How it works
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.
Frequently asked questions
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.
mAh is not energy
Milliamp-hours describe charge, not energy, and are only comparable at the same voltage. Converting to watt-hours — capacity × nominal voltage, so a 3,000 mAh cell at 3.7 V is about 11.1 Wh — is what makes two batteries comparable, and it is why power banks are honestly compared in Wh rather than mAh.
That conversion also explains an apparent discrepancy people notice: a 10,000 mAh power bank does not charge a 3,000 mAh phone three times, because the bank's cells are at 3.7 V while the output is boosted to 5 V and the conversion loses energy.
Why real runtime falls short of the calculation
Usable capacity is less than rated capacity for several compounding reasons: converter efficiency, the voltage cut-off at which a device stops rather than running the cell flat, and capacity that declines with age and cycle count.
Discharge rate matters too — most chemistries deliver less total capacity at high current draw than at low, so a device drawing heavily gets less than a linear calculation suggests.
Temperature is the other large factor. Lithium cells lose significant capacity in cold conditions, and it returns when they warm, which is why runtime in winter can be well below the figure that held in summer.
