Legit Explained
Power Bank mAh, What It Actually Means
A "25,000mAh" power bank will never give a phone 25,000mAh of charge. That's not false advertising — it's basic electrical physics, and the actual math is worth understanding before buying one.
Modern power banks display real-time output percentage — useful, but it doesn't change the underlying energy-loss math.
A power bank's mAh rating is its internal cell capacity, not what reaches your phone. Voltage conversion and circuitry lose roughly 30% to 45%, per the manufacturer data this article cites, so multiply the rating by about 0.65 and divide by your phone's battery size to estimate charges. Buy a rating comfortably above what you think you need.
The Short Version
- A power bank's advertised mAh rating measures its internal cell capacity, not what actually reaches your device.
- Energy is lost converting between voltages and through circuitry inefficiency — typically 30% to 45% of the rated capacity, according to manufacturer testing data.
- A "25,000mAh" power bank realistically delivers roughly 13,750 to 17,500mAh to your actual devices.
- You can estimate real-world charges with a simple formula, rather than trusting the mAh number at face value.
Why the Number on the Box Isn't the Number You Get
Power bank capacity is measured in milliamp-hours (mAh) at the battery cell's native voltage, which is typically 3.6-3.7V for lithium-ion cells. But USB output isn't delivered at that voltage — it's converted, usually to 5V, 9V, 12V, or 20V depending on the charging standard in use. That conversion isn't lossless. Circuitry, heat dissipation, and the physical act of moving current through wires and connectors all consume some of the original energy.
According to Anker's own published FAQ for one of its power banks, that loss typically runs 30% to 45% of rated capacity. In concrete terms: a power bank advertised at 25,000mAh typically delivers about 13,750 to 17,500mAh to your actual devices — meaningfully less than the number printed on the box, and not because of any defect or deception, just how voltage conversion physically works.
The Formula for Estimating Real Charges
Example: a phone with a 3,349mAh battery, charged from a 25,000mAh power bank:
(25,000 × 0.65) ÷ 3,349 ≈ 4.8 charges
The 0.65 multiplier accounts for that same 30-45% efficiency loss, landing on roughly 65% of rated capacity as the realistic usable output. It's not an exact figure — actual efficiency varies by power bank, cable quality, and charging speed — but it's a far more honest starting point than assuming the full mAh number is available to you.
Why This Matters When Comparing Power Banks
Two power banks with the same mAh rating can deliver meaningfully different real-world performance depending on their internal efficiency, output wattage, and how many devices they're charging simultaneously. A power bank rated for 165W total output, for example, typically has to split that power across active ports — charging three or four devices at once usually caps total output lower than charging just one or two, which is a genuine engineering tradeoff, not a marketing footnote.
| Rated Capacity | Realistic Usable Capacity (65%) | ~Phone Charges (3,500mAh battery) |
|---|---|---|
| 10,000mAh | ~6,500mAh | ~1.9 |
| 20,000mAh | ~13,000mAh | ~3.7 |
| 25,000mAh | ~16,250mAh | ~4.6 |
Estimates using the 65%-efficiency formula; actual results vary by power bank model, cable, and device.
The Honest Takeaway
None of this means power bank mAh ratings are meaningless or that manufacturers are misleading buyers — it's an industry-wide physical reality, disclosed by reputable manufacturers in their own technical documentation, not hidden fine print. The practical lesson is simple: when comparing capacity, mentally apply the roughly 65% rule rather than taking the printed number as the literal charge you'll get. A power bank rated well above what you think you need accounts for this gap by design.
How to choose
- If you want about one or two phone charges, a 10,000mAh bank is the row in the table that fits: roughly 1.9 charges for a 3,500mAh phone.
- If you want about three to four charges, look at 20,000mAh: roughly 3.7 charges on the same assumptions.
- If you want several charges or a tablet too, 25,000mAh gives roughly 4.6 phone charges, but the table is an estimate, not a promise.
- If you charge several devices at once, check the total output wattage and how it is split across ports, not just the mAh number.
- Before buying, find your phone's exact battery capacity in its specs and run the 0.65 formula yourself.
Questions and answers
How many times will a power bank charge my phone?
Use (power bank rated mAh x 0.65) / your phone's battery mAh. For example, a 25,000mAh bank and a 3,349mAh phone gives about 4.8 charges. It is an estimate, since real efficiency varies.
Why does a 25,000mAh power bank not deliver 25,000mAh?
The rating is measured at the cell's native voltage, usually 3.6 to 3.7V, but USB output is converted to 5V, 9V, 12V or 20V. That conversion and the circuitry lose some energy as heat.
Is it false advertising if a power bank delivers less than its rating?
No. The rating measures internal cell capacity, which is standard practice, and the gap comes from voltage-conversion physics. This article attributes the loss figure to Anker's published FAQ for one of its power banks; we did not re-verify that source.
Where does the 0.65 number come from?
It reflects a 30% to 45% loss, landing near 65% usable. It is an approximation, and actual efficiency varies by power bank, cable quality and charging speed.
Does a higher-wattage power bank lose less capacity?
Not necessarily. Wattage affects charging speed, not the basic conversion loss, so the 30% to 45% loss applies regardless of output wattage.
Why does charging several devices at once lower the output?
A power bank's total output is usually shared across active ports. A bank rated for 165W total typically delivers less per port when three or four devices are plugged in than when one or two are.
How big a power bank do I need?
Work backward from the estimate: choose a rating that gives the number of charges you want after the 0.65 adjustment, then go a bit higher. See the how-to-choose list above.
Can I take a power bank on a plane?
The TSA says power banks with lithium-ion batteries must go in carry-on bags and are not allowed in checked luggage. Its page does not list watt-hour limits, so check your airline and the FAA guidance for capacity limits.
Does the cable affect how much charge I get?
It can. Cable quality is one of the factors that affects real-world efficiency, along with charging speed and the power bank itself.
Where do I find my phone's battery capacity?
In its technical specifications, usually listed in mAh. Use that number in the formula for the most accurate estimate.
This article contains no affiliate links and no buy buttons. Technical figures and the efficiency-loss formula referenced are sourced from Anker's official published product FAQ and are accurate as of writing. Actual efficiency varies by manufacturer, model, and charging conditions.