Which number on a power bank box tells the truth about how much phone charge it holds? The short answer is that one of them is a rough proxy and the other is real energy. Yet most shoppers buy the bigger mAh number and end up surprised when a 20,000 mAh pack charges a phone fewer times than expected.
The confusion is understandable. mAh (milliamp-hours) and Wh (watt-hours) both describe stored energy, but they do it differently. One depends on voltage, the other does not. Airlines care about one of them. Retailers print the other one in giant letters. This article untangles the two, explains the math, covers what regulators actually check, and shows how to read a spec sheet without getting fooled.
What mAh Actually Measures
mAh is a measure of electric charge, not energy. One milliamp-hour means a current of one milliamp flowing for one hour. It is a useful number, but it is incomplete on its own because charge without voltage tells you nothing about how much work the battery can do.
Think of it like water in a tank. mAh is the volume of water. Voltage is the pressure. A big tank at low pressure and a smaller tank at high pressure can hold the same total energy.
This is why comparing mAh across devices with different voltages is a mistake. A 10,000 mAh pack built on lithium-ion cells at a nominal 3.7 volts holds roughly 37 Wh of energy. The same 10,000 mAh figure at 5 volts would represent 50 Wh. The number on the label looks identical, but the energy is not.
Internal lithium-ion cells are conventionally rated at a nominal voltage of about 3.7 V, with an operating range of roughly 3.0 to 4.2 V depending on chemistry and manufacturer. That 3.7 V figure is the one used in most capacity math, and it is the reason mAh numbers on power banks are not directly comparable to mAh numbers on phone batteries rated at a different voltage.
What Wh Actually Measures
Watt-hours measure energy directly. One Wh is one watt of power delivered for one hour. Because it already folds voltage into the calculation, Wh is a universal yardstick. A 37 Wh pack holds 37 Wh of energy no matter what voltage its cells run at.
This is why Wh is the number regulators and airlines use. It is also why Wh is the fair way to compare two power banks side by side.
The conversion is simple arithmetic:
Wh = (mAh × V) ÷ 1000
Run it on common pack sizes at the standard 3.7 V cell voltage:
| Power bank rating | Voltage used | Energy in Wh |
|---|---|---|
| 5,000 mAh | 3.7 V | 18.5 Wh |
| 10,000 mAh | 3.7 V | 37 Wh |
| 20,000 mAh | 3.7 V | 74 Wh |
| 26,800 mAh | 3.7 V | 99.2 Wh |
| 30,000 mAh | 3.7 V | 111 Wh |
Notice how the 26,800 mAh pack lands just under 100 Wh. That is not a coincidence. Manufacturers deliberately size packs to sit beneath common airline thresholds, which is why you see that odd number so often.
Why the Two Numbers Diverge in Practice
Here is where buyers get burned. A power bank rated at 20,000 mAh does not deliver 20,000 mAh to a phone. That figure describes the internal cells at 3.7 V. The USB port outputs at 5 V, and converting from cell voltage to port voltage loses energy to heat.
On top of that, every conversion step has efficiency losses. In real-world use, a pack typically delivers roughly 60 to 70 percent of its nominal mAh rating to a connected device. So a 20,000 mAh pack might realistically deliver somewhere around 12,000 to 14,000 mAh at the USB port. That range varies by design, so treat it as a ballpark rather than a guarantee.
A practical tip: when comparing packs, convert everything to Wh first, then discount by roughly a third for usable output. Two packs with the same Wh will perform similarly even if their mAh labels look wildly different. And if you are pairing a power bank with larger backup equipment at home, the same energy-first thinking applies when you compare options like the best home generators for reliable backup power.
The Airline Rule That Makes Wh Matter
If there is one place where mAh vs Wh stops being academic, it is airport security. Aviation regulators classify lithium-ion batteries by watt-hour rating, not milliamp-hours. The commonly cited thresholds are:
- 100 Wh or less: generally allowed in carry-on with limits on quantity
- 100 to 160 Wh: typically requires airline approval
- Above 160 Wh: generally forbidden in passenger baggage
These thresholds appear across multiple regulatory frameworks, including IATA dangerous goods rules, US FAA and PHMSA rules for lithium batteries in carry-on baggage, and equivalent rules from China’s CAAC and EU/ICAO authorities. Every one of these frameworks expresses limits in Wh.
A word of caution: these rules are revised periodically, and the exact wording, quantity limits, and approval requirements can differ by airline and by jurisdiction. Always check the current text of the applicable regulation and your airline’s own policy before flying. Do not rely on a blog post, including this one, for the final word on what you can carry.
A practical detail that catches travelers constantly: airline staff usually check the printed Wh on the label. A pack with no visible Wh marking, or with a worn-off label, may be refused even if it is technically under the limit. Before a trip, photograph the label and keep the pack in carry-on, never checked baggage.
What Labels Do and Do Not Tell You
Consumer labeling of power banks is inconsistent. The EU Battery Regulation (EU) 2023/1542, adopted in 2023 and applying from February 2024 with phased obligations, addresses battery labeling and capacity information. Whether it specifically mandates Wh labeling on consumer power banks is something worth verifying against the regulation’s current text rather than assuming. (Electric battery)
In the United States, there is no single federal mandate requiring consumer power banks to print Wh, so far as can be confirmed. Many reputable brands print both mAh and Wh anyway, partly for airline compliance and partly because informed buyers ask for it.
What this means for shoppers is simple: treat the mAh number as marketing and the Wh number as engineering. If a listing shows only mAh, do the conversion yourself. If it shows neither clearly, that is a red flag.
Common Mistakes People Make
A few errors show up again and again when people compare power banks.
Comparing mAh across different voltages. A phone battery rated at 5,000 mAh at 3.85 V and a power bank rated at 5,000 mAh at 3.7 V are close, but a laptop pack rated in mAh at a higher voltage is a different animal entirely. Convert to Wh before comparing.
Assuming rated capacity equals delivered capacity. The 60 to 70 percent rule of thumb exists for a reason. A pack that promises three phone charges often delivers two.
Ignoring the output wattage. A high-capacity pack with a low-wattage port charges slowly. Capacity tells you how much energy is in the tank; wattage tells you how fast it comes out. Both matter.
Buying the biggest pack without checking flight rules. A 30,000 mAh pack converts to about 111 Wh, which crosses the 100 Wh line and may require airline approval or be refused outright.
Trusting the marketing photo over the spec label. The fine print on the back of the unit is where the real Wh figure lives. Check it before you buy, not at the gate.
How to Read a Spec Sheet in 30 Seconds
A quick routine makes any power bank listing easy to judge.
- Find the mAh rating and the cell voltage. If voltage is not stated, assume 3.7 V for lithium-ion.
- Multiply mAh by volts and divide by 1,000 to get Wh.
- Compare that Wh figure against other packs, not the mAh figure.
- Discount by roughly 30 to 40 percent to estimate usable output.
- Check the Wh against airline thresholds if you plan to fly.
- Check output wattage for charging speed, especially for laptops.
This same energy-first discipline pays off whenever you size any backup power system. If you are weighing battery-based options against fuel-powered ones, a solar generator comparison is easier to judge once you are comfortable converting between mAh and Wh. And if you are financing a larger purchase, running the numbers through a home equity loan calculator keeps the budget side as clear as the spec sheet.
When mAh Is Still Useful
None of this makes mAh useless. For comparing two packs built on the same cell chemistry at the same voltage, mAh is a perfectly good shorthand. It is also the number most phone batteries are rated in, which makes rough mental math convenient. (UL Solutions)
The rule of thumb is straightforward: use mAh for apples-to-apples comparisons at the same voltage, and use Wh for everything else. Wh is the number that survives contact with airlines, with mixed-voltage comparisons, and with any honest estimate of real-world performance.
There is a broader lesson here about how battery products are marketed. Big numbers sell. A 30,000 mAh label looks more impressive than a 111 Wh label, even though they describe the same pack. Once you know the conversion, the marketing loses its power over you. You can look at any listing, do two seconds of arithmetic, and know exactly what you are buying.
Conclusion
The mAh vs Wh question comes down to one idea: charge is not energy, and energy is what you actually get. mAh tells you how much charge is moving; Wh tells you how much work it can do. The conversion, Wh = (mAh × V) ÷ 1000, takes seconds and instantly makes any two power banks comparable.
For everyday shopping, convert to Wh, discount for real-world efficiency, and check output wattage for speed. For travel, check the printed Wh against current airline rules and confirm the exact thresholds with your carrier, since those rules are revised regularly and vary by jurisdiction. Do that, and the number on the box stops being a mystery and starts being a tool.
FAQ
Is a higher mAh always better in a power bank?
Not necessarily. Higher mAh means more charge, but only at the same voltage. A pack with a lower mAh rating at a higher cell voltage could hold more total energy. Convert both to Wh before deciding, and factor in output wattage, port selection, and weight.
How do I convert mAh to Wh for a power bank?
Multiply the mAh rating by the cell voltage in volts, then divide by 1,000. For a typical lithium-ion pack at 3.7 V, a 20,000 mAh rating becomes 74 Wh. If the voltage is not printed, 3.7 V is the standard assumption for lithium-ion cells.
Why do airlines use Wh instead of mAh?
Watt-hours measure actual energy regardless of voltage, which makes them a consistent safety metric across every battery design. Milliamp-hours depend on voltage and are not comparable across devices. Aviation regulators build their lithium battery limits around Wh for that reason.
How many times will a 20,000 mAh power bank charge my phone?
It depends on your phone’s battery size and conversion losses. A 20,000 mAh pack holds about 74 Wh, but real-world delivery is typically 60 to 70 percent of the nominal rating. In practice, expect roughly three to four charges for a phone with a 4,000 to 5,000 mAh battery, though results vary.
Can I take a 30,000 mAh power bank on a plane?
A 30,000 mAh pack at 3.7 V converts to about 111 Wh, which sits above the common 100 Wh threshold. That typically means it needs airline approval and may be refused. Always check the current rules and your airline’s policy, and make sure the Wh is printed clearly on the label.
Does a power bank’s mAh rating include conversion losses?
No. The mAh figure describes the internal cells at their nominal voltage, not what comes out of the USB port. Converting cell voltage to the 5 V output involves efficiency losses, which is why delivered capacity is usually well below the rated number.
