A 20,000mAh power bank will not fully charge a smartphone four times. The labeled capacity is calculated at the cell's nominal 3.7V, but when that energy is converted to the 5V that actually leaves the port, circuit loss and cable resistance mean only 60 to 75 percent is usable. So when choosing a power bank, look not at the leading mAh number but at the usable capacity in watt-hours (Wh) and the PD output (W) your device can accept.
The labeled mAh is not the real charge
To compare capacity accurately, convert mAh into Wh (watt-hours). As LG Uplus explains, multiplying by the 3.7V cell voltage gives the true energy: 20,000mAh × 3.7V / 1,000 = 74Wh. But that 74Wh does not all reach your device.
Enuri notes that once you add the boost from the 3.7V cell to 5V USB output, circuit heat, and cable resistance, real usable capacity is about 60 to 75 percent of the label. As a shortcut, multiplying the labeled capacity by roughly 0.7 gives the approximate usable amount.
A power bank's true capacity is measured in Wh, not mAh. And nearly a third of those Wh disappear in the process of changing the voltage.
How many times does 20,000mAh really charge?
Convert the labeled capacity to Wh, apply a 68 percent conversion efficiency, then divide by your phone's battery capacity to get the real number of charges. The table below is calculated directly for a smartphone with a 15.4Wh battery (roughly a 4,000mAh class).
| Labeled capacity | Wh (×3.7V) | Usable (×0.68) | Charges (15.4Wh phone) |
|---|---|---|---|
| 10,000mAh | 37Wh | ~25Wh | ~1.6 |
| 20,000mAh | 74Wh | ~50Wh | ~3.3 |
| 25,000mAh | 92.5Wh | ~63Wh | ~4.1 |
In other words, 20,000mAh realistically means about three charges, and 10,000mAh about 1.5 to 2. Efficiency shifts with cable quality, charging speed, and device condition, so treat these counts as a rough ceiling, not a guarantee.
PD output matters as much as capacity
Even at the same 20,000mAh, a different maximum output changes charging speed sharply. Samsung's EB-P4520 offers 45W PD 3.0 at 20,000mAh, while the EB-P5300 offers 25W PD/PPS at the same capacity. A pack that falls short of what your device demands charges slowly no matter how large it is. With only a 5W USB-A port, charging takes more than twice as long as with 20W PD.
| Device | Recommended PD | Note |
|---|---|---|
| Smartphone | 20-25W | PPS support helps heat and efficiency |
| Tablet | 30-45W | Upper end if charging with screen on |
| Light laptop | 45-65W | Only models with USB-C PD charging |
| High-performance laptop | 100W | When using a single port alone |
Output also depends on the cable. To exceed 60W you need a cable with an e-marker chip. Matching charger and cable specs continues in how to choose a USB-C charger and how to check a USB-C cable.
Can you take it on a plane?
Power banks are allowed only in carry-on, never in checked luggage. Under international rules, 100Wh or less is allowed without special approval, 100 to 160Wh needs airline approval, and above 160Wh is prohibited. As calculated above, 20,000mAh is 74Wh and 25,000mAh is 92.5Wh, both under 100Wh. But large packs above roughly 27,000mAh (about 100Wh) can require approval, so it is safer to convert the labeled capacity to Wh in advance.
Choose in this order
- Usable Wh first: labeled mAh × 3.7 / 1,000 × 0.68 for the energy you can actually use
- Convert to charges: usable Wh / your phone's battery Wh for expected count
- Device PD output: 45W or more if it doubles as a laptop charger, 25W is enough for phone only
- PPS and e-marker cable support for high-speed, high-output use
- Air travel: if frequent, check it is under 100Wh via the Wh conversion