours = (mAh / 1,000) * volts, usable Wh = watt-hours * efficiency / 100, then runtime hours = usable Wh / device watts. The examples on the page are there so you can compare your inputs with a worked example before copying the answer.
Current method: usable mAh = mAh * usable percentage / 100, then runtime hours = usable mAh / average mA. Power method: watt-hours = (mAh / 1,000) * volts, usable Wh = Wh * efficiency / 100, then runtime hours = usable Wh / average watts. For example, 2,400 mAh at 85% usable capacity and a 20 mA average load gives 2,040 / 20 = 102 hours.
How to read the answer
Read runtime first, then check the supporting capacity or energy figure. The current mode reports usable mAh and hours or days. The watts mode reports nominal and usable Wh before showing runtime.
- Estimated runtime is the main answer.
- Usable capacity is the labeled mAh after the percentage allowance in current mode.
- Nominal and usable energy are shown in watt-hours in power mode.
- Runtime days are useful for low-current devices, but they are still estimates rather than a shelf-life promise.
Common mistakes to avoid
Battery estimates fail when peak current is entered as a continuous load, sleep time is ignored, mAh is compared across different voltages, or the full label capacity is treated as usable in every condition.
- Do not enter a short peak current as if the device draws it continuously.
- Do not ignore sleep, idle, transmit, display, motor, or startup states when finding an average load.
- Do not compare batteries by mAh alone when voltage is different.
- Do not enter peak watts if you want average runtime, or average watts if you are checking whether a short peak load will shut the battery down.
- Do not ignore voltage converters, inverters, long cables, low-battery cutoff, or manufacturer discharge limits.
- Do not expect old, cold, hot, damaged, or heavily loaded batteries to match the estimate.
Example: 2,400 mAh battery at a 20 mA average load
At 85% usable capacity, a 2,400 mAh battery provides 2,040 usable mAh. Divide 2,040 mAh by a 20 mA average load to get 102 hours, or 4.25 days.
The 20 mA input must be a battery-side average. If the device alternates between a high-current active state and a low-current sleep state, calculate a time-weighted average current first.
Example: USB power bank runtime from watts
A 10,000 mAh power bank with 3.7 V cells stores 37 Wh before losses. At 85% efficiency, usable energy is 31.45 Wh. If the device averages 8 W, runtime is 31.45 / 8 = 3.93125 hours, or about 3h 55m 53s.
This method is safer than dividing the power bank mAh by an output-current label because the battery cell voltage and USB output voltage can differ.
Choose current mode or power mode
Use current mode when battery capacity and average load are measured at the same battery-side voltage. This is common for sensors, microcontrollers, radios, and small electronic projects.
Use power mode when the load is described in watts, when battery and device voltages differ, or when a converter sits between them. Watts and watt-hours keep energy comparisons consistent across voltage changes.
Why the estimate can still be wrong
Texas Instruments notes that usable battery capacity changes with load and discharge behavior. Real runtime also moves with chemistry, self-discharge, age, temperature, converter efficiency, and the voltage where the device stops working.
If a device has radios, motors, heaters, bright screens, or startup spikes, test a conservative average too. Peak current also needs a separate electrical check because a battery can have enough capacity but still fail to support a short high-current burst.
Useful related checks
Battery runtime, amp-hours, watt-hours, watts, and amps answer related but different questions. Use the neighboring tools when you need an electrical conversion before estimating runtime.
Research and references
These references help check the measurements, units, limits, or safety notes used in this guide.
Worked examples for Device Battery Life Calculator
3h 55m 53s (31.45 usable Wh); voltage and efficiency decide the runtime.
5h 33m 0s (16.65 usable Wh); the low 3 W load stretches runtime.
1h 37m 41s (48.84 usable Wh); the 30 W load drains the pack faster.
102 hours, or 4.25 days, from 2,040 usable mAh.
9 hours from 900 usable mAh.
212.5 hours, or about 8.85 days, from 425 usable mAh.
FAQ in plain language
When should I use the Device Battery Life Calculator?
Use it when your task matches one of these common needs: Estimate how long a power bank may run a tablet, light, router, or camera. Estimate a sensor or small project runtime from battery mAh and average current in mA. It works best when you already know the measurements, amounts, units, or options the page asks for.
What is the Device Battery Life Calculator doing with my inputs?
In plain language: Current method: usable mAh = mAh * usable capacity percent / 100, then runtime hours = usable mAh / average load current in mA. Power method: watt-hours = (mAh / 1,000) * volts, usable Wh = watt-hours * efficiency / 100, then runtime hours = usable Wh / device watts. The examples on the page are there so you can compare your inputs with a worked example before copying the answer.
What do the main Device Battery Life Calculator inputs mean?
Battery capacity mAh: The milliamp-hour rating from the battery, phone, power bank, or small electronics label. Voltage: The nominal battery voltage used to convert capacity into watt-hours. Use the pack or cell voltage from the spec sheet. Device watts: The average power draw of the device while it is running. If you only know amps, multiply volts by amps to estimate watts. Average load current mA: The average current drawn from the battery. For a device that sleeps and wakes, use its duty-cycle average rather than its brief peak current. Efficiency or usable capacity %: The share of labeled energy or capacity you expect to use after conversion loss, heat, battery overhead, aging, and safety cutoffs.
How should I read the Device Battery Life Calculator answer?
Read the headline answer, then check the smaller lines beside it. For everyday tools, those lines usually show the distance, time, cost, units, or setting that made the answer change.
What should I double-check before trusting the answer?
Real battery life depends on battery age, temperature, chemistry, discharge rate, screen brightness, radio use, power spikes, voltage-converter loss, inverter loss, cable loss, low-battery cutoff, and manufacturer limits. Also check the unit, scale, mode, and result limit because small input changes can change the answer.
Why do I need voltage when I already know mAh?
mAh alone does not tell total energy unless voltage is known. A 10,000 mAh battery at 3.7 V stores different energy than 10,000 mAh at 12 V.
What efficiency percentage should I use?
Use 80% to 90% for many USB power bank estimates. Use lower values when voltage conversion, heat, old batteries, or long cables waste more energy.
Related tools
- Amp Hours to Watt Hours CalculatorConvert battery amp-hours and voltage into watt-hours and kilowatt-hours.
- Watts to Amps CalculatorConvert watts to amps at common 12 V, 120 V, and 240 V inputs with AC, DC, three-phase, and power-factor options.
- Electricity CalculatorEstimate electricity use and cost from watts, hours, days, and rate per kWh.
Keep exploring
If this guide is close but not exact, these links keep you near the same kind of problem.
- Everyday ToolsBrowse the full category for related tools that help with the same job.
- All free toolsSearch the complete Access Free Tools library by task, category, or tool name.
- All calculator and utility guidesFind more plain-language examples, formulas, mistakes, and result explanations.
- Free calculator resourcesStart here when you are not sure which calculator page fits.
Privacy and copying results
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Use Copy answer when you want to save the inputs and result in notes, homework, a message, or a project list. Check the units, labels, and limits before copying.