Use the calculator
Enter a battery's capacity, load, voltage when needed, and efficiency. The calculator estimates runtime and shows it in hours, minutes, days, and an approximate hours-and-minutes format.
How to calculate battery runtime
- Enter a value in Battery capacity, then choose Capacity unit: mAh, Ah, or Wh.
- Enter the power draw in Load (power draw), then choose Load unit: mA, A, W, or kW.
- Enter Voltage (V) when the selected units require it. You may also enter voltage for reference when capacity is Wh and load is W or kW.
- Leave Efficiency (%) at its prefilled value of 100, or replace it with a number from 1 to 100. The field is required and cannot be blank.
- Click Calculate.
When voltage is required
Voltage (V) is required when capacity is in mAh or Ah, or when load is in mA or A. It is optional only when capacity is in Wh and load is in W or kW.
Read, copy, or download the result
The result shows Runtime, In minutes, In days, and Approx. It also includes the formula and intermediate conversion steps. Use Copy result to copy the plain-text result, or Download result (.txt) to save it as a text file.
Formula and unit conversions
The calculator finds runtime in hours by dividing usable energy by load power:
t = (E × η) / P
E is energy in watt-hours, η is efficiency as a fraction, and P is load power in watts. Capacity in Ah or mAh is converted to Wh using voltage. Load in A or mA is converted to W using voltage, while a kW load is converted to W.
Validation and common input errors
Capacity and load must be positive, finite numbers. Voltage must be positive when required. Efficiency (%) must contain a number from 1 through 100; leaving it blank produces an error. Empty fields, non-numeric text, zero or negative required values, and values that are too large to calculate are rejected.
Limits of the estimate
The model assumes constant load power and constant discharge voltage. It does not model non-linear discharge curves, temperature effects, internal resistance, cell aging, or Peukert's law. The calculator does not infer missing capacity, voltage, or efficiency values, and its result is not certified electrical engineering, product safety, or professional advice.
Privacy and local calculation
The calculator performs its calculation in the browser. Its script does not submit the calculator input fields.
Frequently Asked Questions
What is the formula for battery runtime?
Runtime in hours equals usable energy divided by load power: runtime (h) = (capacity in Wh × efficiency) / load in W. Capacity in Ah or mAh is converted to Wh using the battery voltage, and load in A or mA is converted to W using the same voltage.
What units does this calculator support?
Capacity can be entered in milliamp-hours (mAh), amp-hours (Ah), or watt-hours (Wh). Load can be entered in milliamps (mA), amps (A), watts (W), or kilowatts (kW). Voltage is in volts (V). The result is shown in hours, minutes, days, and an approximate hours-and-minutes breakdown.
When do I need to enter voltage?
Voltage is required whenever capacity is entered in mAh or Ah, or when load is entered in mA or A, because those units must be converted to watt-hours and watts using the battery voltage. It is optional only when capacity is in Wh and load is in W or kW.
What does the efficiency value represent?
Efficiency represents the percentage of the battery's rated energy that is actually usable, accounting for factors like converter losses, battery age, temperature, and discharge rate. It is prefilled at 100% and must contain a value from 1% to 100%; the calculator does not infer or guess this figure.
Is this an exact prediction of real-world runtime?
No. It is a deterministic estimate based on the fixed formula and the numbers you enter. Real batteries have non-linear discharge curves, capacity that varies with temperature and discharge rate (Peukert's law), and aging effects that this simple linear model does not capture.