Will It Work? · Guide

mAh or Wh: how to read a power bank label

By Bulan Sarkar, founder of ElectronicsInfoline (since 2000)

A power bank sold as 20,000 mAh does not deliver 20,000 mAh to a phone, and nothing is wrong with it. The label counts the charge at one voltage, the phone takes it at another, and a converter sits in between. Here is how to read the two lines that settle it, using a maker's own label.

Wh = mAh ÷ 1000 × volts. One maker's label reads “Cell Capacity 20000mAh 3.7V 74Wh” and “Rated capacity 13000mAh (5V)”. In: 74 Wh. Out at 5 V: 65 Wh. The maker's own figures put the output at 87.8 % of what the cells hold.

The two lines on the label

Cell capacity is what the lithium cells hold, counted at their own voltage. Rated capacity is what comes out of the USB port, counted at 5 V. Both lines from one maker's spec (Xiaomi):

Figure 1. Two lines from a maker's power bank spec: the cells hold 74 Wh; the rated capacity at 5 V is 13,000 mAh × 5 V = 65 Wh. Out over in: 87.8 %.

The maths, on two labels

Label	Cells	Out at 5 V	Out ÷ in
20,000 mAh 3.7 V (Xiaomi)	74 Wh	13,000 mAh × 5 V = 65 Wh	87.8 %
30,000 mAh 3.7 V (Xiaomi)	111 Wh	18,000 mAh × 5 V = 90 Wh	81.1 %

The second label prints its energy as 111 Wh, and 30,000 mAh × 3.7 V ÷ 1000 gives the same 111 Wh. The gap between the in and out columns is the converter at work:

Figure 2. The cells sit near their printed voltage; a converter raises it to what the device needs, and loses some energy as heat on the way. That loss is the efficiency figure in the calculator.

Tips and tricks

Compare power banks in Wh, not mAh. Two banks with the same mAh at different cell voltages hold different energy.
If the label prints a rated capacity at 5 V, use it: the converter’s loss is already in it.
A phone’s own battery is also counted at its cell voltage. Divide the bank’s output Wh by the phone battery’s Wh for an upper bound on full charges.
For a 9 V or 12 V device the rated capacity at 5 V does not apply directly. Use the Wh and the maker’s efficiency, if given.

What we would do

Look for a power bank whose spec lists both lines, cell capacity and rated capacity, so you know what you are buying. Then take the rated figure in Wh to the power-cut backup calculator to see what it runs and for how long.

Questions people ask

How do I convert mAh to Wh? Divide the mAh by 1000 and multiply by the voltage printed beside it: 20,000 mAh at 3.7 V is 74 Wh.

Why do I get fewer phone charges than the mAh suggests? The mAh on the box is counted at the cell voltage, about 3.7 V. The output runs at 5 V or more, and the converter loses some energy on the way. One maker rates its 20,000 mAh bank at 13,000 mAh at 5 V.

Which number should I compare between two power banks? Watt-hours, or the rated capacity at 5 V if both labels print it. mAh only compares fairly when both are counted at the same voltage.

Sources

Each figure on this page is quoted from the source below or worked out from them. Sources checked 20 October 2026, each quote read again against the source in a second pass.

Xiaomi, Xiaomi Xiaomi Power Bank 20000 (Integrated Cable) PB2020MI: Cell Capacity 20000mAh 3.7V 74Wh Rated capacity 13000mAh (5V) mi.com/global/product/xiaomi-power-bank-20000/specs/
Xiaomi, Xiaomi Xiaomi 18W Power Bank 30000mAh PB3018ZM: Battery power 111Wh 3.7V 30000mAh Rated capacity 18000mAh (5V/3.6A) mi.com/global/product/xiaomi-18w-power-bank-30000mah/specs/