Power bank spec sheets are designed to confuse. The headline number is almost always mAh, because bigger numbers sell better. A 30,000 mAh bank sounds twice as good as a 15,000 mAh bank. The actual story is more complicated, and once you understand the relationship between mAh, watt-hours, and output wattage, the size and price differences across the category start making a lot more sense.
The three numbers that actually matter: capacity in watt-hours, output power in watts, and input power in watts. mAh is a marketing number that hides the real capacity. Once you can convert between them, sizing a bank for your devices and travel plans takes less than a minute.
The three numbers that matter
Watt-hours (Wh). The actual energy capacity. One watt-hour is the amount of energy delivered by one watt of power for one hour. A 100 Wh battery can deliver 100 watts for one hour, or 50 watts for two hours, or any other combination of power and time multiplying to 100. This is the universal unit and what airlines use.
Output watts. The maximum power the bank can push to a device at any moment. A 30W output bank can charge a 30W phone at full speed but a 100W laptop at only 30W (one third of full speed). Higher output means faster charging, up to the device’s own maximum input rate.
Input watts. The maximum power the bank itself can accept from a wall charger. Determines how quickly the bank recharges. Often much lower than output, which is why some banks take 6 to 8 hours to recharge while premium ones do it in 90 minutes.
mAh is a fourth number that you can mostly ignore once you have Wh. The conversion: Wh equals mAh times battery voltage divided by 1000. Most lithium-ion cells are 3.7V or 3.6V. So a 10,000 mAh bank is 37 Wh, a 20,000 mAh bank is 74 Wh, and a 27,000 mAh bank is 100 Wh (the airline carry-on limit).
Why mAh alone is misleading
The mAh number is measured at the cell voltage of about 3.7V. When the bank charges a device over USB, it has to convert that voltage up to 5V, 9V, 15V, or 20V depending on the negotiated charging profile. Voltage conversion always loses some energy as heat. Real-world conversion efficiency runs 80 to 90 percent for quality banks.
So a 20,000 mAh bank holds 74 Wh of stored energy but delivers only 60 to 65 Wh to your devices. An iPhone 16 with a 13.6 Wh battery should theoretically get 4.5 full charges from this bank. In practice you get 3.5 to 4, and the last charge is partial.
The discrepancy is not a defect or false advertising. It is just the physics of voltage conversion. A bank advertised at 20,000 mAh genuinely contains 20,000 mAh at 3.7V. The energy you can extract is always less than the energy stored.
This is why two power banks with the same mAh rating can deliver noticeably different real-world performance. The one with better voltage conversion (higher efficiency) wins.
Capacity sizing by device
Once you have Wh, sizing a power bank for your needs is straightforward.
iPhone or modern Android phone. Roughly 12 to 18 Wh of battery. A 10,000 mAh (37 Wh) bank delivers two to three full charges after losses. Sufficient for a weekend trip.
Tablet (iPad Pro 13, Samsung Galaxy Tab S10). Roughly 35 to 45 Wh. A 20,000 mAh (74 Wh) bank delivers one and a half tablet charges, or one tablet plus two phone charges.
Ultrabook (MacBook Air 13, Dell XPS 13). 50 to 60 Wh batteries. A 20,000 mAh bank gives you one charge, a 27,000 mAh bank gives you one and a half.
Performance laptop (MacBook Pro 14, ROG Zephyrus G14). 70 to 100 Wh batteries. A 27,000 mAh (100 Wh) bank delivers a single full charge with little to spare.
High-end laptop (MacBook Pro 16). 99 to 100 Wh battery. You need a 100 Wh bank to do one full charge, and you need a 140 Wh or larger to do any meaningful top-up plus other devices. This is also where you hit airline restrictions.
The airline rule that limits your choice
The FAA caps lithium-ion batteries in carry-on baggage at 100 Wh without airline approval. Above 100 Wh and up to 160 Wh, most airlines allow up to two batteries with prior approval. Above 160 Wh, prohibited on commercial flights.
That 100 Wh ceiling translates to 27,000 mAh at 3.7V. Most power banks rated up to 27,000 mAh stay safely under the limit. A few aggressive 27,000 mAh banks come close to 100 Wh and you should check the explicit Wh rating, which is required to be printed on the bank itself by FAA rules.
Above 27,000 mAh you are looking at portable power stations (Jackery, EcoFlow, Bluetti, Anker SOLIX) rather than traditional power banks. These typically run 200 Wh to 2,000 Wh and are for car camping, RV use, or home backup, not airplane carry-on.
The 100 Wh limit is per battery, not per bag. You can carry two 99 Wh banks in the same bag without approval. They just have to be in carry-on, with terminals protected (in the original packaging or with tape over the contacts).
Output wattage and what it means for fast charging
The output watt rating sets the ceiling for how fast your devices charge. Match the bank’s output to your device’s max input for full-speed charging.
20W output: enough for fast charging iPhones (up to 27W peak) and most Android phones. Bottlenecks anything beyond a phone.
30W output: fast charging for phones and most tablets. Still bottlenecks laptop charging.
45W to 65W output: fast charging for ultrabooks. Sufficient for a MacBook Air 13 or Dell XPS 13. Still slow for larger laptops.
100W output: covers most laptops up to 15-inch. MacBook Pro 14 and 16 (base) charge at full speed.
140W to 240W output: needed for the highest-tier laptops (16-inch MacBook Pro with M-series, high-end Windows creator and gaming laptops). Rare in the power bank category, more common in wall chargers and dedicated 240W EPR cables.
Multiple-port banks split the output. A 100W bank with two USB-C ports might do 100W on a single port but only 65W plus 30W when both are used simultaneously. Check the per-port specs if you plan to charge multiple devices at once.
Input wattage and recharge speed
The bank itself has to recharge, and how fast it recharges depends on input wattage.
10W input: cheap banks. 20,000 mAh bank takes 7 to 10 hours. 30W input: common mid-range. 20,000 mAh bank takes 2.5 to 3 hours. 60W input: premium tier. Same bank recharges in 90 minutes. 100W input: top tier (Anker 737, Baseus Blade). Recharges a 25,000 mAh bank in about 90 minutes.
A high-output bank with low input wattage is annoying because you can drain it faster than you can refill it. Read both numbers before buying.
Quality and safety markers
Look for:
- USB-PD 3.0 or higher, the modern fast-charging standard
- An explicit Wh number printed on the bank (required for flying)
- A reputable brand (Anker, Baseus, Ugreen, Mophie, Belkin, RAVPower)
- Battery type: lithium-polymer (LiPo) for newer designs, lithium-ion (Li-ion) for traditional cylindrical-cell banks
- Built-in cables on travel-focused models, separate ports for higher-power banks
- Display: a small OLED or LED shows percentage remaining and output wattage, useful for actually knowing what you have left
Avoid banks with no Wh markings, banks from unknown brands at suspiciously low prices, and banks that get noticeably hot during normal use.
A simple buying decision
Daily commute, phone only: 10,000 mAh (37 Wh), 20W output, USB-PD. About 30 to 50 dollars from a known brand.
Weekend trip, phone and tablet: 20,000 mAh (74 Wh), 30W output. About 50 to 90 dollars.
Travel including laptop: 27,000 mAh (100 Wh), 100W output, 60W input minimum. About 100 to 150 dollars.
Camping or off-grid, multiple devices over days: portable power station (Jackery, Anker SOLIX, EcoFlow), not a power bank. Different category.
For more on testing charging and cable performance, see our methodology.
Frequently asked questions
What does mAh actually measure?+
Milliamp-hours measure electric charge capacity at a specific voltage, typically 3.7V or 3.6V inside the battery cell. A 10,000 mAh power bank holds 37 Wh of energy at 3.7V (10,000 times 3.7, divided by 1000). The mAh number alone is meaningless without knowing the voltage.
What is the FAA limit for power banks on planes?+
100 watt-hours per battery in carry-on, no checked baggage. Most consumer power banks under 27,000 mAh fall under this limit. Banks between 100 and 160 Wh require airline approval (most allow up to two). Above 160 Wh banks are prohibited on commercial flights entirely.
Why is my 20,000 mAh power bank only giving my phone 3 full charges?+
Voltage conversion losses and the phone's larger effective battery capacity. A 20,000 mAh power bank holds about 74 Wh. After conversion losses (15 to 20 percent typical) you get about 60 Wh of usable output. An iPhone 16 has a 13.6 Wh battery, so you get roughly 4.4 charges in theory and 3 to 4 in practice.
How many watts do I need for fast charging?+
Phones: 20 to 30W USB-PD. Tablets: 30 to 45W. Laptops: 65W for ultrabooks, 100W for 14 to 15 inch performance laptops, 140W to 240W for 16-inch creator and gaming laptops. The bank, the cable, and the device all need to support the same wattage for fast charging to work.
Why does my power bank charge so slowly itself?+
Most power banks accept input at a lower wattage than they output. A 100W output bank might only accept 30W input, so it takes 3 hours plus to recharge. Premium banks (Anker 737, Baseus Blade) accept 100W input and recharge in about 90 minutes. Check both the input and output wattage on the spec sheet.
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