How to Pick a Power Bank
If you're learning how to pick a power bank, the first decision is how to choose a power bank capacity that matches your devices, and the number on the box can be misleading. A power bank's mAh rating doesn't translate directly into phone charges, because part of the energy is lost in conversion. This guide explains how to choose a power bank capacity in real terms, translating mAh into actual phone recharges, then covers fast charging, troubleshooting and the airline carry-on limits for 2026.
Key Takeaways
- Expect roughly 60 to 70% of the rated capacity to reach your phone.
- A 10,000 mAh power bank gives about 1.5 to 2 phone charges; 20,000 mAh about 3 to 4.
- Choose USB-C PD fast charging if you want quick top-ups or to charge a laptop.
- Power banks go in carry-on bags only, and 100 Wh (about 27,000 mAh) is the usual limit.
- Many airlines now restrict how many you carry and ban using them on board.
How to Choose a Power Bank Capacity: Quick Guide
Match capacity to how many charges you need between wall sockets.
| You need | Choose | Notes |
|---|---|---|
| Top up your phone once on a busy day | About 5,000 mAh | Pocket-sized; often magnetic |
| One or two full phone charges | About 10,000 mAh | Good everyday balance |
| A weekend away or a long travel day | About 20,000 mAh | Several phone charges; heavier |
| Laptop, tablet or several devices | 20,000 mAh with high-wattage USB-C PD | Check the wattage your laptop needs |
| Flying | Under 100 Wh (about 27,000 mAh) | Carry-on only; check your airline |
Why Rated Capacity Never Fully Transfers
Power bank capacity is rated at the battery's internal voltage, usually 3.7 volts. Phones charge at 5 volts or higher, so the power bank has to convert the voltage up, and some energy is lost as heat in the conversion, the cable and your phone's own charging circuit.
The real-world rule
Expect roughly 60 to 70% of the rated capacity to end up in your phone. A 20,000 mAh power bank holds about 74 Wh (20,000 x 3.7 / 1,000), and after losses delivers around 48 Wh. Fast charging and wireless charging tend to lose more.
mAh vs Wh
Watt-hours (Wh) measure energy and allow fair comparisons. To convert: Wh = mAh x 3.7 / 1,000. Airlines use Wh for their limits, so it's useful to know both.
20000mAh: How Many Charges?
Estimates below assume about 65% efficiency. Your results will vary with your phone, cable and charging speed.
| Power bank | Rated energy | Delivered (approx.) | 3,000 mAh phone | 4,000 mAh phone | 5,000 mAh phone |
|---|---|---|---|---|---|
| 5,000 mAh | 18.5 Wh | About 12 Wh | About 1 | About 0.8 | About 0.6 |
| 10,000 mAh | 37 Wh | About 24 Wh | About 2 | About 1.6 | About 1.2 |
| 20,000 mAh | 74 Wh | About 48 Wh | About 4 | About 3 | About 2.5 |
| 27,000 mAh | About 100 Wh | About 65 Wh | About 5.5 | About 4 | About 3.4 |
How Many mAh Do I Need?
Use this simple formula: your phone's battery (mAh) x charges you want / 0.65. For two full charges of a 4,000 mAh phone: 4,000 x 2 / 0.65 = about 12,300 mAh. In that case, a 20,000 mAh power bank leaves headroom, while a 10,000 mAh one would fall a little short.
Balancing capacity and weight
Higher capacity means more weight. A 5,000 mAh bank slips into a pocket; a 20,000 mAh bank is more like a small paperback. Choose the smallest capacity that covers your typical day or trip.
Fast Charging PD Explained
USB Power Delivery (PD) is a fast-charging standard over USB-C. The power bank and your device agree on a higher voltage and power, so charging is quicker.
- Phones: many fast-charge at around 18 to 30 W.
- Tablets and laptops: often need 30 to 65 W or more.
- A 66 W power bank can charge some laptops as well as phones, if the laptop supports USB-C PD charging.
- Shared ports: charging several devices at once often splits the total power.
- Cables: use a USB-C cable rated for the wattage you need.
Power Bank Airline Rules (2026)
Lithium batteries can overheat and catch fire, so airlines treat power banks strictly, and rules tightened through 2025 and 2026.
| Rule | What it means |
|---|---|
| Where to pack | Carry-on or on your person only; never in checked bags |
| Up to 100 Wh (about 27,000 mAh) | Generally allowed without airline approval |
| 100 to 160 Wh | Needs airline approval at best; some current guidance doesn't allow power banks this size at all |
| Over 160 Wh | Not allowed on passenger flights |
| How many | Many airlines now limit passengers to two, and some to one |
| Using it on board | Many airlines now ban charging devices from a power bank, or charging the power bank, during the flight |
Before you fly
- Check the capacity in Wh on the power bank; if only mAh is shown, calculate it (mAh x 3.7 / 1,000).
- Check your airline's current rules on quantity and on-board use, as they vary and change.
- Keep power banks in your carry-on, within reach, not in checked luggage.
- If you're asked to gate-check a bag, remove any power banks first.
A 20,000 mAh capacity (about 74 Wh) is comfortably under the 100 Wh limit, which makes it a popular travel size.
Power Bank Not Charging Phone? Troubleshooting
| Problem | What to try |
|---|---|
| Phone doesn't charge at all | Try another cable and port; check the power bank has charge |
| Charges very slowly | Use a fast-charging cable; check both devices support the same fast-charging standard |
| Stops after a few minutes | Small devices draw too little current and trigger auto-off; use a low-current mode if available |
| Wireless charging won't start | Align the phone with the magnet or coil; remove thick or metal cases |
| Power bank gets hot and stops | Overheat protection; let it cool in a ventilated spot |
| Power bank won't recharge | Try a different charger and cable; if it's swollen or damaged, stop using it |
Wireless, Magnetic and Solar Power Banks
Magnetic wireless power banks
They snap onto the back of compatible phones and charge without a cable. They're convenient, but wireless charging is less efficient and creates more heat, so you'll get fewer charges from the same capacity.
Solar power banks
Built-in solar panels are small and charge slowly, so treat them as an emergency top-up rather than a main charging method. Larger foldable solar panels are much more practical for camping and outdoor use.
Power Bank Safety
- Buy from reputable sources and avoid damaged or swollen units.
- Don't leave power banks in hot cars or direct sun.
- Charge on a hard surface, not on a bed or sofa.
- Stop using a power bank that swells, smells or gets unusually hot.
- Recycle old power banks properly; don't put them in household waste.
Now you know how to choose a power bank capacity, you can pick one that covers your day, charges at the right speed and complies with airline rules.
Power Banks From Global Swanky
Browse more in our Consumer Electronics collection.
1. PD66W Fast Charge 20000mAh Power Bank
The PD66W Fast Charge 20000mAh Power Bank for Travel Use is $21.24, with 20,000 mAh (about 74 Wh) and 66 W fast charging.
2. Large Capacity PD66W 20000mAh Power Bank
The Large Capacity Mobile Power Bank PD66W Fast Charging 20000mAh is $15.32, a budget-friendly 20,000 mAh option with fast charging.
3. 20000mAh Portable Charger
The 20000mAh Power Bank Portable Charger is $16.79, for several phone charges on longer days.
4. Portable 5000mAh Magnetic Wireless Power Bank
The Portable 5000mAh Magnetic Wireless Power Bank is $16.75, a pocket-sized top-up with wireless charging.
5. Portable Solar Charger Foldable Panel
The Portable Solar Charger Foldable Panel for Outdoor Power Needs is $28.74, for topping up devices while camping.
Frequently Asked Questions
How many mAh do I need in a power bank?
Multiply your phone's battery capacity by the number of charges you want, then divide by about 0.65 to allow for conversion losses. For two charges of a 4,000 mAh phone, that's roughly 12,000 mAh, so a 10,000 to 20,000 mAh bank suits most people.
What are the power bank airline rules?
Power banks must go in carry-on baggage, never checked bags. Those up to 100 Wh (about 27,000 mAh) are generally allowed; larger ones need approval or may be banned. Many airlines now limit the number you can carry and ban using them on board, so check your airline before flying.
What is fast charging PD?
USB Power Delivery (PD) is a USB-C fast-charging standard. The power bank and device agree on a higher voltage and wattage, such as 20 W for many phones or 45 to 65 W for some laptops. You need a PD-capable power bank, device and cable.
Why is my power bank not charging my phone?
Common causes are a faulty cable or port, a flat power bank, auto-off with low-power devices, overheating protection or poor wireless alignment. Try another cable, check the charge level and let a hot power bank cool.
How many charges does a 20000mAh power bank give?
After conversion losses, a 20,000 mAh power bank delivers roughly 48 Wh, which is about 4 charges of a 3,000 mAh phone, 3 of a 4,000 mAh phone or 2.5 of a 5,000 mAh phone. Fast charging and wireless charging reduce this.