The mAh number on a power bank's box is not the number airlines regulate, and it's not even the number that tells you how much energy the bank actually holds. That number is watt-hours, and most listings either bury it or don't publish it at all.
mAh Measures the Wrong Thing
Power bank capacity is almost universally advertised in milliamp-hours, a figure measured at the battery cell's internal voltage, typically around 3.6 to 3.7 volts. But your phone or laptop charges at 5 volts or higher, meaning a power bank never actually delivers its full advertised mAh figure to your device. The real measure of stored energy is watt-hours, calculated by multiplying capacity by voltage, and it's the only figure that lets you meaningfully compare two power banks or two different devices against each other.
A 20,000mAh power bank typically works out to roughly 72 to 74 watt-hours depending on the specific cell voltage used. A 40,000mAh bank comes in around 140 to 145 watt-hours. These aren't simple doubling relationships because the conversion depends on the exact voltage, which varies slightly between products and isn't always published either.
Why This Matters at the Airport
Airline lithium battery rules are written in watt-hours, not milliamp-hours. Most airlines allow power banks up to 100 watt-hours without any special approval, permit 100 to 160 watt-hours only with airline approval and typically a limit of two spare batteries, and prohibit anything above 160 watt-hours from flights entirely. A traveler shopping by mAh alone has no direct way to check which bracket their power bank falls into, since the rule is written in a unit most product listings don't feature prominently.
This creates real confusion at the higher end of the market. A 40,000mAh power bank sounds like a reasonable travel companion, but depending on its actual watt-hour rating, it can land close to or even past the 100Wh threshold that triggers airline restrictions, and a buyer relying on the mAh figure alone has no way to know that without doing the conversion themselves.
What to Actually Check Before Buying
Look specifically for a watt-hour figure on the product listing or manufacturer spec page rather than relying on the mAh number alone, since watt-hours is the number that determines airline eligibility and gives you an honest comparison against other power banks. If only mAh is published, you can estimate watt-hours by multiplying the mAh figure by roughly 3.6 to 3.7 and dividing by 1,000, though a manufacturer-published figure is always more reliable than an estimate. And if you travel frequently, staying comfortably under the 100 watt-hour mark avoids any airline approval hassle altogether.
Winner
Watt-hours, not milliamp-hours, is the number that actually determines airline eligibility and gives you an honest comparison between power banks. Look for it specifically before buying, especially for anything in the 20,000mAh-and-up range.
Pros and Cons
High-mAh power banks: Sound more impressive on the shelf and do hold more usable charge, but without a published watt-hour figure you can't verify airline eligibility or make an honest comparison to a competing product.
Products that publish watt-hours clearly: Let you check airline rules and compare capacity honestly, but this figure is inconsistently published across the category, especially on budget models.
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