Converting mAh to watt-hours, and why Wh is the figure to compare

Milliamp-hours (mAh) tell you the charge a cell can deliver at its rated voltage. Watt-hours (Wh) tell you the energy. To convert, multiply the amp-hours by the volts, or divide the milliamp-hours by 1,000 and then multiply by the volts. A 10,000 mAh bank at 3.7 V therefore stores 37 Wh. Watt-hours are the figure to compare with carry-on rules, with the energy a device needs, and with other batteries.

The conversion, with the arithmetic shown once

A milliamp-hour is one thousandth of an amp-hour, so the formula divides by 1,000 before the voltage is applied:

Wh = mAh ÷ 1000 × V
Wh = Ah × V          (Ah = mAh ÷ 1000)

For a 10,000 mAh bank with a 3.7 V cell, 10,000 ÷ 1,000 = 10 Ah, and 10 × 3.7 = 37 Wh. The same steps give 74 Wh for 20,000 mAh and 111 Wh for 30,000 mAh, all at 3.7 V. The conversion does not depend on how the bank is used. It is a property of the cell as rated.

Why the voltage on the label changes the answer

Most lithium-ion cells are rated at 3.6 V or 3.7 V, and the two ratings give different watt-hours for the same milliamp-hour figure. The difference is 0.1 V out of 3.6 V, or 2.8%. The table shows the same capacities at both ratings.

Use the voltage printed on the label when you have it. Some labels print a watt-hour figure directly. Check it against the formula with the printed milliamp-hours and voltage. If the two disagree, the label may round the number or quote a different rating, so the manufacturer’s specification is the place to settle it.

Why a USB output figure is a different number

The cell in a power bank works at its own voltage, but the USB port is usually 5 V after a converter. The same stored energy therefore appears as a smaller milliamp-hour figure when it is read at 5 V. Divide the 37 Wh of a 10,000 mAh bank by 5 V:

37 Wh ÷ 5 V = 7.4 Ah = 7,400 mAh at 5 V, before conversion losses

A bank marked 10,000 mAh therefore delivers about 7,400 mAh at USB voltage in the ideal case. That is not a fault. It is what happens when a fixed amount of energy is spread over a higher voltage. Only the watt-hour figure stays the same whichever voltage you read it at, which is why it is the one to compare.

Scaling between two banks

At one voltage, watt-hours are proportional to milliamp-hours, so the ratio of two banks is the ratio of their capacities. A 27,000 mAh bank at 3.7 V holds 99.9 Wh. A 10,000 mAh bank holds 37.0 Wh. The larger bank has 99.9 ÷ 37.0 = 2.7 times the energy. Banks at different voltages compare only in watt-hours. A 10,000 mAh bank at 3.7 V (37.0 Wh) and a 7,400 mAh bank at 5 V (37.0 Wh) hold the same energy, although their milliamp-hour figures differ.

The same proportion works in reverse when you need a target energy. If a trip needs 99.9 Wh of stored energy and your banks are rated 10,000 mAh at 3.7 V, you need 99.9 ÷ 37.0 = 2.7 banks, so three banks, which store 111 Wh in total. The margin is the point. The next section explains why the stored figure is not the energy that reaches the device.

Turning stored energy into usable energy

Stored watt-hours are not all delivered. The power budget tool multiplies the stored figure by an assumed end-to-end efficiency, which covers the bank’s conversion, the cable and the device’s charging circuit. The default is 80%. It is an assumption, not a measurement, and the methodology page describes the same simplified efficiency for batteries and for solar.

Wh stored = 20,000 ÷ 1,000 × 3.7 = 74.0 Wh
usable    = 74.0 × 0.80 = 59.2 Wh

A lower assumption, such as 60%, gives 74.0 × 0.60 = 44.4 Wh. The difference between 59.2 Wh and 44.4 Wh is 14.8 Wh, a quarter of the 80% figure. Efficiency is the largest unknown in this chain, and it is the one most worth measuring. A USB power meter placed between the bank and the device shows the energy that actually arrives, and that is a better input than a guessed percentage.

What the watt-hour figure leaves out

• Conversion and cable losses between the bank and the device.
• Ageing. A cell holds less charge after many charge cycles, and a printed figure describes a new cell.
• Temperature. A capacity quoted for normal conditions is not the capacity in the cold. The batteries in cold weather guide covers this.
• Protection circuitry, which can stop output before the cell is fully empty.

None of these enter the formula. The formula gives the energy in the cell as rated, and every later number depends on how much of that energy you can get out. The honest way to use a watt-hour figure is to treat it as the ceiling and then apply a smaller number for planning.

Reading a battery label before a trip

Look for four things on the bank or in its specification: the capacity in mAh, the cell voltage, any watt-hour figure, and the output ports with their ratings. Note whether the mAh figure refers to the cell or to a USB output. If the voltage is missing, use a stated nominal value and write it in your notes, because the watt-hour result moves by about 3% between 3.6 V and 3.7 V.

Keep the label figures with the trip plan so that the same numbers feed the power budget each time. Changing one input between two plans, such as the efficiency or the number of days, is then a clear comparison rather than a guess about what changed.

The 100 Wh and 160 Wh airline limits

The US Federal Aviation Administration writes its rules for spare lithium-ion batteries in watt-hours, and the FAA's own guidance gives the same conversion: divide mAh by 1,000 and multiply by the volts. Spare batteries and power banks travel in carry-on baggage, not checked bags. Up to 100 Wh per battery is allowed. From 101 to 160 Wh a spare needs the airline's approval, with a limit of two such spares per person. Above 160 Wh, spares are forbidden for passengers.

The table above shows where common sizes fall at 3.7 V: 26,800 mAh is 99.2 Wh and 27,000 mAh is 99.9 Wh, just under 100 Wh, while 30,000 mAh is 111 Wh and sits in the approval band. Airlines can set stricter limits, including on how many power banks each passenger may carry, and other countries' regulators publish their own rules, so check the airline and route before flying.

Where the watt-hour figure goes next

Once a bank is in watt-hours, the next question is how many charges it gives a phone, which is covered in how many phone charges a power bank gives. For a day-by-day budget that includes sunlight, the solar panel guide shows how a panel adds to the stored energy. A UV water treatment device draws from a battery as well, so the same conversion applies when you check its runtime; see UV water treatment. For the full plan, the offline trip planning checklist shows where batteries fit alongside the other items.