Battery Capacity Converter

Battery capacity can be measured in amp-hours (Ah) or watt-hours (Wh). Converting between them requires knowing the voltage.

Core formulas:

• Wh = Ah × Voltage
• Ah = Wh / Voltage
• mAh = Ah × 1,000
• kWh = Wh / 1,000

Common battery voltages:

Example conversions:

• Phone: 5,000 mAh at 3.7V = 18.5 Wh
• Power bank: 20,000 mAh at 3.7V = 74 Wh
• Laptop: 56 Wh at 11.1V = 5,045 mAh
• Tesla Model 3: 60 kWh at 350V = 171.4 Ah

Airline limit: Most airlines allow batteries up to 100 Wh in carry-on. Batteries 100-160 Wh need airline approval.

The number printed on a power bank is almost always its internal cell capacity at 3.7 volts, not what reaches your phone. Charging happens at 5 volts, and the boost converter that steps the voltage up wastes 10 to 20% as heat, so a “10,000 mAh” bank realistically delivers around 6,000 to 6,800 mAh to a 3.7V phone battery. That isn’t quite false advertising, it’s just two different voltages being quoted, which is the whole reason watt-hours is the unit that doesn’t lie.

This is why you should compare batteries in Wh, never mAh, unless the voltages are identical. A 5,000 mAh phone cell and a 5,000 mAh drone cell can hold very different energy if one is 3.7V and the other is 11.1V. Electric cars skip mAh entirely and rate packs in kWh for the same reason. One more thing worth knowing: rated capacity is for a fresh battery. Lithium cells fade with every charge cycle, losing roughly 20% after a few hundred cycles, so a two-year-old power bank delivers noticeably less than its label ever claimed.