mAh to Wh

Watt-hours = (milliamp-hours ÷ 1,000) × volts. Milliamp-hours measure charge; watt-hours measure energy. You cannot compare two batteries on mAh alone unless they run at the same voltage, which is exactly why the aviation rules are written in watt-hours and not in mAh.

The part that trips people up is which voltage to put in. A lithium-ion cell has a nominal voltage of about 3.7 V — some chemistries are quoted at 3.6 V, and lithium iron phosphate (LiFePO4) cells are 3.2 V. The capacity printed on a power bank is the capacity of the cells inside it, measured at that cell voltage. It is not measured at the 5 V the USB port delivers.

Use 5 V by mistake and you get 100 Wh from a 20,000 mAh unit instead of 74 Wh — a 35% overstatement that can push a perfectly legal battery over a threshold on paper, or, worse, make an illegal one look compliant.

If the battery has a watt-hour figure printed on it, use that and skip the arithmetic entirely. Regulators expect the marked Wh rating, and any reputable manufacturer prints it precisely because the rules are written in that unit.

This is the same fact seen from the customer's side, and it is the single most common complaint about power banks anywhere in the world.

The cells hold 74 Wh of energy. To charge a phone the power bank must convert that from 3.7 V up to 5 V (or to whatever a fast-charge protocol negotiates), and conversion is never free — real-world efficiency is usually somewhere in the region of 85–90%, and it falls further at higher output currents or in cold weather. So the energy that actually reaches the phone is meaningfully less than the energy in the cells.

Expressed back in mAh at 5 V, a 20,000 mAh power bank typically delivers something in the region of 12,000–13,000 mAh. Nothing is faulty and nobody is necessarily lying: the box states the cell capacity, which is a real number measured in the standard way. But a buyer who expects to fill a 5,000 mAh phone four times will get roughly two and a half charges and will write a bad review about it.

For anyone selling these, the practical lesson is to publish both figures — the cell rating and a realistic output figure — rather than only the larger one. It costs a slightly smaller number on the box and removes an entire category of return.

These apply to a passenger carrying a battery, and both IATA and the FAA state them in the same terms.

Up to 100 Wh — normally permitted in carry-on baggage without prior approval. At 3.7 V that is about 27,000 mAh, which covers the overwhelming majority of consumer power banks.

Between 100 Wh and 160 Wh — permitted only with the airline's approval, and limited to two spare batteries per passenger. At 3.7 V that band runs from roughly 27,000 mAh to about 43,000 mAh.

Above 160 Wh — forbidden on passenger aircraft. Lithium-powered generators and most portable power stations sit here, which is why they cannot be flown at all as passenger baggage.

Two rules apply regardless of size. Spare batteries and power banks go in the cabin, never in checked baggage, and terminals must be protected against short circuit. Many airlines also impose their own stricter limits on how many power banks a passenger may carry and whether they may be used in flight, so the operator's own conditions are the final word.

Everything above concerns a person carrying a battery. If you are importing power banks, none of it is the rule that applies to you, and this is where a lot of first-time importers lose a shipment.

Power banks sent on their own are UN 3480, "Lithium ion batteries", and are handled under Packing Instruction 965. They are Class 9 dangerous goods. They are forbidden as cargo on passenger aircraft entirely — they move on cargo aircraft only.

Within PI 965 the watt-hour figure decides the section: a unit rated at or below 100 Wh is prepared under Section IB, and above 100 Wh under Section IA, which is the more demanding of the two. So the same conversion you did for the cabin rule also determines your freight paperwork.

The batteries must also be shipped at a state of charge not exceeding 30% of rated capacity. That is a requirement, not a recommendation, and it comes from Special Provision A331; shipping above 30% needs written approval from both the State of Origin and the State of the Operator. Signing the Shipper's Declaration is a legal statement that this has been complied with.

The classification changes with the packing. A power bank sent with a device is UN 3481 under PI 966; one already fitted inside a device is UN 3481 under PI 967. Earbuds shipped in their charging case fall under PI 966 — but the same case shipped on its own becomes UN 3480 again. And a box holding batteries plus only a charger and cables is still UN 3480, because the charger is not "equipment" for this purpose.

The 30% state-of-charge limit used to apply as a hard requirement only to batteries shipped by themselves. For batteries packed alongside equipment it was a recommendation.

From 1 January 2026 that recommendation became a requirement: lithium-ion batteries packed with equipment under PI 966, where the cells are rated above 2.7 Wh, must be offered for air transport at no more than 30% state of charge unless the relevant States have approved otherwise.

Batteries already installed inside equipment, under PI 967, are still governed by a recommendation rather than a requirement — no more than 30% state of charge, or an indicated capacity of no more than 25%. IATA recommends shipping at that level anyway, and given that the direction of travel has been one-way for several years, treating it as though it were mandatory is the cheaper assumption.

If you are placing orders now, the practical effect is that a factory which used to ship "packed with equipment" consignments fully charged has to change what it does, and you should confirm that it has.

Ask for the watt-hour rating, not the mAh. It is the number every regulation is written in, and a supplier who cannot state it immediately has probably never prepared an air shipment.

Ask to see the UN 38.3 test summary. Without it the goods cannot legally move by air at all, and it is specific to the cell — a factory that changes cell supplier needs a new one.

Ask how the batteries will be presented for shipment, and expect the answer to include the 30% state of charge. If the reply is vague, that is worth more than any brochure as a signal about how much exporting they have actually done.

And check the arithmetic yourself on the first order. A 20,000 mAh unit should be declared at about 74 Wh. If a document says 100 Wh for the same product, somebody has multiplied by 5 V, and that error will follow the shipment all the way to the airline that rejects it.