Joules to Watt-hours: Battery Wh and Airline Limits

Turning a Power Bank's mAh Rating Into the Watt-hours an Airline Wants

Every rule that governs a lithium battery in the cabin is written in watt-hours, and almost nothing sold to consumers is labelled that way. The case of a power bank shouts a capacity in milliampere-hours, a drone pack quotes milliampere-hours and a cell count, a laptop battery hides its rating on a sticker underneath the machine. Watt-hours are what a check-in agent asks about, so the number has to be built before you reach the airport: divide the milliampere-hours by 1 000 to get ampere-hours, multiply by the pack's nominal voltage, and that is the figure the rule applies to. The joule is the same energy in SI, and it is the column a physics-minded reader recognises.

Conversion factor: 1 J = 0.000 277 777 778 Wh, because one watt-hour is exactly 3 600 J — a watt sustained through an hour of 3 600 seconds. So a 20 000 mAh power bank built on 3.7 V cells holds 20 × 3.7 = 74 Wh, which is 74 × 3 600 = 266 400 J.

Four Things the Label Does Not Tell You

The Printed mAh Is Counted at Cell Voltage

Marketing capacity is measured where the charge actually sits, inside the lithium cells at about 3.7 V — not at the 5 V the USB socket delivers. Multiplying 20 000 mAh by 5 V gives 100 Wh and lands you on the wrong side of a limit the same pack comfortably clears at 74 Wh.

Two Thresholds Decide the Whole Question

Under 100 Wh a spare battery travels in hand baggage without asking anyone. Between 100 and 160 Wh it needs the operator's approval and is normally capped at two spares per passenger. Above 160 Wh it cannot be carried as baggage at all and has to move as declared cargo.

Nominal Voltage Comes From the Chemistry

Use 3.6 or 3.7 V for a lithium-ion cell, 3.85 V for the high-voltage cells in recent phones, 3.2 V for lithium iron phosphate, 1.2 V for nickel-metal hydride and 1.5 V for alkaline. Cells wired in series add their voltages, so a four-cell drone pack sits at 14.8 or 15.4 V rather than 3.7.

A Bank's Contents Are Not Its Output

Stepping cell voltage up to 5 V or negotiating a USB-C profile costs something, so a 74 Wh pack typically hands over 55 to 65 Wh at the socket. The safety rule is written on what the cells contain, which is why the declared figure is always the larger of the two.

From a Cell Label to a Number You Can Show at the Gate

The arithmetic is short, but it moves across three units — milliampere-hours on the case, watt-hours in the regulation, joules in any physics-flavoured datasheet. The converter covers the last two and keeps the digits clean while you work through a bag full of devices.

Build the watt-hour figure from the case

Take the milliampere-hours, divide by 1 000, multiply by the nominal voltage of the chemistry inside. A 10 000 mAh bank on 3.7 V cells is 10 × 3.7 = 37 Wh; a 5 000 mAh phone at 3.85 V is 19.25 Wh.

Type a joule rating in when that is what the sheet gives

Cell datasheets, teardown reports and lab notes often state stored energy in joules or kilojoules instead. Put that value in the left box and the watt-hour equivalent builds as the digits land, so a 266 400 J figure resolves to 74 Wh with nothing to press.

Flip the arrows when the watt-hours are the known side

Most of the time you already hold a watt-hour figure and want the joules behind it. The swap control puts the watt-hour box on the left; typing straight into the right-hand box works equally well, because neither side is read-only.

Lift the bare figure for a declaration form

The copy control above either box returns the digits alone, and Ctrl+C inside a field does the same. Approval requests and shipping paperwork want a plain number in the energy field, with no unit text trailing behind it.

The limit applies per battery, not per bag: three separate 74 Wh banks are three articles under 100 Wh, not a single 222 Wh one. Non-rechargeable lithium metal cells are judged on grams of lithium rather than watt-hours, and many operators now require power banks to stay accessible in the cabin and not be charged during the flight. Read the airline's own page before a long trip.

Consumer Packs by Capacity, Voltage and Airline Category

Each row multiplies the capacity by the nominal voltage of that chemistry, then converts to joules at 3 600 per watt-hour. The last column reads the result against the 100 Wh and 160 Wh thresholds.

Battery or pack	Capacity	Nominal V	Energy (Wh)	Energy (J)	Cabin category
AA alkaline cell	2 500 mAh	1.5 V	3.75	13 500	Not a lithium article
18650 lithium-ion cell	3 500 mAh	3.6 V	12.6	45 360	Under 100 Wh
Smartphone battery	5 000 mAh	3.85 V	19.25	69 300	Under 100 Wh
Power bank, mid size	10 000 mAh	3.7 V	37	133 200	Under 100 Wh
Power bank, large	20 000 mAh	3.7 V	74	266 400	Under 100 Wh
Laptop battery, three cells	6 000 mAh	11.4 V	68.4	246 240	Under 100 Wh
Drone flight pack, four cells	5 870 mAh	15.4 V	90.4	325 433	Under 100 Wh
Travel power station, LiFePO4	12 500 mAh	12.8 V	160	576 000	Approval needed
E-bike battery	10 000 mAh	36 V	360	1 296 000	Not carried as baggage

Notice how much of the work the voltage column is doing. The e-bike battery holds the same 10 000 mAh as the mid-size power bank, yet its 36 V assembly stores nearly ten times the energy and falls outside what any passenger aircraft will take in a bag. The drone row is the one worth memorising: at 90.4 Wh it clears the threshold, but a 6 800 mAh version of the same 15.4 V pack reaches 104.7 Wh and suddenly needs a phone call to the airline.

What the Converter Adds at the Packing Stage

Each Battery in the Bag Checked in Turn

Overtyping the value recomputes immediately, so a camera pack, two spares and a laptop can be run through the same pair of boxes one device after another without clearing anything in between.

A Bare Wh Number for the Approval Request

The copy control hands back digits with no unit and no spacing, which is what an airline's dangerous-goods form or a courier's lithium declaration expects in its energy field.

kWh Is One Search Away for Oversized Packs

Search either dropdown for the kilowatt-hour, the kilojoule or the megajoule and the rest of the energy list follows, which covers portable power stations and e-bike packs quoted well beyond the cabin range.

Million-Joule Pack Totals Stay Grouped

Thousands are separated with spaces, so the 1 296 000 J of an e-bike battery stays countable, and anything past ten billion joules moves to exponent form instead of filling the field with zeros.

Cabin-Baggage Questions About Battery Watt-hours

My power bank says 20,000 mAh — what watt-hour figure does the airline want?

Seventy-four watt-hours, assuming the usual 3.7 V lithium-ion cells: 20 000 mAh is 20 Ah, and 20 × 3.7 = 74 Wh, or 266 400 J. That sits well under the 100 Wh mark, so it travels in hand baggage without permission. If the maker used 3.6 V cells the answer is 72 Wh, and a high-voltage 3.85 V design gives 77 Wh — none of which changes the category. A pack has to be near 27 000 mAh of 3.7 V cells before it crosses 100 Wh at all.

Why is the mAh on the case measured at 3.7 V and not at the 5 V USB output?

Because charge is a property of the cells, and those cells sit at roughly 3.7 V for most of their discharge. Quoting the count at that point makes the headline number as large as possible, which is why every manufacturer does it. The boost converter that lifts the output to 5 V cannot create charge, so at 5 V the same pack delivers far fewer milliampere-hours — around 11 000 to 13 000 for a nominal 20 000 mAh bank once step-up losses are taken out. Energy is the honest measure, which is exactly why transport rules are written in watt-hours instead of milliampere-hours.

The pack prints no Wh figure at all — can I still fly with it?

Usually yes, but you have to be able to justify the number. Work it out from the capacity and the chemistry, write it on a label stuck to the pack, and keep the calculation in mind. Staff are trained to look for a watt-hour marking, and where one is absent they fall back on the printed capacity and voltage — so a pack showing both is far easier to clear than one showing only a capacity. Anything with no legible markings whatsoever, or an obviously homemade cell assembly, is often refused outright regardless of how small it turns out to be.

Two 10,000 mAh banks, and one clearly holds more — how?

Milliampere-hours count charge, not energy, so the answer depends on the voltage each pack counts at. A bank whose cells are wired in series to 7.4 V and rated 10 000 mAh at that voltage carries 74 Wh, double the 37 Wh of an identical-sounding 3.7 V unit. Lithium iron phosphate at 3.2 V gives 32 Wh for the same capacity. Usable output then varies again with converter efficiency and with how generously the maker rounded. Comparing two packs only becomes meaningful once both are expressed in watt-hours or joules.

How many joules is a phone battery, and why is that number never printed?

A 5 000 mAh phone cell at 3.85 V holds 19.25 Wh, and 19.25 × 3 600 gives 69 300 J. Nobody prints it because the joule is inconveniently small for anything that runs for hours — five figures on a phone, six on a laptop, seven on an e-bike — and because the marketing department and the regulator each settled on other units long ago. The joule is still the right unit for comparison, though: it places a phone battery on the same scale as the 120 J or so a 70 kg person spends climbing one 175 mm stair, which makes the density of a lithium cell rather easier to appreciate.

Joules to Watt-hours