Question 1

How do I convert mAh to Wh?

Accepted Answer

Watt-hours (Wh) = mAh × voltage ÷ 1000. Power bank cells are rated at 3.7V, so a 20,000 mAh bank = 20000 × 3.7 ÷ 1000 = 74 Wh. Airlines use the Wh figure, not mAh.

Question 2

Is my power bank allowed on a plane?

Accepted Answer

Most airlines allow power banks up to 100 Wh in carry-on with no approval (about 27,000 mAh at 3.7V). From 100–160 Wh you usually need airline approval, and over 160 Wh they're banned. Power banks must go in carry-on, never checked luggage.

Question 3

How many times will a power bank charge my phone?

Accepted Answer

Roughly: (power bank mAh × 3.7 × 0.85 efficiency) ÷ (phone battery mAh × 3.85). A 20,000 mAh bank charges a typical 4,000 mAh phone about 4 times. Conversion loss means you never get the full rated capacity.

Question 4

Why doesn't a 10,000 mAh bank give my phone 10,000 mAh?

Accepted Answer

The bank's cells are 3.7V but USB output is 5V, and converting between them loses ~15–20% energy. So a 10,000 mAh power bank delivers roughly 6,000–6,800 mAh at 5V.

5,000 mAh	18.5 Wh · ~1 phone charge · ✈️ carry-on OK
10,000 mAh	37 Wh · ~2 charges · ✈️ carry-on OK
20,000 mAh	74 Wh · ~4 charges · ✈️ carry-on OK
27,000 mAh	~100 Wh · ~5–6 charges · ✈️ at the limit
>27,000 mAh	over 100 Wh · ✈️ needs airline approval

Power Bank Calculator

Common power bank sizes