Accessories

Power Bank mAh Ratings, Explained: Why You Never Get the Number on the Box

The physics of USB power delivery means advertised capacity and usable capacity are never the same number. Here's the real math, and three real power banks run through it.

F

Flossa

Published August 14, 2026 · 16 min read

Value Score

The complaint every power bank gets

Look through the reviews on almost any power bank listed as "20,000mAh," and within the first page you'll find someone convinced they got scammed. They plugged it into their phone, watched the charge percentage climb, and did the math: a 20,000mAh power bank should charge their roughly 4,000–4,500mAh phone battery about four and a half times over. In practice, they got somewhere between two and three charges, and assumed the battery inside was fake or degraded.

It isn't. The power bank is doing exactly what its internal cells are rated to do. The confusion is baked into the industry's own labeling convention, and once you understand it, you can read any power bank listing — Anker, Baseus, a no-name brand on a marketplace, doesn't matter — and predict roughly what you'll actually get before you buy it.

Two voltages, one label

The mAh figure printed on a power bank's packaging describes the capacity of the lithium-ion cells inside it, measured at the cell's native voltage — typically 3.6V to 3.7V nominal for standard Li-ion or Li-polymer cells. That's the voltage at which the battery chemistry actually stores and releases charge.

But nothing you own charges at 3.7V over USB. USB power delivery operates at 5V as a baseline, stepping up to 9V, 12V, 15V, or 20V for higher-wattage fast charging. To get power out of the bank and into your phone, the power bank has to run the current through a boost converter — a small circuit that steps the voltage up from ~3.7V to 5V (or higher).

Stepping voltage up necessarily steps current down for the same amount of energy, because power is voltage multiplied by current. And no converter is 100% efficient — some energy is lost as heat every time it happens. Both of those facts mean the mAh figure you see at 5V output is mathematically smaller than the mAh figure measured at 3.7V inside the cell, before you even account for conversion losses.

Worked example: a 20,000mAh bank

Here's the actual math, using round numbers:

1. Rated capacity: 20,000mAh at 3.7V (cell nominal voltage)
2. Convert to energy (watt-hours): 20,000mAh × 3.7V ÷ 1000 = 74Wh
3. Typical boost-conversion efficiency: power bank literature and third-party teardown testing generally puts USB-C power bank conversion efficiency in the 80–90% range for modern PD circuitry, lower for older or cheaper designs.
4. Usable energy at, say, 85% efficiency: 74Wh × 0.85 = 62.9Wh
5. Convert back to mAh at 5V output: 62.9Wh ÷ 5V × 1000 = ~12,580mAh

So a "20,000mAh" power bank, once you account for the voltage step-up and realistic conversion losses, typically delivers somewhere in the 12,000–14,000mAh range measured at 5V — commonly cited industry-wide as roughly 60–70% of the printed rating. That's not a defect. It's the same 20,000mAh cells doing the same job at a different voltage.

Where the rest of the energy goes

If you're doing the subtraction, roughly 30–40% of the printed mAh figure "disappears" between the cell and your phone. That gap is made up of a few real, physical factors:

• Voltage step-up loss — converting 3.7V to 5V (or higher, for fast charging) is the single biggest factor, and it's unavoidable with current battery chemistry.
• Heat — every boost converter wastes some energy as heat; this is why power banks get warm during use, especially at higher wattages.
• The bank's own standby draw — the internal circuitry, any LED display, and Bluetooth or Qi wireless charging coils (if present) draw a small amount of power just to operate.
• Cable and connector resistance — a longer or lower-quality cable loses a small amount of additional energy as heat along its length.
• Charge/discharge cycle inefficiency — some energy is lost every time a lithium-ion cell is charged and then discharged, independent of the USB circuitry.

mAh and watts are answering different questions

The second most common confusion is mixing up capacity (mAh) with charging speed (W, watts). They measure completely different things, and a bigger number in one doesn't mean a bigger number in the other:

A 10,000mAh power bank with a 65W USB-C port will charge a laptop faster than a 20,000mAh power bank with only an 18W port — but it will run out of total energy sooner. Neither bank is "better" in the abstract; they're built for different jobs.

Three real 20,000mAh power banks, compared

To make this concrete, here's how the math plays out on three actual, currently-listed 20,000mAh power banks, using each manufacturer's own published specifications:

*Estimated using the 3.7V cell-to-5V-output conversion described above at a representative 80–88% efficiency band; actual usable capacity varies by unit, ambient temperature, and which port/protocol is used.

Notice that all three list the same 20,000mAh capacity, but the higher-wattage models (30W and 65W) will hit that usable-capacity ceiling faster on compatible devices, and the 65W Baseus model can additionally top up a laptop — something the 18W Anker unit can't do at a meaningful speed. If your priority is charging a phone twice while barely noticing the weight in your bag, the wattage difference matters less. If you want one bank that can also rescue a dying laptop, it matters a lot.

The one mAh number that's actually regulated

There's a mAh threshold that isn't marketing at all — it's aviation regulation. The FAA and equivalent international bodies restrict lithium-ion batteries in carry-on luggage based on watt-hours (Wh), not mAh directly, which is why power bank listings for larger-capacity units often convert the number for you:

• Under 100Wh: generally allowed in carry-on without airline approval
• 100–160Wh: allowed only with prior airline approval, and typically limited to two spare batteries per passenger
• Over 160Wh: prohibited on most passenger aircraft entirely

Using the same 3.7V conversion from earlier, a 20,000mAh power bank sits at about 74Wh — comfortably under the 100Wh line. This is also exactly why you rarely see power banks marketed much above the 25,000–27,000mAh range from mainstream brands: past roughly 27,000mAh at 3.7V, a bank starts approaching the 100Wh threshold and becomes a harder sell for travel-focused buyers.

How to read a listing in under a minute

Next time you're comparing power banks, skip past the giant mAh number first and check these three things instead:

1. What's the max output wattage, and on which port? A 65W-capable bank is only fast if you're using the port and cable that supports 65W — secondary ports are almost always lower-wattage.
2. What charging standards does it support? USB-PD (Power Delivery) is the modern cross-brand standard; Qualcomm Quick Charge (QC) is common on older or budget Android-focused devices. A bank that only lists one may charge your specific device more slowly than the wattage number implies.
3. Does the wattage match what your device can actually accept? A 65W-capable power bank charging a phone that maxes out at 20W will charge at 20W, full stop — the extra wattage only matters if you also own something (a laptop, a tablet) that can use it.

Quick-reference cheat sheet

Bottom line

A power bank that "only" delivers 12,000–14,000mAh from a 20,000mAh rating isn't broken and isn't lying to you in any way that violates industry convention — it's reporting cell capacity at cell voltage, the same way every major brand does. The number worth comparing across products isn't the mAh on the box in isolation; it's mAh combined with max wattage, port type, and price, because those three together tell you how the bank will actually behave in your bag. Once you know the conversion math, you can mentally discount any power bank's advertised capacity by roughly a third and get a realistic expectation before you ever click "buy."