Everything you need to know about how to choose a power bank: wh vs mah, wattage, and what actually matters, explained plainly.
mAh (milliamp-hours) measures charge at a specific voltage.
The problem: manufacturers can choose different voltages to inflate the apparent mAh number.
A battery pack with 20,000 mAh at 3.7V contains 74 Wh.
A battery pack with 20,000 mAh at 3.85V contains 77 Wh.
Both have "20,000 mAh" on the label.
Formula: Wh = (mAh × V) / 1,000 Where V is the battery cell voltage.
If the manufacturer does not state the voltage (many do not), use 3.7V as the standard lithium-ion nominal voltage.
Example: 26,800 mAh × 3.7V / 1,000 = 99.16 Wh Better: look for the Wh printed on the bank itself.
Required for airline compliance and usually printed on the label.
Rule of thumb: multiply the Wh capacity of everything you want to charge by 1.25 to account for conversion losses (typically 80% efficiency).
That is your minimum Wh target.
A power bank with 20,000 mAh and 18W output will not charge a MacBook Pro.
The laptop requires at minimum 67W, and 18W is below the threshold w here the laptop even begins charging.
Match output wattage to device requirements: Note: "minimum" means the laptop charges, but slowly.
If you are using the laptop while charging, you may need the higher wattage to keep up with power consumption.
Multi-port power banks split output wattage when multiple devices are connected.
A bank rated at 100W total may deliver: 100W to a single USB-C device 65W to USB-C + 18W to USB-A = 83W total when two devices are connected 45W + 25W + 18W when three devices are connected This is not deceptive, it is physics.
Check the bank's documentation for the actual split configuration if you need to charge multiple high-wattage devices simultaneously.
No power bank delivers 100% of its rated Wh to devices.
Energy is lost to heat during the voltage conversion from 3.7V battery cells to the 5V/9V/20V output voltage.
Good quality banks (Anker, Baseus, Zendure): 80-87% Average quality banks: 70-78% Poor quality banks: 60-70% A 100 Wh bank at 85% efficiency delivers 85 Wh to your devices.
A 100 Wh bank at 65% efficiency delivers 65 Wh, 23% less, and you cannot tell from the marketing.
The IATA standard that most airlines follow: Under 100 Wh: carry-on only, no approval needed (limit usually 2 per person) 100-160 Wh: carry-on only, airline approval usually granted on request Over 160 Wh: not allowed on commercial aircraft No power banks in checked luggage, this applies regardless of Wh
A power bank is a lithium battery in a housing.
Quality differences in the battery cells and protection circuits matter for both safety and longevity.
Wh rating printed on the unit UL, CE, or FCC certification marks Current-generation protection circuits (over-charge, over-discharge, short-circuit, temperature) Manufacturer warranty of 12+ months
Our picks
As an Amazon Associate I earn from qualifying purchases. Picks are based on manufacturer specs, independent reviews and owner reports.
Baseus
A 20,000mAh power bank with 65W USB-C output that can charge many laptops and handheld consoles.
Anker
Anker's long-running high-capacity power bank with two USB-A ports.
Anker
A compact 10,000mAh power bank with USB-C, enough for about two phone charges.
FAQ
Wh (watt-hours) is the more useful spec because it directly measures energy stored regardless of battery voltage. mAh (milliamp-hours) only measures energy if you also know the voltage, and manufacturers use different voltages to inflate apparent mAh ratings. Wh is also the unit airlines use for carry-on compliance.
Fewer than the math suggests. A 20,000 mAh / 72 Wh bank delivers about 56 Wh to devices after conversion losses (78% efficiency assumed). An iPhone 14 has a 12.68 Wh battery. So 56 / 12.68 = 4.4 full charges in ideal conditions. With real cables, heat, and partial charges, expect 3.5 to 4 full charges in practice.
Check your laptop's USB-C power adapter label for output wattage. That number is your minimum requirement. MacBook Air 13 M3: 30W minimum (67W faster). MacBook Pro 14: 67W minimum (96W rated). Dell XPS 13: 45W minimum. Gaming laptops typically need 100W+.
Lithium batteries degrade with each charge cycle. Most power banks reta in 80% of rated capacity after 300 to 500 full cycles. For daily use, this is roughly 1 to 1.5 years before noticeable capacity reduction. High heat and frequent deep discharges accelerate degradation.
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