Quick start
- Enter battery capacity in mAh and the nominal voltage from the product label.
- Enter the device average power draw in watts.
- Use efficiency to account for conversion loss, heat, cables, and imperfect battery use.
Best uses
These are the situations this tool is meant for. If your task is close to one of these, the examples and notes below can help you choose the right inputs.
- Estimate how long a power bank may run a tablet, light, router, or camera.
- Convert mAh and volts into watt-hours.
- Add realistic loss instead of assuming 100% battery use.
- Compare two batteries that use different voltages.
What this calculator is solving
The Device Battery Life Calculator converts battery capacity into watt-hours, applies a realistic efficiency loss, and divides by device power draw to estimate runtime.
You do not need to memorize the formula first. Start by matching each input label on the calculator to the number, date, unit, or setting you actually have.
The formula in plain language
In plain language: The calculator converts milliamp-hours and voltage into watt-hours, applies an efficiency percentage, then divides usable watt-hours by device watts. The examples on the page are there so you can compare your inputs with a filled-out calculation before copying the answer.
If that sounds abstract, use the example cards on the calculator page. They show a complete set of inputs and the kind of answer you should expect.
How to read the answer
Read the headline result first. Then look at the smaller supporting lines because they explain the parts behind the answer, such as totals, units, ranges, or formula steps.
- Estimated runtime is the main answer.
- Nominal energy is the battery watt-hours before efficiency loss.
- Usable energy is the watt-hours after the efficiency percentage.
Common mistakes to avoid
If the answer looks strange, the most likely cause is a small input mismatch: the wrong unit, date, weight, scale, mode, or policy assumption.
- Do not compare batteries by mAh alone when voltage is different.
- Do not assume a device draws the same watts all the time.
- Do not expect old, cold, hot, damaged, or heavily loaded batteries to match the estimate.
Research and references
These references shaped the calculator assumptions, unit choices, or safety notes.
Examples from the calculator
Runtime estimate
Longer runtime estimate
Watt-hour based runtime
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. Convert mAh and volts into watt-hours. It works best when you already know the values, dates, units, or settings the page asks for.
What is the Device Battery Life Calculator doing with my inputs?
In plain language: The calculator converts milliamp-hours and voltage into watt-hours, applies an efficiency percentage, then divides usable watt-hours by device watts. The examples on the page are there so you can compare your inputs with a filled-out calculation before copying the answer.
What do the main Device Battery Life Calculator inputs mean?
mAh: Battery capacity in milliamp-hours from the label. Voltage: Nominal voltage used to convert capacity into watt-hours. Device watts: Average power draw of the device while it is running.
How should I read the Device Battery Life Calculator answer?
Read the main answer first, then check the supporting lines and examples to understand how the calculator got there. If one input changes, rerun the tool and compare the new answer instead of guessing.
What should I double-check before trusting the answer?
Real battery life depends on battery age, temperature, power spikes, screen brightness, radio use, inverter losses, and manufacturer limits. Also check that you used the right unit, date, scale, or mode because small input changes can change the result.
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.
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Privacy and copying results
Recent answers stay visible only while you work in the current browser tab. They are not sent to a server.
Use Copy answer when you want to paste the expression and result into notes, homework, a message, or another document. Check the units and assumptions before copying.