Enter values and calculate runtime
Start with Battery capacity and Load (power draw). Choose a unit for each, add Voltage (V) when a conversion needs it, then set Efficiency (%) before selecting Calculate. The result shows runtime in hours, minutes, days, and an approximate hours-and-minutes format. Formula & steps lists the conversions used, while Copy result and Download result (.txt) act on the displayed result.
- Enter a number in Battery capacity.
- Choose Milliamp-hours (mAh), Amp-hours (Ah), or Watt-hours (Wh) in Capacity unit.
- Enter the draw in Load (power draw), then choose Milliamps (mA), Amps (A), Watts (W), or Kilowatts (kW) in Load unit.
- Enter Voltage (V) when the selected units need a conversion.
- Set Efficiency (%) and click Calculate. The field starts at 100 and accepts values from 1 through 100.
How the formula converts capacity and load
The calculator converts capacity to watt-hours and load to watts, applies Efficiency (%), then divides usable energy by load power. An efficiency of 85% contributes 0.85 of the converted capacity.
Battery capacity: 100 Ah
Load (power draw): 120 W
Voltage (V): 12
Efficiency (%): 85
Runtime: 8.5 hours
In minutes: 510 min
Approx.: 8 h 30 minThe example uses runtime (h) = (capacity in Wh x efficiency) / load in W. Capacity in Ah becomes Wh with capacity (Ah) x voltage (V). A load in A becomes watts with current (A) x voltage (V); a load in kW is multiplied by 1,000.
When Voltage (V) is required
Enter Voltage (V) when Capacity unit is Milliamp-hours (mAh) or Amp-hours (Ah), or when Load unit is Milliamps (mA) or Amps (A). The field can stay blank when capacity is Watt-hours (Wh) and load is Watts (W) or Kilowatts (kW). If you enter voltage, it must be greater than zero.
Efficiency (%) accepts values from 1 through 100. Battery capacity and Load (power draw) must be greater than zero. Blank required fields, invalid number text, and values whose absolute size is above 1e12 trigger an error.
Input combinations the calculator handles
- For 100 Ah, 120 W, 12 V, and 85% efficiency, the displayed runtime is 8.5 hours, or 8 h 30 min in the approximate breakdown.
- A capacity in Milliamp-hours (mAh) and a load in Milliamps (mA) both use Voltage (V) for conversion.
- Watt-hours (Wh) with Watts (W), or Kilowatts (kW), can be calculated without entering voltage.
Battery Runtime Calculator and related science tools
Use Science Calculators for another physics, chemistry, electrical, optics, wave, fluid, gas, or radioactive-decay calculation. The Electrical Calculator includes watts, volts, amps, resistance, voltage drop, wire-size estimates, energy consumption, and battery-bank sizing. The Physics Calculator covers quantities such as force, acceleration, momentum, energy, pressure, and wavelength.
What happens to the entered values
The calculation runs in the browser. The calculator script does not send the input fields through fetch, XMLHttpRequest, sendBeacon, or a form submission. Copy result copies the displayed text, and Download result (.txt) creates a text file from it.
Frequently asked questions
How do I calculate battery runtime?
Divide usable battery energy in watt-hours by load power in watts. The calculator converts mAh or Ah with Voltage (V), applies Efficiency (%), and shows hours, minutes, days, and an approximate hours-and-minutes breakdown.
What units does the Battery Runtime Calculator support?
Capacity unit has Milliamp-hours (mAh), Amp-hours (Ah), and Watt-hours (Wh). Load unit has Milliamps (mA), Amps (A), Watts (W), and Kilowatts (kW). Voltage (V) and Efficiency (%) are separate inputs.
Do I need voltage to estimate battery runtime?
Enter Voltage (V) when Capacity unit is Milliamp-hours (mAh) or Amp-hours (Ah), or when Load unit is Milliamps (mA) or Amps (A). It is optional for Watt-hours (Wh) with Watts (W) or Kilowatts (kW).
Is the battery runtime estimate exact?
No. The calculator applies a fixed formula to the values you enter, so the result is an estimate rather than a battery measurement.