Battery Runtime Calculator
Find out how long a project or pack will actually last. Enter your battery's capacity and voltage, a realistic usable percentage, converter efficiency, and the load, and get runtime in hours and minutes.
Usable is often ~80% for lithium, ~50% for lead-acid. Set efficiency to 100% for a direct DC load with no regulator.
Watts = volts × amps. A 5 V device pulling 1 A draws 5 W.
Formula & how it works
Energy is Wh = mAh ÷ 1000 × V. Runtime is (Wh × usable% × efficiency%) ÷ load(W), in hours. Hours convert to hours and minutes for readability.
Worked example
A 3000 mAh, 3.7 V cell holds 11.1 Wh. At 80% usable and 90% efficiency, about 8 Wh reaches the load. Running a 5 W device, that is 8 ÷ 5 ≈ 1.6 hours, or roughly 1 hour 36 minutes.
Estimating battery life
Watt-hours are the great equalizer
Comparing batteries by milliamp-hours alone is misleading because it ignores voltage. Converting to watt-hours — capacity times voltage — puts every chemistry and pack size on the same footing, and dividing by the load in watts gives a runtime you can trust across different systems.
Be honest about usable capacity
Rated capacity is a best case measured under gentle conditions. In practice you rarely drain a pack fully, both to protect it and because devices cut out early. Dialing in a usable percentage — lower for lead-acid, higher for lithium — turns an optimistic number into a realistic one.
Heavy loads cheat the rating
Batteries deliver less total energy when discharged quickly, an effect that grows with current. For a demanding load, lean toward the lower end of your usable estimate and add margin. Steady, modest loads come closest to the rated figures.