Calculator KB-25
Power bank actual capacity calculator
Why does a 20,000 mAh power bank charge a 4,000 mAh phone fewer than five times?
Result
In brief: How many phone charges a power bank provides: actual capacity at the 5 V output, energy in Wh, converter losses, and rules for taking it on a plane. Example: Full phone charges: 3.7 times.
Calculation example
| Capacity on the label | 20000 mAh |
|---|---|
| Cell voltage | 3.7 V |
| Converter efficiency | 85 % |
| Phone battery | 4000 mAh |
| Its voltage | 3.85 V |
| Full phone charges | 3.7 times |
|---|---|
| Energy in the cells | 74 Wh |
| Available at the output | 62.9 Wh |
| Actual capacity at 5 V | 12 580 mAh |
| On a plane | allowed, up to 100 Wh |
How it is calculated
Eout = E · η
N = Eout · 0,9 / Ephones
The label shows capacity at the cell voltage, while the output is 5 V or higher. Some energy is lost in the power bank's converter, and the phone's charge controller uses about another 10%.
What to look for
- Compare power banks by Wh: different manufacturers specify mAh at different voltages.
- If the actual capacity is much lower than calculated, the power bank is probably overrated on its label. You can check it with a USB tester.
Frequently asked questions
How many times can a 20000 mAh power bank charge a phone?
About 3.7 times for a 4000 mAh phone: the cells store 74 Wh, and at 85% efficiency, 62.9 Wh is available at the output. Another 10% or so is lost charging the phone itself.
Why is a power bank’s actual capacity lower than advertised?
The label gives the capacity at 3.7 V, but the output is 5 V or higher. After losses, 20000 mAh works out to about 12580 mAh at 5 V.
Can I take a power bank on a plane?
Usually, yes, but only in carry-on baggage and up to 100 Wh (about 27000 mAh at 3.7 V). From 100 to 160 Wh, you need the airline’s approval. Check the exact rules with your carrier.
