Battery Life Calculator
📊 Battery and Load Details
Use the rating printed on the battery or device specification sheet. Select Wh when watt-hours are provided directly.
Required for mAh or Ah. It is not used when capacity is entered directly in Wh.
Enter average power draw, not only the maximum adapter or charger rating.
Accounts for conversion, regulator, inverter, wiring, and other electrical losses.
Reduces the rated capacity for reserve charge, cut-off voltage, aging, or a desired discharge limit.
📱 Example Presets
Presets are illustrative starting points. Replace them with the specifications and measured average load of the actual device.
⏱️ Estimated Runtime
The bar represents usable capacity after the selected efficiency and capacity limits.
Battery Life Calculator: Estimate Runtime from Battery Energy
A battery runtime estimate must compare energy with power. Battery capacity stated in milliamp-hours (mAh) is an electrical charge value, while device consumption stated in watts (W) is a power value. Dividing mAh directly by watts does not produce a reliable runtime. The calculator therefore converts battery capacity to watt-hours (Wh) by using the battery’s nominal voltage, or accepts watt-hours directly when that value is already available.
Battery Life Formula
When capacity is entered in mAh, it is first converted to amp-hours:
Capacity (Ah) = Capacity (mAh) ÷ 1,000
The battery’s rated energy is then calculated as:
Battery energy (Wh) = Capacity (Ah) × Nominal voltage (V)
Estimated runtime is calculated after applying the usable-capacity and efficiency settings:
Runtime (hours) = Battery energy (Wh) × Usable capacity × Efficiency ÷ Average load (W)
Information Required for the Calculation
- Battery capacity: Enter the battery rating in mAh, Ah, or Wh.
- Nominal voltage: Required when capacity is entered in mAh or Ah. It is not needed when the battery energy is entered directly in Wh.
- Average device consumption: Enter the device’s average power draw in watts, rather than assuming that the maximum charger or adapter rating is the normal load.
- System efficiency: Accounts for losses in converters, voltage regulators, inverters, wiring, and related electronics.
- Usable capacity: Accounts for reserve charge, cut-off voltage, battery aging, or a chosen discharge limit.
How to Use the Battery Life Calculator
- Enter the battery capacity and select mAh, Ah, or Wh.
- Enter the nominal battery voltage when using mAh or Ah.
- Enter the device’s estimated average consumption in watts.
- Adjust system efficiency and usable capacity to reflect expected losses and discharge limits.
- Select Calculate Battery Life to view the estimated runtime, rated battery energy, and estimated delivered energy.
Example Calculation
A 5,000 mAh battery with a nominal voltage of 3.85 V stores approximately 19.25 Wh of rated energy. With 90% usable capacity and 90% system efficiency, the estimated delivered energy is about 15.59 Wh. If the device draws an average of 3 W, the estimated continuous runtime is approximately 5 hours and 12 minutes.
Why Actual Battery Life May Be Different
The result is an engineering estimate, not a guaranteed operating time. Real battery performance changes with the device and operating conditions.
- Changing current draw: Screens, radios, processors, motors, heaters, and other components may create short or sustained load spikes.
- Cut-off voltage: A device may shut down before the battery has released all of its rated energy.
- Temperature: Cold or excessive heat can reduce available capacity and conversion efficiency.
- Battery age and condition: Used batteries normally store less energy than their original rating.
- Discharge rate and chemistry: Available capacity can change with current draw and battery chemistry.
- Measurement accuracy: A runtime estimate is only as reliable as the entered average power value.
Using Device Presets
The preset buttons provide example values for common device categories, but they are not specifications for every smartphone, tablet, laptop, or gaming laptop. Battery voltage, watt-hour capacity, and average load vary considerably between models. For a meaningful estimate, replace the preset values with information from the actual battery label, manufacturer specifications, or a measured average power reading.
Conclusion
A useful battery life estimate requires battery energy in watt-hours and device consumption in watts. By including capacity, nominal voltage, efficiency, and usable capacity, this calculator avoids directly comparing incompatible units and provides a more defensible estimate. The result should still be treated as approximate because real runtime depends on load behavior, battery condition, temperature, cut-off limits, and other operating factors.
Frequently Asked Questions
Can mAh be divided directly by watts to calculate runtime?
No. Milliamp-hours measure electrical charge, while watts measure power. The battery’s nominal voltage is needed to convert mAh or Ah to watt-hours before runtime can be estimated.
Do I need to enter voltage when the battery is rated in Wh?
No. Watt-hours already express battery energy, so the calculator can use that value directly. Voltage is required only when capacity is entered in mAh or Ah.
What should I use for system efficiency?
Use a measured or manufacturer-provided figure when available. Otherwise, the default value can serve as a general estimate, but the correct value depends on the device’s power electronics and operating conditions.
What does usable capacity mean?
Usable capacity is the portion of the battery’s rated energy expected to be available before the device reaches its discharge limit. It can also be reduced to account for battery aging or to preserve a reserve charge.
Will the calculated runtime be exact?
No. The result is an estimate based on average values. Actual runtime may differ because of changing current draw, temperature, cut-off voltage, battery chemistry, state of health, and measurement accuracy.
