Power Bank Real Capacity (mAh to Wh), Efficiency Loss & 100Wh Flight Limit Calculator (2026)

Manufacturers advertise capacity based on the internal lithium cells' nominal voltage of **3.7V** ($20,000\text{ mAh} \times 3.7\text{V} = 74\text{ Wh}$). However, USB ports output at **5V** (or 9V/15V/20V for USB-PD). Stepping 3.7V up to 5V immediately changes the theoretical capacity to $74\text{ Wh} / 5\text{V} = 14,800\text{ mAh}$. After ~10% DC-DC boost converter loss in the power bank and ~8% charging circuit heat loss inside the phone, roughly **62–65 Wh (12,500–13,000 mAh)** of net energy reaches the phone battery—giving $\approx 2.5$ full charges.

Global aviation regulators (FAA, TSA, EASA, IATA, DGCA, CAAC) regulate lithium batteries by **Watt-hours (Wh)**, not mAh, setting the unrestricted carry-on limit at **100 Wh**. At a standard 3.7V cell voltage, $100\text{ Wh} = (100 \times 1000) / 3.7 = \mathbf{27,027\text{ mAh}}$. This is why flagship laptop power banks are specifically engineered to 26,800 mAh (99.16 Wh) or 27,000 mAh (99.9 Wh). Remember: power banks are strictly forbidden in checked luggage and must always go in your cabin carry-on.

Regulatory standards (especially China's GB/T 35590 and CCC certification) require manufacturers to print two numbers on the casing: **Cell Capacity** (e.g., `20,000 mAh / 74 Wh at 3.7V`) and **Rated Capacity** (e.g., `12,600 mAh at 5V=3A`). Rated Capacity is the true measured energy delivered out of the USB port at 5V.

Running the power bank's synchronous boost converter at high wattage (e.g., 65W–140W at 20V) generates slightly more inductor and MOSFET switching heat (reducing conversion efficiency by ~3%–5% compared to gentle 5V/1A trickle charging), though high voltage reduces $I^2R$ copper loss across the USB-C cable.

Aviation security officers (especially at CAAC airports in China and European/Asian hubs) are instructed to confiscate any lithium power bank that lacks a clearly legible manufacturer print showing either Watt-hours (Wh) or both Voltage (V) and Milliamp-hours (mAh) so they can verify it is under 100 Wh.