A power bank listing that says 20,000mAh sounds precise. It is not, on its own, because mAh measures charge, not energy, and it depends on the voltage it is quoted at.
The formula
Wh = mAh × V ÷ 1,000. Power bank cells are normally rated at about 3.7 V, so 20,000mAh at 3.7 V is 20,000 × 3.7 ÷ 1,000 = 74 Wh. Wh is the number you can compare across products.
Why you get less than the label suggests
Your phone charges at 5 V or higher, so the power bank has to convert its cell voltage up. That conversion loses some energy as heat, so the usable output is lower than the rated figure. How much lower depends on the product, which is another reason a listing should state Wh and voltage.
What to look for on a listing
- Capacity in Wh, or mAh with the voltage it is quoted at.
- The output wattage per port and the charging protocols supported.
- A brand with a traceable store or distributor. Power banks are high-risk products in our rubric, so a listing with no rated capacity is not ranked.
Flying with a power bank
Airlines commonly limit power banks to 100 Wh in carry-on baggage, which is one reason Wh matters. Rules differ by airline, so check yours before you travel.