What Is a Power Bank and How Does It Work?
A power bank is essentially a portable battery pack with built-in charging circuitry. You charge it from a wall outlet or other source, then it stores that energy and delivers it to your devices via USB ports. The quality of the internal cells and the efficiency of the circuitry determine how well it actually performs.
Understanding Capacity: mAh vs. Wh
Capacity is typically listed in milliampere-hours (mAh). A higher number means more stored energy — in theory. But there's an important caveat: power banks lose energy during the conversion process (typically 10–20% depending on quality). So a 10,000 mAh power bank won't deliver 10,000 mAh to your phone.
Watt-hours (Wh) is a more accurate measure of real-world capacity, especially for comparing across different voltages. To convert: Wh = mAh × Voltage ÷ 1000.
Rough Capacity Guidelines
- 5,000–10,000 mAh: One to two full charges for a typical smartphone. Slim and pocketable.
- 10,000–20,000 mAh: Multiple phone charges or one full tablet charge. Good all-around option.
- 20,000–30,000 mAh: Heavy travelers, multiple devices, or charging a laptop. Heavier and bulkier.
Charging Speed: What Wattage Do You Need?
Wattage determines how fast the power bank can charge your devices (output) and how quickly it can recharge itself (input).
Key Standards to Know
- 5W (standard USB-A): Slow, but fine for overnight charging or low-priority devices.
- 18W–20W (USB Power Delivery / Quick Charge): Fast charging for most smartphones.
- 30W–65W: Fast laptop charging. Essential if you want to charge a MacBook or Windows ultrabook.
- 100W+: High-performance laptops and simultaneous fast charging for multiple devices.
Make sure the power bank supports the fast-charging standard your device uses — USB-C Power Delivery (PD) is the most universal modern standard.
Ports: More Isn't Always Better
Consider how many devices you need to charge simultaneously. A mix of USB-A and USB-C ports offers the most flexibility. Note that most power banks reduce charging speed when multiple ports are in use at the same time — check the specs carefully.
Size and Weight Trade-offs
Higher capacity means more weight. A 10,000 mAh unit might weigh around 200g, while a 26,800 mAh unit can exceed 600g. Think about whether you're packing light for day trips or need maximum capacity for long hauls.
Features Worth Paying For
- Pass-through charging: Charge the power bank and a device simultaneously.
- LED or digital display: Know exactly how much charge remains.
- Low-current mode: Essential for charging earbuds, smartwatches, and other low-draw devices that standard power banks may not detect.
- Wireless charging: Convenient but inefficient — expect 15–25% more capacity loss.
Airline Rules on Power Banks
If you're flying, power banks must go in carry-on luggage — not checked bags. Most airlines follow IATA guidelines capping power banks at 100Wh without prior approval, and up to 160Wh with airline approval. Check your specific airline before traveling with a large unit.
Quick Decision Guide
| Use Case | Recommended Capacity | Min. Output Wattage |
|---|---|---|
| Everyday phone top-ups | 5,000–10,000 mAh | 18W |
| Weekend travel, multiple devices | 15,000–20,000 mAh | 30W |
| Laptop charging, extended trips | 20,000–30,000 mAh | 65W+ |
The Bottom Line
The right power bank depends on your specific devices and travel habits. Prioritize real wattage output and compatibility with your device's fast-charging standard over raw mAh numbers — and factor in weight if portability matters to you.