Power Bank Numbers Lie: The 30-Second Check Before You Buy

The biggest number on the box is the least useful one. A "20,000mAh" power bank does not deliver 20,000mAh to your phone — that figure is measured at the battery cell's own 3.7 volts, before conversion losses, before cable losses, before the electronics take their cut.

The 30-second check

Convert to watt-hours: mAh × 3.7 ÷ 1000 = Wh. A 20,000mAh bank is 74Wh. Expect to receive roughly 60–70% of that at the device. A phone with a 20Wh battery gets about two full charges from that "five charge" bank — and now you know before buying, not after.

Why Wh is the number that matters

• Airlines regulate in Wh. Up to 100Wh flies in carry-on, no questions. 100–160Wh needs airline approval. Checked luggage: never.
• Laptops charge in watts. A bank can have huge capacity and still trickle a laptop. For real laptop charging you need 60W+ sustained output — capacity and output are separate specs.
• Wh makes brands comparable. Some list mAh at 3.7V, some at 5V. Watt-hours cuts through it.

What we would actually buy

For a bag that goes everywhere: something in the 74Wh class with 100W output — the Anker Solix line and its rivals hit full-laptop territory while staying airline-legal. For an emergency drawer at home, a small 200-dollar-class power station like the Jackery Explorer 300 runs a router and a laptop through an outage, which is the use case people discover they had only when the power goes.