A power bank's advertised capacity in mAh consistently overstates how much charge it can actually deliver to a device — a real, predictable gap worth understanding before assuming the number on the box is the number you'll actually get.
Advertised mAh overstates real usable charge
Power banks lose a meaningful portion of their rated capacity to energy conversion inefficiency — voltage conversion between the power bank's internal battery and a connected device's charging needs isn't perfectly efficient, and some energy is lost as heat in the process. In practice, a power bank typically delivers somewhere around 60-70% of its advertised mAh rating as usable charge to a connected device. This isn't false advertising — the mAh rating reflects the internal battery's actual capacity — but it does mean a 10,000mAh power bank won't fully charge a 10,000mAh phone battery from empty, which surprises people expecting a direct one-to-one match.
Output wattage determines charging speed, separate from capacity
A power bank's total capacity (mAh) and its output speed (measured in watts, related to voltage and amperage) are separate specs answering different questions — capacity is how much charge it holds in total; output wattage is how fast it can deliver that charge to a connected device. A high-capacity power bank with low output wattage charges a device slowly despite having plenty of total charge available, while a lower-capacity bank with high output wattage can charge quickly but for a shorter overall duration. Both numbers matter, and checking that the output wattage supports fast charging for the specific device intended to be charged is a separate check from total capacity.
Airlines restrict power banks by watt-hours, not mAh
Airlines set carry-on restrictions for power banks based on watt-hours (Wh), not the mAh rating printed on most consumer packaging — this conversion isn't always straightforward for a traveler to calculate on the spot, and can catch people by surprise at airport security. Checking a specific power bank's watt-hour rating (sometimes printed on the device itself, or calculable from mAh and voltage) against airline restrictions before air travel avoids a bank being confiscated or a scramble at security.
Pass-through charging isn't universally supported
Some power banks support "pass-through" charging, letting the bank charge a connected device while it's itself being charged from a wall outlet simultaneously — genuinely convenient for keeping devices topped up overnight without a separate charging step. This feature isn't universal, and using pass-through charging on a power bank not actually designed for it can generate excess heat and potentially degrade the battery faster over time, so it's worth confirming a specific model genuinely supports it before relying on that use pattern regularly.
The one thing people forget
Check the number and type of output ports against actual devices needing to be charged, not just the total port count — some power banks split their maximum output wattage across multiple ports when charging more than one device simultaneously, meaning each device charges slower than it would from a dedicated single port, which is worth knowing before assuming multiple ports means full-speed charging on all of them at once.