10,000 mAh is not 10,000 mAh: where a third of your power bank goes

The biggest number is the one that matters least

On a shelf of power banks, the mAh figure is printed larger than everything else, and it is the only number anyone compares. It is also the one number on the box measured in a place where you will never use that energy.

It is not a lie. It is an honest measurement of something other than what you care about. Capacity is counted inside the cell, where the voltage is 3.7. The USB socket puts out 5. Two different units wearing the same name.

The arithmetic, in three steps

• From the box to real energy. Multiply the mAh by the cell voltage: 10,000 mAh × 3.7 V = 37 watt-hours. That is everything that is in there.
• From the cell to the socket. Getting to 5 volts means converting, and converting spends some of it — the manufacturers themselves quote 80–85% efficiency. About 30 watt-hours survive, which is 6,070 mAh if you read it back at 5 volts.
• From the socket to the battery. Your phone loses something too while charging, usually 10–20%. Roughly 26 watt-hours actually reach the cell inside the phone.

The rule of thumb: of every ten you read, about six leave the socket and about five reach the battery. When a box implies two and a half charges, expect one and a half.

The unit that does not lie: watt-hours

On the back of every power bank there is a small number in Wh. That is the real energy, already worked out, and it does not change meaning depending on where you measure it. Which is exactly why it is the only figure airlines accept.

• The threshold is 100 Wh. Below it, a power bank goes in the cabin with no questions; between 100 and 160 you need the airline’s approval; above 160 it does not fly. A hundred watt-hours is roughly 27,000 mAh at 3.7 V, so a 20,000 sits comfortably inside.
• Cabin only, never the hold. That is a safety rule, not a baggage rule: nobody is watching a lithium cell in the hold if it goes wrong.
• Using one in flight is a separate question, and it is changing. Over the past year a number of airlines have banned recharging a power bank from the aircraft’s sockets, and some have banned using one at all in flight. It varies by carrier and the rules are recent — check yours before you fly rather than assuming.

If you are comparing two power banks, compare the Wh. It is the only number that does not change meaning depending on where it was measured.

What to read instead of mAh

• Watts, not milliamps. How fast it charges is decided by output watts: 18 W is the decent minimum, 30 W fills a modern phone properly, 65 W is what you need if a laptop is also in the bag. A huge slow brick is worse than a small quick one for almost every use.
• USB-C with Power Delivery, on both ends. One socket for everything and no dedicated cable. Careful: "USB-C" on the box does not promise Power Delivery, and a power bank that still recharges over micro-USB is an old design at any price.
• How many watts survive two devices. The headline figure is almost always one port on its own. Plug in two and they share, often unevenly: if you need both at once, look for the "total output" line in the specifications.
• How long it takes to refill itself. The number nobody checks, and the one that decides whether you actually use it. A 10,000 that refills in under two hours goes back in the bag; one that takes six stays flat on the nightstand.
• Weight per watt-hour. A 20,000 weighs about what two 10,000s weigh, and that is honest — energy has mass. The question is not how big, it is how much of it you are willing to carry every day.
• Pass-through charging. Being able to refill the bank and the phone from the same wall socket is worth an extra socket in a hotel room. Not every model does it, and it is rarely printed large.

Three sizes, side by side

And then there is the price

Power banks are a category where a struck-through price works especially well, because the product has no visible expiry: the same model stays listed for years and its launch price stops meaning anything after the first few months.

• Ask what the discount is calculated against. A percentage taken off a list price from three years ago is not a saving.
• Use the reference the store itself shows. On Amazon the typical price paid by customers over the past 90 days is often displayed next to the current one, and the FTC expects a struck-through "was" price to be a price the item genuinely sold at recently. We always say which one we are using.
• Be suspicious of mAh that are too large for the price. Energy costs money and cells cost by weight: a 30,000 for ten dollars either does not hold what it claims, or holds cells you would not want in your pocket.
• Check that a Wh figure is printed somewhere on the shell. It is what airline staff look for, and if it is not there, proving how big it is becomes your problem at the gate.

It is the same check we run before publishing. If a price does not pass it, the deal does not go out — which is why some days we publish less.