How many times will a power bank charge my phone?
A 20,000 mAh power bank doesn't charge a 5,000 mAh phone four times. Enter both batteries to see the realistic number, and why the difference exists.
By Zubair Abid. Updated .
Quick answer
Charges ≈ power bank Wh × efficiency ÷ phone battery Wh. A 20,000 mAh bank (74 Wh at 3.7 V) delivering about 65% of its energy gives about 48 Wh, enough for roughly 2.5 full charges of a 5,000 mAh phone (19.25 Wh). A 10,000 mAh bank gives about 1.2 charges.
Power bank
Phone, earbuds or tablet
How do you calculate power bank charges?
Charges = (Bank mAh × Bank V) × Efficiency ÷ (Phone mAh × Phone V)
Comparing mAh directly is misleading because the two batteries run at different voltages: power bank cells at about 3.7 V, modern phone batteries at about 3.85 V. Converting both to watt-hours puts them on equal terms.
Efficiency is the share of stored energy that actually reaches your phone. The power bank steps its 3.7 V up to 5 V or 9 V for USB, the phone steps it back down, and the cable and both circuits lose some as heat. 60 to 70% is realistic for most power banks; the default here is 65%.
How many charges do common power banks give?
| Power bank | Stored energy | Delivered | Full phone charges |
|---|---|---|---|
| 5,000 mAh | 18.5 Wh | 12.0 Wh | About 0.6 |
| 10,000 mAh | 37 Wh | 24.1 Wh | About 1.2 |
| 20,000 mAh | 74 Wh | 48.1 Wh | About 2.5 |
| 26,800 mAh | 99.2 Wh | 64.5 Wh | About 3.3 |
Smaller phones (3,300 to 4,000 mAh) get proportionally more charges. Earbuds cases hold only 1 to 2 Wh, so even a small bank charges them many times.
Why do you get fewer charges than the mAh suggests?
- Different voltages. The rating is for 3.7 V cells; your phone's battery runs higher.
- Conversion losses. Stepping up to USB voltage and back down wastes energy as heat.
- Fast charging is less efficient. High-watt charging generates more heat than slow charging.
- Using the phone while charging. The screen and radios draw power that never reaches the battery.
- Age and cold. Older power banks and cold weather both reduce usable capacity.
Planning to fly with it? A 20,000 mAh bank is 74 Wh, under the 100 Wh limit. Check any size with the power bank airline checker.
Which power bank size should you buy?
- 10,000 mAh: one full phone charge with some spare; pocket-sized. Good for a day out.
- 20,000 mAh: two to three phone charges, or a partial laptop top-up with USB-C PD. Good for travel.
- 25,000 to 27,000 mAh: the largest size that stays under the 100 Wh airline limit.
For laptops, check the power bank's USB-C output (for example 45 W, 65 W or 100 W) against your laptop's charger with the laptop charger checker.