A milliamp-hour rating describes charge, and charge only translates into energy when you also know the voltage. A power bank's cells sit at a nominal 3.7 V, while its output is typically 5 V or higher, and the conversion involves losses. A power bank marketed as 10,000 mAh holds roughly 37 watt-hours and delivers roughly 6,000 to 6,500 mAh at 5 V. Watt-hours are the honest basis for comparison because they do not change with voltage.
Why the number does not mean what it looks like
Milliamp-hours measure charge. Energy is charge multiplied by voltage. A rating that gives only charge is therefore incomplete, and the missing half is the part that varies.
Inside a power bank, the cells sit at a nominal 3.7 V. At the USB output, the voltage is 5 V or higher, depending on what the connected device negotiated. The conversion between the two is not lossless.
So there are three different numbers in play, and a specification sheet may quote any of them without saying which:
- The charge the cells hold, at cell voltage
- The energy the cells hold, in watt-hours
- The charge the product can deliver at its output voltage, after losses
The third is always the smallest, and it is the only one a user actually experiences.
The arithmetic, worked through
Take a power bank described as 10,000 mAh.
Step one — energy in the cells. 10,000 mAh is 10 amp-hours. At a nominal 3.7 V, that is 10 × 3.7 = 37 watt-hours.
Step two — account for conversion. Raising 3.7 V to 5 V in a switching converter is efficient but not free. A well-designed product might achieve around 85 per cent. That leaves 37 × 0.85 ≈ 31.5 watt-hours available at the output.
Step three — express it at output voltage. 31.5 Wh ÷ 5 V ≈ 6.3 amp-hours, or roughly 6,300 mAh.
So a product sold as 10,000 mAh delivers in the region of 6,300 mAh at 5 V. Nothing improper has happened. The 10,000 mAh figure is a real statement about the cells. It is simply not a statement about what comes out of the port.
The same power bank delivering at 9 V would be described as roughly 3,500 mAh at that voltage — the same energy, expressed at a higher voltage. This is why output capacity quoted in milliamp-hours without stating the voltage is close to meaningless.
Rated, typical, and the figure that gets printed
Cell manufacturers publish two capacity figures. The typical capacity is the average across a production batch. The rated capacity is the floor the manufacturer guarantees every cell will meet.
The rated figure is the one that matters when a specification is being enforced, because it is what the supplier is committed to. The typical figure is the one that tends to appear in marketing, because it is larger. Where a supplier quotes only one number and will not say which it is, that is useful information about the supplier.
Comparing two products honestly
Compare watt-hours, and compare rated rather than typical figures where both are available.
Watt-hours are the right unit because they are voltage-independent. Two power banks described in watt-hours can be compared directly. Two described in milliamp-hours cannot, unless you know the voltage each figure refers to — and if they refer to different points in the circuit, the comparison is meaningless.
Where only milliamp-hours are given, you can derive an approximate watt-hour figure from the nominal cell voltage, but a derived figure is not the same as a declared one. The declared watt-hour rating is what appears in transport documentation, and it is the figure that determines which shipping rules apply.
The 100 watt-hour line
Lithium batteries above 100 Wh face substantially stricter transport rules, and most airlines refuse them in passenger baggage without prior approval. That threshold is written in watt-hours, and it is the reason a particular capacity keeps appearing at the top of product ranges.
A 20,000 mAh cell pack at 3.7 V holds 74 Wh. A 27,000 mAh pack holds just under 100 Wh — which is why 27,000 mAh is a common ceiling in the category, and why products above it are rare and marketed on a different basis. The limit is a transport rule, not a technical one, and it explains a product range boundary that otherwise looks arbitrary.
Three checks worth doing
Ask for the watt-hour figure. It is required on the product marking and in the transport documentation, so a supplier who cannot state it has a documentation gap that will matter later.
Ask which capacity figure is being quoted. Rated or typical, cell or output. A supplier who answers precisely is one who understands their own product.
Measure it. A USB power meter and a resistive load will tell you what a power bank actually delivers, in watt-hours, in about an hour. This is the only test that settles the question, and it costs less than a lunch. In a category where capacity claims are routinely ambiguous, being the buyer who measures is a durable advantage.