# mAh vs Wh: The Power Bank Number That Actually Matters at the Airport Every power bank advertises its milliamp-hours — 10,000mAh, 20,000mAh, 26,800mAh — but **mAh vs Wh** is the comparison that decides whether your battery board passes airport security, how many phone charges you really get, and why two "20,000mAh" banks can differ wildly in real capacity. The short version: **watt-hours (Wh) measure energy; milliamp-hours (mAh) measure charge at a specific voltage.** Airlines, laptops, and physics all speak Wh. Marketing speaks mAh because the number looks bigger. ## What mAh Actually Measures (and Why It's Voltage-Dependent) Milliamp-hours describe how much electric charge a battery holds — but energy depends on voltage too, and mAH ratings hide that. A 10,000mAh power bank rated at its internal cell voltage (3.7V) holds roughly **37 watt-hours** of energy. The same charge expressed at 5V USB output would be 50,000mAh; at a laptop's 20V it's 1,850mAh. Same energy, three different mAh numbers. That's why comparing mAh across devices is meaningless unless the voltages match: - **Phone batteries** are rated at ~3.85V — close enough to a power bank's cell voltage that mAh comparisons roughly work - **Laptop batteries** run 11–15V internally, so their mAh numbers can't be compared to a power bank's - **USB power banks** advertise the 3.7V cell figure — the most flattering number available ## The Wh Conversion You'll Actually Use One formula covers everything: **Wh = (mAh × V) ÷ 1000** A 20,000mAh bank at 3.7V = 74Wh. That single number unlocks the airport question, because **lithium battery airline rules are written entirely in Wh**: - **≤100Wh** — carry-on allowed, no approval needed (covers virtually every consumer power bank; 27,000mAh is the practical ceiling) - **100–160Wh** — airline approval required, usually capped at two spares - **>160Wh** — banned from passenger flights entirely - **Checked luggage** — power banks are prohibited in checked bags on every major airline, carry-on only The 100Wh line is why 26,800mAh appears everywhere in product listings: at 3.7V that's 99.16Wh — the largest battery that squeaks under the limit with margin to spare. When a listing quotes mAh only, do the 3.7V conversion before flying; assuming the seller's number is at a higher voltage to dodge scrutiny is not a risk worth taking at security. ## Why Your 20,000mAh Bank Charges a Phone "Only" 3–4 Times The marketing math says a 20,000mAh bank should charge a 5,000mAh phone four times. Real-world result: closer to three, and the arithmetic explains it. Two losses stack: 1. **Voltage conversion.** The bank stores energy at 3.7V but outputs at 5V (or 9V/12V/20V for fast charging). Boosting voltage trades current for voltage with ~10% conversion loss on the way up. 2. **Cable and thermal losses.** Every watt-hour passing through wires and charging circuits loses a few percent as heat — phone charging hovers around 80–90% end-to-end efficiency on a good day. Convert properly: 20,000mAh × 3.7V = 74Wh stored. A 5,000mAh phone battery holds 5,000 × 3.85V ≈ 19.25Wh. At 85% real-world efficiency, usable energy is about 63Wh — roughly **three full phone charges**. Buying decisions get much easier once you run this math instead of dividing the marketing numbers. ## Reading a Power Bank Label Like an Auditor Serious manufacturers print both figures. The label you want to see: - **Capacity**: "20000mAh / 74Wh (3.7V)" — both units, with the cell voltage stated - **Output ratings** per port: 5V⎓3A, 9V⎓3A, 20V⎓5A style — tells you the actual fast-charge capability - **Wh printed directly** — required by airline rules for batteries over a certain size, and its absence on a "26,800mAh" bank is a yellow flag If a listing refuses to state watt-hours anywhere — description, spec table, or label photos — assume the energy content is less flattering than the mAh headline, and search the model name plus "Wh" before buying. Regulators require Wh marking precisely because mAh is easy to inflate. ## Conclusion mAh vs Wh isn't trivia — it's the difference between a power bank that flies and one that gets confiscated, and between honest capacity math and marketing arithmetic. Wh measures energy and governs airline limits (100Wh for carry-on, no approval needed); mAh measures charge at a hidden voltage. Convert with Wh = mAh × V ÷ 1000, expect about three real phone charges from a "four-charge" 20,000mAh bank, and buy from labels that print both numbers. ## FAQ **How many watt-hours is a 10,000mAh power bank?** At the standard 3.7V cell voltage, 10,000mAh equals 37Wh — comfortably under the 100Wh carry-on limit. The formula: 10,000 × 3.7 ÷ 1000 = 37Wh. **Can I bring a 26,800mAh power bank on a plane?** Yes — 26,800mAh at 3.7V is about 99Wh, just under the 100Wh no-approval limit, and it must ride in your carry-on, never checked luggage. Confirm the watt-hour figure is printed on the device before flying. **Why doesn't my 20,000mAh power bank charge my phone 4 times?** Energy conversion losses: the bank boosts its 3.7V cells to 5V+ for output (losing ~10%) and charging circuits lose a few more percent as heat. Real-world usable energy is about 85% of the label, which turns the theoretical four charges into roughly three. **Is a higher mAh always better?** Higher mAh at the *same voltage* means more energy, but across different voltages the numbers aren't comparable — a 30,000mAh laptop battery at 15V holds far more energy than a 20,000mAh phone bank at 3.7V. Watt-hours is the only figure that compares fairly across devices. You can also cross-check your own numbers with the Spravs tool — it takes about a minute.