What Size Power Bank Do You Need for Travel? (mAh Explained)

Your phone hits 40 percent right as boarding starts, so you grab the power bank from your bag. It gets you to 65 percent before it goes cold, and you spend the flight wondering how a battery with "20,000mAh" printed on the side did so little. That gap between the number on the box and the charge in your hand is the whole problem, and it is fixable with a little math.

Why the mAh number alone misleads you

Manufacturers print milliamp-hours because it is a bigger, more impressive number than watt-hours. A 20,000mAh power bank sounds enormous next to your phone's 4,000mAh battery. But mAh only measures capacity at the battery's internal voltage, usually 3.7 volts, not at the 5 volts your phone actually charges from. Airlines do not use mAh at all. They regulate energy in watt-hours, and that single unit conversion is where most travelers get confused, and occasionally get their power bank pulled at security.

The mAh to Wh math, and why it decides if you can fly

The conversion is one line: mAh x 3.7V / 1000 = Wh. A 20,000mAh power bank works out to about 74Wh. A 27,000mAh power bank lands close to 100Wh, which is not a coincidence. Manufacturers size their biggest "travel-legal" models right at that line on purpose, because 100Wh is where airline rules change.

Most power banks list the Wh figure on the casing next to the mAh number, so once you know what it means, you rarely have to calculate it yourself.

What airlines actually allow

The rule, as covered in more detail in our power bank flight rules guide, breaks into three bands:

• Up to 100Wh: allowed in carry-on baggage on essentially every airline, no approval needed

• 100 to 160Wh: allowed only with the airline's advance approval, usually capped at two batteries per passenger

• Over 160Wh: banned from passenger flights entirely, no exceptions

This is why almost every power bank marketed for travel tops out around 25,000 to 27,000mAh. Go bigger than that and you are buying a battery that either needs a phone call to the airline before you fly, or cannot fly with you at all.

The real capacity trap

Here is the part the spec sheet does not mention: you never get the full rated capacity out of a power bank, even in ideal conditions. Converting the battery's 3.7V charge up to the 5V or higher your device needs costs energy, and the circuitry, cable, and heat generated along the way cost more. In practice, a power bank delivers roughly 60 to 70 percent of its printed mAh figure as actual, usable charge.

That means a 20,000mAh power bank realistically hands over something closer to 12,000 to 14,000mAh at the USB port, not 20,000. It is still enough for around three full phone charges for most modern smartphones, but if you were expecting four or five based on the box, you will run out earlier than planned. Budget on the lower number, not the printed one, and a mid-range bank will never disappoint you.

How much power bank you actually need, by trip

Match the battery to the trip, not to whichever number looks biggest on the shelf.

• Day trips and city breaks: a 10,000mAh bank is plenty. It weighs almost nothing, clears security without a second look, and delivers roughly one and a half to two and a half phone charges, enough to survive a day of maps, photos, and transit apps until you are back at a plug.

• Long-haul flights and long travel days: step up to 20,000mAh, which lands around 74Wh and flies without any airline approval anywhere. It covers a phone through a full day of delays and layovers, and still leaves enough for a tablet or a set of earbuds. This is the size worth pairing with the routines in our guide to keeping your electronics charged abroad, since a long flight is exactly when sockets are scarcest.

• Backpacking or trips with unreliable power: go up to 25,000 to 27,000mAh, right at the edge of the 100Wh line, so it still flies without a fight while giving you the most capacity legally available. This is the range digital nomads working from trains, hostels, and off-grid guesthouses actually need, since a dead laptop mid-deadline is a different problem than a dead phone.

PD output: the number that saves a laptop, not just a phone

Capacity is only half the spec. The other half is output wattage, listed as USB PD (Power Delivery) on the box. A basic power bank rated for phones alone often maxes out around 18 to 20 watts, plenty for a phone, useless for anything bigger. Tablets generally want 30 to 45 watts. Laptops are the real test: most modern ultrabooks need 45 to 65 watts to charge at a normal pace, and larger machines want closer to 100 watts.

If you plan to use your power bank as a genuine laptop backup, not just a phone top-up, check for USB-C PD output rated at 65 watts or higher before you check the mAh number at all. A 27,000mAh bank with only 18W output will trickle-charge a laptop so slowly it barely keeps pace with the screen draining it. Before you decide, it is worth checking how to confirm your devices are voltage compatible with whatever charger and region you are traveling through.

For most travelers, a mid-size 20,000mAh bank with USB-C PD covers phones, earbuds, and the odd tablet top-up without drama. A general-purpose power bank with USB-C output in that range is the sensible default if you are not sure yet which category you fall into.

Buy for the trip you actually take most often, not the worst-case trip you take once a year. Check the Wh number before the mAh number, confirm the PD wattage if a laptop is involved, and remember that whatever is printed on the casing, you are getting about two-thirds of it in real charge.

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