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Everything is normalised to watt-hours before dividing. Capacity becomes Wh as mAh × V ÷ 1000, as Ah × V, or is used directly when you select Wh. The load becomes watts as mA × V ÷ 1000, as A × V, or straight through when you select W. Runtime in hours is then Wh × efficiency ÷ watts. The mAh readout is the reverse trip, Wh × 1000 ÷ V, which is why voltage matters even for a Wh entry.
Battery voltage (V) is the input people get wrong most often, and it silently rescales both conversions at once; the field's note offers Li-ion ≈ 3.7 V, AA ≈ 1.5 V and lead-acid ≈ 12 V. Efficiency (%) defaults to 85 and stands in for regulator and heat losses. The table beneath re-runs the identical calculation at 100, 90, 85, 75 and 60% so you can read the spread instead of trusting one assumption.
This is a constant-current, constant-voltage model and real batteries are neither. Voltage sags as a pack empties, so usable watt-hours fall short of the nameplate figure; cold weather cuts capacity further; and alkalines in particular surrender a large share of their rated mAh at anything above a gentle draw. The device presets are illustrative starting points rather than measured discharge profiles, so treat every result as an optimistic upper bound.
الأسئلة الشائعة
How long does a 3000 mAh battery last at 200 mA? +
About 12 hours 45 minutes at the defaults. 3000 mAh at 3.7 V is 11.1 Wh, a 200 mA draw is 0.74 W, and the 85% efficiency setting leaves 9.4 Wh usable. Without that haircut the ideal figure would be 15 hours.
How do I convert mAh to watt-hours? +
Multiply by the pack voltage, then divide by 1000. A 10000 mAh bank at 3.7 V holds 37 Wh. That is why airlines quote watt-hours rather than mAh, and why two packs with identical mAh at different voltages store different energy.
What efficiency should I enter for a USB power bank? +
Around 80 to 85%. Boosting a 3.7 V cell up to 5 V costs real energy, which is why a 10000 mAh bank typically delivers nearer 6000–6500 mAh at 5 V. The 85% default is a fair starting point for most regulated devices.
Why does my device die before the calculated time? +
The model assumes a steady draw at a steady voltage. Real packs sag as they empty, cold weather cuts usable capacity, and most devices spike well above their average load. Alkaline cells suffer worst, losing a large share of rated capacity at higher currents.
Do 5000 mAh at 3.7 V and 5000 mAh at 12 V last equally long? +
No. The second holds over three times the energy, 60 Wh against 18.5 Wh. Milliamp-hours only compare meaningfully at the same voltage, which is exactly why the calculator converts both capacity and load to watt-hours before dividing.
