mAh to Wh Calculator

Multiply the capacity by the voltage and divide by 1,000: Wh = mAh x V / 1000. A 20,000 mAh power bank with 3.7 V lithium-ion cells stores 20,000 x 3.7 / 1000 = 74 Wh. The voltage is essential: mAh alone is a charge figure, not an energy figure, so two batteries with identical mAh ratings but different voltages store different amounts of energy.

Use the nominal cell voltage printed on the battery, typically 3.6 or 3.7 V for lithium-ion and lithium polymer cells. Do not use the 5 V USB output voltage of a power bank: manufacturers quote mAh at cell level, so converting at 5 V overstates the stored energy. For multi-cell packs, use the pack voltage, for example 11.1 V for a 3-cell lithium polymer battery.

Most airlines follow the IATA rules: lithium batteries up to 100 Wh are allowed in carry-on baggage without approval, batteries between 100 and 160 Wh need airline approval, and spare batteries are never allowed in the hold. A 20,000 mAh pack at 3.7 V is 74 Wh, well inside the limit, while a 27,000 mAh pack at 99.9 Wh sits right at the threshold. Always check your airline's own policy before flying.

Because energy depends on voltage as well as charge. The mAh rating counts charge, and Wh = mAh x V / 1000. A 5,000 mAh pack at 3.7 V holds 18.5 Wh, while a 5,000 mAh pack at 11.1 V holds 55.5 Wh, three times the energy. This is why watt-hours, not milliamp-hours, is the honest unit for comparing batteries across devices with different cell arrangements.

The rating describes the energy stored in the cells, not the energy delivered to your device. Boosting the 3.7 V cell voltage up to the 5 V or higher USB output loses some energy as heat in the converter, and the receiving device loses a little more in its own charging circuitry. Cell capacity also fades with age and is lower in cold conditions, so usable output is always somewhat below the printed figure.