How to Use Battery Capacity Calculator
The Battery Capacity Calculator estimates how large a battery must be to power a device for a target runtime. It converts watts and hours into watt-hours, then into mAh at the selected voltage.
- Enter the average device power in watts. If a device has peaks and idle periods, use a realistic average rather than the maximum rating.
- Enter the desired operating time in hours.
- Enter the nominal battery voltage, such as 3.7V for a single lithium cell or 12V for many small battery packs.
- Read the required mAh and Ah values, then add a safety margin for real-world losses.
Formula & Theory - Battery Capacity Calculator
The Battery Capacity Calculator uses the following formula or calculation model:
Battery capacity (mAh) ≈ (device power (W) × use time (hours) × 1000) / voltage (V)
Energy needed (Wh) = power × timeBattery capacity in amp-hours is energy divided by voltage. Because many small batteries are labeled in milliamp-hours, the calculator multiplies by 1000 after converting watt-hours into amp-hours. The supporting energy result in Wh is useful when comparing battery packs with different voltages.
Assumptions and Limits
Converters, inverters, cold temperature, aging, discharge limits, and manufacturer capacity ratings can reduce usable runtime.
Use Cases for Battery Capacity Calculator
Specific use cases include:
- Size a battery for an IoT sensor, camera, fan, router, or LED project.
- Compare 3.7V and 12V pack requirements for the same power load.
- Estimate whether a power bank has enough usable capacity.
- Plan backup runtime before buying cells or modules.