A power bank advertised as 20000mAh may deliver a smaller milliamp-hour figure at its USB output without anything being missing. The catch is voltage. Milliamp-hours describe electrical charge, while useful energy comparisons also need voltage. Read the watt-hour rating and the stated output-test conditions before deciding whether a capacity claim is realistic.
Milliamp-hours need a voltage beside them
A charge capacity cannot be compared fairly across different voltages by looking at the mAh number alone. Multiplying amp-hours by nominal voltage gives a nominal energy estimate in watt-hours. The nominal voltage is an accounting reference, not a claim that battery voltage stays perfectly constant through discharge. Power banks often describe the internal cells in one part of the label and delivered output capacity under specified conditions in another. Those quantities refer to different points in the system.
The conversion changes the number even before losses
Take an original example with 20 Ah of nominal capacity referenced to 3.7V. That corresponds to 74 Wh. At an ideal 5V output with no losses, the same energy would correspond to 14.8 Ah, or 14800mAh. A smaller mAh figure at 5V is therefore expected from voltage conversion alone. It would be wrong to call the difference between 20000 and 14800mAh entirely wasted energy. The voltage reference changed, so the charge quantity changed with it.
Real output includes conversion and operating limits
The power bank converter is not lossless, and its electronics consume some energy. Cutoff behavior, battery condition, temperature, and output load affect what can be delivered. If our hypothetical 74Wh bank delivered 63Wh at the output under a defined test, that would be about 85% of the nominal cell energy. At 5V, 63Wh corresponds to 12.6Ah. These are illustrative calculations, not a promised efficiency or capacity for a particular product.
Phone refill counts add another conversion
Dividing the bank mAh by the phone mAh mixes two battery systems and ignores their voltage references and losses. The phone also uses energy while charging. For a better estimate, compare energy quantities and allow for the complete charging path. Even then, a refill from 20% to 80% is not the same task as empty to full. Be explicit about the starting percentage, ending percentage, and whether the device is operating during the test.
What a meaningful capacity check looks like
Use documented test conditions and equipment suited to the supported output mode. A USB meter can measure energy passing its location, but it does not directly inspect internal cell capacity. Do not open a power bank to verify its label. Compare delivered watt-hours under a representative load with the manufacturer specification, including its voltage and cutoff conditions. If a product has inconsistent labeling or dramatically poor performance, seek support through the seller or manufacturer with the test conditions recorded.
What to check before you act
- Find the watt-hour rating and its voltage reference.
- Keep cell capacity separate from rated output capacity.
- Compare delivered energy at the same load and output mode.
- Estimate phone refills using comparable energy figures and clearly stated assumptions.
Common questions
Does a lower USB-output mAh number prove false advertising?
No. Voltage conversion alone changes the mAh quantity. The label still needs to describe its reference and test conditions accurately.
Does using a higher USB voltage destroy more capacity?
Not simply because the output mAh number falls. Compare watt-hours and efficiency, not charge capacity at different voltages.
The practical takeaway
Watt-hours make the comparison much clearer. Once you know where each capacity figure is measured, ordinary voltage conversion stops looking like missing battery capacity.
References and further reading
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
