A 20,000mAh power bank should charge a 4,000mAh phone five times over. In practice it manages two full charges and change, and the reason has nothing to do with a manufacturer lying on the box. The printed number and the number that actually reaches your phone are measured under different conditions, and the gap between them is basic electrical math, not false advertising.
mAh Measures the Cell, Not What Comes Out the Port
The 20,000mAh figure describes the capacity of the lithium-ion cells inside the power bank at their native voltage, which is around 3.7 volts. USB output, though, runs at 5 volts, and often higher for fast-charging profiles. Milliamp-hours aren't a fixed unit of energy on their own; watt-hours are, and watt-hours equal volts multiplied by amp-hours. A 20,000mAh cell at 3.7V holds about 74 watt-hours of energy no matter what number is printed on the shell.
Boosting the Voltage Costs Capacity
To push that energy out at 5V, the power bank runs it through a boost converter, and because energy is conserved rather than amp-hours, raising the voltage necessarily lowers the current available at the same total energy. Do the math on that same 74 watt-hours at 5V instead of 3.7V, and the theoretical maximum drops to roughly 14,800mAh, a 26% loss before anything else is even accounted for. That conversion isn't a flaw in a specific product, it's a direct consequence of Ohm's law applied to a voltage step-up.
Then Real-World Inefficiency Takes Another Bite
Boost converters aren't perfectly efficient either. Even a well-designed one typically runs at 85 to 90 percent efficiency, and cheaper ones fall lower than that. Layer that loss on top of the voltage conversion math, and a 20,000mAh power bank realistically delivers somewhere around 12,000 to 13,000mAh at 5V, roughly 60 to 65 percent of the number on the packaging. None of this shows up as a lie on the spec sheet, because the 20,000mAh figure is technically accurate. It's just measuring something other than what most people assume it measures.
Why Two Power Banks With the Same Rating Can Perform Differently
This is also why two power banks both rated at 20,000mAh can charge a phone a noticeably different number of times. A pack with a more efficient boost converter and better internal battery management keeps more of that theoretical 14,800mAh ceiling intact, while a cheaper one loses more of it to heat during conversion. The printed mAh number tells you almost nothing about that difference, because it was never measuring the converter in the first place.
The Actual Takeaway
Divide a power bank's advertised mAh rating by roughly 1.6 to get a realistic estimate of what actually reaches a 5V device, and treat anything closer to the full number as a best-case figure rather than a promise. The energy is genuinely in there. Getting it out at a different voltage than it went in at is what quietly costs you a third of it before your phone ever sees a single milliamp.
