What the Numbers on a Power Bank Actually Mean

A 20,000 mAh battery does not put 20,000 mAh into your phone. Here is the arithmetic that explains the gap.

Portable batteries are sold on one big number, and that number is almost always misunderstood. A 20,000 mAh power bank will not charge a 5,000 mAh phone four times. It will usually manage two and a half charges, sometimes fewer. Nothing is being faked. The specification is measured at a different voltage than the one your phone charges at, and the conversion costs energy.

Capacity is measured at the cell, not at the port

Lithium-ion cells inside a power bank run at roughly 3.7 volts. Your phone charges over USB at 5 volts or higher. Converting from 3.7 volts up to 5 volts loses energy as heat, and the advertised milliamp-hour figure is measured at the lower cell voltage.

To compare capacity honestly, convert to watt-hours. Multiply the milliamp-hour rating by 3.7 and divide by 1,000. A 20,000 mAh bank holds about 74 watt-hours. Real-world delivery after conversion losses runs about 60 to 70 percent of that, so plan on roughly 45 to 52 watt-hours actually reaching your devices. A phone with a 5,000 mAh battery at 3.85 volts holds about 19 watt-hours, so two and a half charges is the honest expectation.

Watt-hours also matter at the airport. Airlines in the United States generally allow spare lithium batteries up to 100 watt-hours in carry-on baggage without approval, which is why nearly every consumer power bank tops out around 26,800 mAh. A bank without a printed watt-hour rating can be refused at the gate.

Output wattage decides charging speed, not capacity

Capacity tells you how long the bank lasts. Wattage tells you how fast it charges. These are independent, and a large bank with weak output is a common disappointment.

Look for the output specification printed on the body, which reads something like "5V/3A, 9V/2.22A, 12V/1.67A, 20W max." The maximum wattage is what matters. Rough guidance:

10W to 12W: fine for earbuds and older phones, slow for anything modern
20W to 30W: full-speed charging for most current phones
45W: enough to charge a lightweight laptop while it sleeps
65W to 100W: enough to run and charge most laptops

If a bank has several ports, check the shared budget. Many 65W banks deliver 65W only from one port alone, and drop to 45W plus 18W when two devices are plugged in.

Protocols: USB Power Delivery and the rest

Fast charging requires the device and the charger to agree on a protocol. USB Power Delivery, usually written USB PD, is the standard worth insisting on because it is what phones, tablets, laptops, and handheld consoles have converged on. PD 3.0 with Programmable Power Supply, listed as PPS, is what several Android phones need for their top charging speeds.

Proprietary protocols still exist, and a bank that lists only a vendor-specific fast-charge mode may fall back to a slow 5V/2A with your hardware. If the specification sheet does not say "USB PD," assume it does not have it.

Pass-through, recharge time, and the things nobody lists

Pass-through charging means the bank can charge your phone while itself being charged from the wall. It is convenient in hotel rooms with one outlet, and many banks do not support it. It is rarely on the front of the box.

Recharge time is the other omission. A 20,000 mAh bank with a 18W input takes roughly five hours to fill. The same capacity with 65W input takes under two. If you top up overnight it does not matter. If you refill between flights, it matters a great deal.

What to actually buy

For a phone-only carry, a 10,000 mAh bank with 20W to 30W USB PD output is the sweet spot for size and weight. For a phone plus tablet weekend, 20,000 mAh at 30W. For a laptop, look for 20,000 mAh or more with 65W or 100W PD output and confirm the input matches, so you are not waiting half a day for the bank itself.