A 20,000 mAh power bank rating usually describes the capacity of its internal cells at about 3.7 V—not the amount of charge it can deliver from a 5 V USB port. That cell rating represents 74 Wh of energy. The bank converts that energy to USB voltage, and conversion losses reduce what reaches the port, so its rated USB output capacity is lower. Compare watt-hours and the bank’s output rating at its stated voltage and current, rather than expecting 20,000 mAh at the port.
Why the label and USB output use different numbers
mAh measures electric charge, not energy by itself. To compare capacity meaningfully, you need voltage too: watt-hours (Wh) = amp-hours (Ah) × volts (V). A 20,000 mAh pack is 20 Ah; at a nominal cell voltage of 3.7 V, that is 74 Wh.
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The cells operate at a different voltage from a typical USB output. As Anker Support explains, “To charge devices, a power bank must convert its 3.7V cell voltage to 5V output (or higher).” The conversion circuitry uses some energy, and the same energy expressed at a higher voltage corresponds to fewer mAh. So cell mAh and USB-output mAh are not directly interchangeable.
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Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →A useful estimate is: output mAh ≈ stored Wh × conversion efficiency ÷ output voltage × 1,000. For example, Anker illustrates the calculation with 74 Wh and an assumed 82% conversion efficiency: 74 Wh × 0.82 = 60.7 Wh; at 5 V, that is about 12,140 mAh. The 82% is an example, not a guaranteed efficiency for every model. Start with Wh, apply efficiency once, then convert to output mAh; do not count the voltage conversion as a second separate loss.
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What manufacturers’ capacity figures look like
These Xiaomi specifications show how a 20,000 mAh cell rating can coexist with a lower output rating. They are model- and market-specific examples, not a universal output figure for power banks.
| Model and market page | Cell capacity | Rated output capacity |
|---|---|---|
| Redmi Fast Charge Power Bank, PB200LZM (United States) | 20,000 mAh at 3.7 V; 74 Wh | 12,000 mAh at 5.1 V / 3.6 A |
| Power Bank 20000 with integrated cable, PB2020MI (Singapore) | 20,000 mAh at 3.7 V; 74 Wh | 13,000 mAh at 5 V |
Both figures describe different stages: energy stored in the cells and rated capacity available at an output under specified conditions. A lower output mAh rating alone is not evidence that the cell rating is deceptive. These specifications also do not independently verify the actual capacity of every unit; that requires testing the particular product under defined conditions.
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Why the number of phone recharges varies
Even a bank’s rated output capacity does not translate directly into a fixed number of full phone charges. The phone’s battery capacity is another voltage-dependent mAh figure, and energy is lost during charging inside the phone as well as during the power bank’s conversion. Results also depend on the bank’s selected output voltage and current, the load, and the device’s charging behavior.
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For a real-world comparison, look for tests that state the port, output voltage, current or wattage, and method. TechRadar reports that its VRURC T2055 test delivered 65.3 Wh of usable capacity at 20 W (9 V, 2.22 A), with 85% efficiency; its 10 W and 18 W tests produced different results. That is an editorial result for one model and test condition, not a general efficiency rate for 20,000 mAh banks. Its power-bank test guide notes that measurements vary by test condition and may not represent every use case.
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How to compare power banks fairly
- Compare stored energy: Wh is more useful than mAh alone when cell voltages differ. If a specification gives only mAh, check the voltage it refers to.
- Compare usable output: Use the stated rated capacity or output Wh, and note the voltage and current attached to that figure.
- Check charging compatibility separately: Ports, supported output voltages, and wattage tell you about compatibility and charging rate, not how much energy the bank stores.
- Check test conditions in reviews: A measured figure is meaningful only alongside the output voltage, load, port, and method. Do not assume results at one condition apply to another.
When a low output rating merits closer scrutiny
A lower USB mAh figure is expected when it is stated at a higher voltage than the cell rating, especially after conversion losses. To assess a particular power bank, first check whether its label or product page distinguishes cell capacity from rated output capacity and specifies the relevant voltage and current. If the stated output seems inconsistent with the product’s own specifications, or you need to verify the advertised cell capacity, a controlled load test is needed; the label alone cannot establish actual performance.
Keep the endpoint clear: power-bank output is energy delivered at its port. It is not the same as energy ultimately stored in the phone, because the phone also incurs charging losses.
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