mAh to Wh: What Battery Capacity Really Means and the 100 Wh Flight Limit

Milliamp-hours measure charge, watt-hours measure energy. How to convert between them, why mAh alone tells you nothing, and what the 100 Wh airline limit works out to.

Introduction

Every power bank, phone and rechargeable cell is sold on a milliamp-hour figure, and it is the least useful number on the box. Airlines regulate batteries in watt-hours, laptops are specified in watt-hours, and a milliamp-hour rating cannot be compared between two batteries unless they happen to run at the same voltage. Converting between the two takes one multiplication and removes most of the confusion.

The Core Mathematics

Wh = (mAh × V) ÷ 1000

mAh to Wh: multiply by the voltage, then divide by 1,000.
Wh to mAh: multiply by 1,000, then divide by the voltage.

The 1,000 is only there because the rating is in milliamp-hours. In amp-hours the relationship is simply watt-hours = amp-hours × volts, and 1 Ah = 1,000 mAh.

Most lithium-ion cells have a nominal voltage of 3.7 V, so that is the figure to use unless the battery states otherwise. A 5,000 mAh phone battery is therefore 5,000 × 3.7 ÷ 1,000 = 18.5 Wh.

Why mAh Alone Tells You Nothing

This is the heart of it. Milliamp-hours measure charge; watt-hours measure energy. They are different physical quantities, and voltage is what connects them.

Compare two cells with identical 3,000 mAh ratings:

A lithium-ion cell at 3.7 V holds 11.1 Wh
A NiMH rechargeable at 1.2 V holds 3.6 Wh

Same number on the label, roughly three times the energy in one of them. Comparing batteries by milliamp-hours alone is only valid when the voltages match, which across different battery chemistries they usually do not.

The 100 Wh Flight Limit

This is where the conversion stops being academic. Lithium battery limits for air travel are set in watt-hours, and the thresholds applied by most regulators and airlines are:

Up to 100 Wh: permitted in carry-on baggage without prior approval
100 to 160 Wh: airline approval required, usually limited to two spares
Above 160 Wh: not permitted in passenger baggage at all

Spare batteries and power banks must travel in the cabin rather than in checked luggage, because a fire in the hold cannot be reached.

At the usual 3.7 V, the 100 Wh ceiling works out to about 27,000 mAh. That is why so many power banks stop just short of that figure: a 26,800 mAh pack is 99.2 Wh, comfortably legal, while a 30,000 mAh pack at the same voltage is 111 Wh and needs permission.

Laptop manufacturers do the same thing. A great many premium laptops carry a battery of 99 Wh or 99.6 Wh, which looks arbitrary until you notice the number it is staying below.

Why a 20,000 mAh Power Bank Does Not Charge a 5,000 mAh Phone Four Times

A common and entirely reasonable expectation that reality does not meet. The arithmetic explains why.

A 20,000 mAh power bank at 3.7 V holds 74 Wh. The phone battery at 5,000 mAh and 3.7 V is 18.5 Wh. On paper that is four full charges.

What intervenes is voltage conversion. The pack's cells sit at 3.7 V but USB delivers 5 V, so the power bank must step the voltage up, and the phone then steps it back down to charge its own cell. Each conversion loses energy as heat. At around 85 percent overall efficiency you get about 3.4 charges; in practice most people see between 2.5 and 3.

This is also why the honest way to read a power bank's rating is in watt-hours. Expressed at USB's 5 V rather than the cells' 3.7 V, that 74 Wh pack is only about 14,800 mAh before losses, and roughly 13,000 mAh after them.

A Sense of Scale

AA alkaline cell: about 2,500 mAh at 1.5 V = 3.75 Wh
Phone: 5,000 mAh at 3.7 V = 18.5 Wh
Tablet: around 30 Wh
Laptop: 50 to 99 Wh
Power bank: 10,000 mAh at 3.7 V = 37 Wh
Car starter battery: 60 Ah at 12 V = 720 Wh
Electric car: 60,000 to 100,000 Wh, which is why they are quoted in kilowatt-hours instead

The jump at the end is the reason the unit changes. A 75 kWh car battery is 75,000 Wh, about four thousand phone charges, and writing that in watt-hours would serve nobody. Our guide to kilowatt-hours and megajoules covers that end of the range.

What the Charge Rating Really Measures

Worth knowing what an amp-hour is, since it is a slightly odd unit. Current is charge per second, so current multiplied by time gives charge. One amp-hour is one amp flowing for one hour, which is 3,600 coulombs.

Manufacturers use it because it is what a battery test measures directly: discharge the cell at a known current and time how long it lasts. Energy in watt-hours has to be derived from that by bringing in the voltage, and voltage is not constant during a discharge, which is why a nominal figure such as 3.7 V is used rather than a real one.

Quick Reference Conversion Table

At the standard lithium-ion 3.7 V:

2,000 mAh = 7.4 Wh
5,000 mAh = 18.5 Wh
10,000 mAh = 37 Wh
20,000 mAh = 74 Wh
26,800 mAh = 99.2 Wh (just under the flight limit)
27,027 mAh = 100 Wh exactly
43,243 mAh = 160 Wh (the absolute ceiling)
1 Ah = 1,000 mAh = 3,600 coulombs

Common Mistakes Worth Avoiding

Comparing mAh across different voltages

The figure is only comparable at equal voltage. Convert both to watt-hours before deciding which battery is larger.

Assuming a power bank delivers its full rated capacity

The rating describes the cells at 3.7 V, not what reaches your phone at 5 V. Expect to lose roughly a fifth to conversion.

Using 5 V in the conversion

A power bank's output is 5 V but its cells are 3.7 V, and the capacity is rated at the cell voltage. Using 5 V inflates the watt-hour figure by a third and can make a legal battery look like one that needs airline approval.

Putting spare batteries in checked luggage

Power banks and loose cells belong in the cabin. This is a safety rule rather than a formality.

Conclusion

Multiply milliamp-hours by voltage and divide by 1,000 to get watt-hours, using 3.7 V for lithium-ion unless told otherwise. Milliamp-hours measure charge and watt-hours measure energy, so only the watt-hour figure lets you compare two batteries or check one against the 100 Wh limit that governs taking it on a plane.

Try our watt-hours to kilowatt-hours converter for quick and accurate conversions.