How Do Power Banks Report Capacity? The mAh-vs-Wh Truth Behind the Label

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A 20,000mAh power bank stores 74 watt-hours of energy. By the time that energy has been boosted from the cells' 3.7 volts up to the USB port's 5 volts, pushed through a cable, and pulled into your phone's own charging circuit, roughly 52 watt-hours actually arrive — about 2.7 full charges of a 5,000mAh phone, as a detailed MakeUseOf breakdown from July 2026 worked through step by step. That gap between the number on the box and the charge in your phone confuses millions of buyers, and it all starts with how power banks report capacity in the first place.

Quick answer: Power banks report capacity as the combined milliamp-hours (mAh) of their internal lithium cells, measured at the cells' nominal voltage of 3.7V — not at the 5V your devices actually receive. A 20,000mAh label means 20,000mAh at 3.7V, which is 74Wh. Usable output at the USB port typically lands at 60–70% of that printed figure. Better brands also print a lower "rated capacity" at 5V alongside watt-hours (Wh), and 2026 regulations in China and the EU now require Wh on the label.

About the author: Chester Takau is an independent tech reviewer who synthesises professional testing data with real user experience to cut through marketing claims.

The Three Numbers on a Power Bank Label

Pick up any decent power bank and you'll find up to three capacity figures, each describing something different:

Cell capacity (mAh at 3.7V) — the big number in every ad. It's simply the sum of the cells inside: four 5,000mAh cells make a "20,000mAh" bank. Because lithium cells sit at a nominal 3.7 volts, that's the voltage attached to the figure, even though nothing you own charges at 3.7V.
Rated capacity (mAh at 5V) — what the USB port can actually deliver, and always a lower number. HowToGeek calls printing only the cell figure a "marketing white lie" and singles out Anker as one of the few brands that voluntarily prints both numbers on the box.
Watt-hours (Wh) — total energy, independent of voltage. This is the figure regulators and airlines care about, and the most honest way to compare two banks.

"Rated capacity is 10,400mAh" — what does that actually mean?

This exact question comes up constantly on Quora, and the answer is short: the cells inside total 10,400mAh at 3.7V, which is about 38.5Wh of energy. Converted to an ideal 5V output that's roughly 7,700mAh; after normal conversion losses, expect around 6,500–7,000mAh at the port — a bit under one and a half full charges of a 5,000mAh phone. The label isn't lying. It's just speaking a different voltage than your phone.

Why Are Power Banks Rated in mAh Instead of Wh?

Two reasons, one innocent and one less so. The innocent one is convention: battery cells have been specified in milliamp-hours for decades, and early power banks were little more than one or two 18650 cells in a plastic shell, so quoting the cell's mAh was the natural label. The less innocent one is marketing. "20,000" looks far better on a product page than "74Wh," and every brand knows it.

Threads debating why the industry uses mAh rather than Wh have run for years. The physics answer is clear: watt-hours are the better unit because they don't change with voltage, which is exactly why regulators are now forcing Wh onto labels — and why some commentators want mAh banned from advertising altogether. More on those rules below.

Where the "Missing" Capacity Goes

The gap between the printed number and your phone's battery percentage comes down to a chain of losses, each one ordinary physics:

Voltage conversion. The cells operate between roughly 3.0V and 4.2V; the USB port delivers 5V (or 9V/20V for fast charging). Energy is conserved, current is not. A 20,000mAh bank at 3.7V holds 74Wh, which works out to 14,800mAh at 5V — before a single loss.
Converter efficiency. The boost circuit that steps 3.7V up to 5V runs at roughly 85–90% efficiency. The rest leaves as heat.
Cable and phone intake. Your phone's own charging circuitry wastes more energy as warmth. End to end, the MakeUseOf worked example lands at about 70% — 52Wh of the original 74Wh reaching the phone's battery.

The consensus rule of thumb across testing sites like Power Bank Expert is that usable capacity is roughly 60–70% of the advertised cell figure; Jackington's video explainer suggests a quick "×0.6" mental check. Most articles stop at explaining the concept, so we did the arithmetic for the four sizes people actually buy:

Advertised capacity	Energy (Wh)	Ideal output at 5V	Realistic output at the port	Full charges of a 5,000mAh phone
10,000mAh	37Wh	7,400mAh	≈6,500mAh	≈1.3
20,000mAh	74Wh	14,800mAh	≈13,000mAh	≈2.7
26,800mAh	99.2Wh	19,800mAh	≈17,500mAh	≈3.6
50,000mAh	185Wh	37,000mAh	≈32,600mAh	≈6.7

Assumptions: realistic output uses an 85–90% efficient boost converter; charge counts use ~70% end-to-end efficiency (the MakeUseOf model) into a 5,000mAh phone battery of about 19.25Wh. Better converters and shorter cables push you toward the top of the range.

For a visual walkthrough of the same arithmetic, Jackington's February 2026 explainer works through the 3.7V-to-5V conversion and the ×0.6 shortcut in plain terms:

Honest Physics vs. Fake Capacity

Everything above is honest physics — a 20,000mAh bank delivering about 13,000mAh at the port is behaving exactly as designed, and the label is technically true. Fraud is a different thing entirely, and conflating the two lets scammers hide behind the "it's just physics" excuse.

Fake capacity has its own physics check: weight. Lithium-ion cells pack roughly 250Wh per kilogram, so a genuine 50,000mAh bank (185Wh) carries around 700–750 grams of cells alone — call it close to a kilo with the housing and electronics. A real 20,000mAh bank weighs roughly 350–450 grams. When a palm-sized, 300-gram listing claims 50,000mAh or 100,000mAh, the number is fiction, as investigations into fake-capacity listings have shown for years. The problem keeps resurfacing: in May 2026, a viral train video appeared to show a vendor selling a counterfeit bank — sand and dead batteries under a real brand's logo.

Practical red flags: weight far below the figures above, no Wh marking anywhere on the label, a price that undercuts every brand-name equivalent, and a huge mAh claim with no rated capacity printed. The safest defense is buying from vetted roundups — our budget tech picks worth the price — rather than the cheapest listing on the results page.

One more thing worth separating: trust in capacity claims took a hit from safety news when Anker recalled power banks over fire and burn hazards — roughly 481,000 units across five models plus about 1.16 million of another, per the CPSC. Overheating cells and inflated capacity labels are separate issues, but the recalls made a lot of buyers read every line of the label for the first time.

How to Measure a Power Bank's Real Capacity at Home

If your "20,000mAh" bank feels suspiciously small or light — a question that comes up word for word on Quora — you can settle it for under $20 with a USB power meter, the same inline gadget reviewers use. The method:

Charge the power bank fully, until its own indicator reads 100%.
Plug the meter into the bank, then connect a steady load — a phone works, a constant-current USB load is better.
Let the bank drain until it shuts itself off.
Read the meter's accumulated mAh and Wh.
Compare: the Wh reading should land around 85–90% of the label's Wh, and the mAh reading around 60–70% of the printed mAh. Far below that, and the label was fiction.

This is exactly what The Hook Up did in his 18-bank Amazon capacity test, running models from Anker, Baseus, UGREEN, ROMOSS, Shargeek and others down to zero on a meter. It's worth a watch before trusting any listing's numbers.

The 2026 Labeling Rules: What They Mean for the Bank You're About to Buy

Two big markets changed the rules this year, and coverage so far has mostly lived on compliance blogs rather than anywhere shoppers will see it.

In China, updated CCC certification rules that took effect March 1, 2026 require power banks to display both mAh and Wh on the mandatory mark — formatted like "20,000 mAh / 74 Wh" — plus a traceable QR code, following a wave of battery-fire incidents. In the EU, Battery Regulation 2023/1542 labeling duties took effect August 18, 2026: every battery sold in the bloc, power banks included, must show its category, chemistry, and capacity on the label, with a QR-code requirement following on February 18, 2027.

The practical payoff is a built-in lie detector: Wh ÷ 3.7 × 1,000 should equal the printed mAh. A label claiming 74Wh and 30,000mAh contradicts itself, and you should walk away. Meanwhile, 2026 industry trend pieces note the marketing conversation is slowly shifting from raw mAh toward output wattage and newer semi-solid-state cells — a healthier set of numbers to compete on.

mAh vs. Wh at the Airport

Airlines think in watt-hours, which is the other reason this conversion matters. The formula is simple: Wh = mAh × 3.7 ÷ 1,000. Under 100Wh, a power bank flies in carry-on with no paperwork — that ceiling works out to about 27,000mAh, which is why 26,800mAh (99.2Wh) models exist. Between 100Wh and 160Wh you need airline approval, and anything above 160Wh is banned from passenger cabins outright — meaning a genuine 50,000mAh bank at 185Wh can't fly at all, as airline power bank rules guides lay out.

If you're assembling a full kit for a trip — chargers, adapters, connectivity and the rest — our travel tech guide pulls the whole picture together.

How Many mAh Should a Good Power Bank Have?

Match the bank to the job, not to the biggest number on the shelf. A 5,000–10,000mAh bank is pocketable and good for one to one-and-a-bit phone charges. A 20,000mAh unit is the weekend sweet spot at roughly 2.7 charges. Flyers should stop at 26,800mAh. Past that, weight climbs fast and airline rules start to bite — and remember to check output watts too, since a laptop wants 45–65W or more regardless of capacity. Our travel tech packing list for 2026 pairs power bank sizes with trip length if you'd rather have the decision made for you.

FAQ

Can Amazon listings for power bank capacity be trusted?

Brand-name listings are usually honest but optimistic — they're using the 3.7V cell convention, so expect 60–70% of the headline figure. The real danger is no-name banks claiming 50,000–100,000mAh in small, light housings. Check the weight, look for a Wh marking, and favor listings backed by meter-tested reviews.

Is my power bank defective if it delivers less than its label?

Almost certainly not. Delivering 60–70% of the printed mAh at the USB port is normal physics. Suspect a defect or a fake only when real output falls well short of that range, or when the bank loses charge just sitting in a drawer.

Which number should I compare when shopping — mAh or Wh?

Wh. It's the only figure that doesn't change with voltage, it's what regulators now force onto labels in China and the EU, and it doubles as the number airlines check.

How do I calculate Wh if only mAh is printed?

Multiply the mAh by 3.7 and divide by 1,000. A 20,000mAh bank is 74Wh; a 26,800mAh bank is 99.2Wh — just under the 100Wh carry-on limit.

Sources

Updated September 2026.

Transparency note: This article was researched and written by Chester Takau with AI assistance for research gathering and drafting. All recommendations reflect the author's own editorial judgment.