mAh Is Marketing, Wh Is Physics
Amp-hours counts charge — how many electrons the battery can push. But electrons alone do not tell you how long a device runs, because devices consume power, and power is charge flow times voltage. A 5 Ah battery at 3.7 V holds 18.5 Wh; at 12 V it holds 60 Wh. Same amp-hour label, more than three times the energy. That is why every phone power bank advertises mAh (the bigger number) while every airline regulation, shipping label, and engineering spec uses Wh (the comparable one). When you convert Wh to Ah you are working backwards from the honest number to the marketing number — and the voltage is the key that unlocks it.
amp hours = watt hours ÷ volts
Ah = Wh / V
Exact, by the definition of the watt-hour — use the battery's nominal voltage.
Real Batteries, Real Numbers
| watt hours | volts | amp hours | What it is |
|---|---|---|---|
| 15 Wh | 3.7 V | 4.05 Ah | Flagship phone battery (4,050 mAh). |
| 37 Wh | 3.7 V | 10 Ah | The 10,000 mAh power bank — legal on any flight. |
| 100 Wh | 3.7 V | 27 Ah | Airline carry-on ceiling — the 27,000 mAh limit. |
| 500 Wh | 12 V | 41.7 Ah | Camping/solar lithium pack — roughly a 50 Ah rated battery. |
| 1,200 Wh | 48 V | 25 Ah | E-bike pack — 25 Ah of cells at 48 V. |
Look at the flight rows: 100 Wh is the carry-on ceiling set by IATA, and it lands at 27 Ah only because phone cells run at 3.7 V. The same 100 Wh in a 12 V camping battery is just 8.3 Ah. Two batteries, identical energy, amp-hour labels that differ by more than three times — which is precisely why the industry moved to Wh for anything regulated. When a solar installer says "match your 500 Wh pack to a 50 Ah 12 V battery," they are running this exact conversion with a margin for depth-of-discharge.
Engineering Context
The forward direction — battery Ah known, energy in Wh wanted — is the amp hours to watt hours page, the one that computes total pack energy. Since Wh is itself a unit of energy, the result here plugs straight into the energy hub: watt hours to kWh for the bill, kWh to joules for science. The current a battery must deliver at a given power is the watts to amps relation at your system voltage, and the voltage itself is set by cell count — see the electric hub for the whole family.
More: Ah to Wh · Wh to kWh · W to A · Electric Hub
Related Unit Converters
Frequently Asked Questions
How do I convert watt hours to amp hours?
Divide by the battery's voltage: amp hours (Ah) = watt hours (Wh) ÷ volts (V). A 500 Wh battery at 12 V holds 500 ÷ 12 = 41.7 Ah. The voltage is essential — the same 500 Wh at 48 V would be only 10.4 Ah, even though the energy is identical.
Why do power banks advertise mAh instead of Wh?
Because mAh is the bigger, more impressive number. A 10,000 mAh pack sounds larger than 37 Wh, but mAh without voltage is meaningless — the pack's cells run at 3.7 V, so the honest energy is 10 Ah × 3.7 V = 37 Wh. Airlines and shipping regulations use Wh for exactly this reason: it is the number that is actually comparable across different voltages.
What voltage do I use for a 12 V lead-acid or lithium battery?
Use the nominal voltage — 12 V for a '12-volt' battery, 3.7 V for a lithium-ion cell, 48 V for an e-bike pack. Nominal is the midpoint of the working range (a 12 V lead-acid actually swings 10.5–14.8 V), and it is the number battery sellers use for the rated capacity. Converting with nominal voltage gives capacity figures consistent with the datasheet.