How to Choose a Power Bank: Wh vs mAh, Wattage, and What Actually Matters

Why mAh Is a Misleading Spec

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.

Converting mAh to Wh

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.

How Much Capacity Do You Need?

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.

Phone only (2 full charges): ~28 Wh → 10,000 mAh bank is sufficient Phone (5 full charges): ~60 Wh → 20,000 mAh bank Laptop (MacBook Air) + phone: ~80 Wh → 25,000 mAh / 90 Wh bank Laptop (MacBook Pro 16) + phone: ~120 Wh → 35,000 mAh bank (exceeds airline carry-on limit)

Output Wattage: The Spec Most People Miss

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 where 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.

The Port Configuration Matters

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.

Real Capacity vs. Rated Capacity

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.

Airline Rules

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

Build Quality Indicators

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