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.
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.
Frequently Asked Questions
Is it false advertising if a power bank doesn't deliver its full rated mAh?
No — mAh ratings measure internal cell capacity at the battery's native voltage, which is standard industry practice. The gap between rated and delivered capacity comes from voltage conversion physics, not deceptive marketing, and reputable manufacturers disclose the efficiency loss in their own technical FAQs.
Does a higher-wattage power bank lose less capacity to inefficiency?
Not necessarily — wattage affects charging speed, not the fundamental voltage-conversion loss. A higher-wattage power bank charges devices faster, but the roughly 30-45% capacity loss applies regardless of output wattage.
Why does charging multiple devices at once reduce total output?
A power bank's total output wattage is typically shared across active ports. Charging four devices simultaneously often caps total output lower than charging one or two, because the internal circuitry has a fixed maximum total power delivery.
How can I estimate how many times a power bank will charge my specific phone?
Use the formula: (power bank's rated mAh × 0.65) ÷ your phone's battery mAh. Check your phone's exact battery capacity in its technical specifications 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.