How to convert mAh to Wh
A 20,000 mAh power bank with 3.7 V cells holds 20,000 × 3.7 ÷ 1,000 = 74 Wh.
How many phone charges will I get?
Expect roughly 60–70% of a power bank's rated watt hours to reach your device, because the voltage is boosted to 5 V or higher for USB and some energy is lost as heat. A 74 Wh bank delivers around 45–50 Wh, which is about three to four charges of a phone with a 13–15 Wh battery.
Flying with batteries
Most airlines follow IATA guidance: spare lithium batteries go in hand luggage, packs up to 100 Wh are normally fine, 100–160 Wh need the airline's approval, and anything bigger is generally not allowed in passenger luggage. Always check your airline's own rules before you fly.
Frequently asked questions
Is a 20000mAh power bank allowed on a plane?
Usually yes. At 3.7 V it's 74 Wh, under the common 100 Wh limit, but it must go in your hand luggage.
Why is my power bank's USB output lower than its mAh rating?
The mAh rating is at the internal cell voltage (about 3.7 V). Output at 5 V or higher, after conversion losses, is a smaller number of mAh.