Formulas
- Energy (Wh) = mAh × voltage ÷ 1,000
- Battery life (h) = Wh ÷ power in watts, or mAh ÷ current in mA
- Power bank charges = power bank mAh × efficiency ÷ device mAh
Examples
A 5,000 mAh battery at 3.7 V stores 18.5 Wh. Using 1.5 W on average, it lasts about 12 h 20 min.
A 10,000 mAh power bank (37 Wh) with 65% efficiency gives about 1.3 full charges to that 5,000 mAh phone.
Power banks on planes
Under the common airline rules (IATA), power banks always go in carry-on bags. Up to 100 Wh they are allowed; from 100 to 160 Wh you need the airline’s approval; above 160 Wh they are usually forbidden. A 20,000 mAh power bank has 74 Wh.
Frequently asked questions
Why does my 10,000 mAh power bank not give 2 full charges to a 5,000 mAh phone?
The mAh on the label is at 3.7 V. Converting to USB voltage and the heat lose 30% to 40% of the energy.
How do I find the voltage?
Most phone and laptop cells are 3.6 to 3.85 V. Laptop batteries often show the Wh directly on the label.
See also
Results are estimates for information only and are not financial, legal or medical advice. Rates, taxes and fees vary — confirm with a professional or your lender.