For most people a 10,000 mAh power bank is the sweet spot: small enough for a coat pocket or bag, and enough to recharge a typical smartphone roughly twice. Go up to 20,000 mAh if you charge a tablet, share with others or go several days without a socket. Go smaller only if weight matters more than anything else.
Capacity is only half the story. The output decides how fast your phone or tablet charges, and the ports decide what you can plug in. A large power bank with a weak 5 W output will take hours to fill a modern phone, while a smaller one with USB-C Power Delivery can top it up quickly.
This guide explains how to read capacity, what to expect in real use, which output and ports to look for, and the rules for taking a power bank on a plane. You can compare models in the power banks range.
Capacity: mAh, Wh and what you actually get
Power banks are sold by milliamp hours (mAh), but that figure is measured at the voltage of the lithium cells inside, usually around 3.6 to 3.7 V. Your phone charges at 5 V or more, so some energy is lost in converting the voltage and as heat. In practice you get noticeably less than the headline figure suggests.
The more useful number is watt hours (Wh), which is what airlines use. To convert:
Wh = mAh × cell voltage ÷ 1,000
At 3.7 V, a 10,000 mAh power bank holds about 37 Wh and a 20,000 mAh model about 74 Wh. Many manufacturers print the Wh figure on the case; if they do, use that rather than your own calculation.
| Capacity | Approx. energy at 3.7 V | Typical use |
|---|---|---|
| 5,000 mAh | 18.5 Wh | One phone top-up, very light |
| 10,000 mAh | 37 Wh | Around two phone charges, everyday carry |
| 20,000 mAh | 74 Wh | Tablets, several phones, weekends away |
| 26,800 mAh | About 99 Wh | Laptops, just under the 100 Wh flight limit |
The number of phone charges depends on your phone’s battery and on losses, so treat any “charges per bank” claim as a rough guide. A 20,000 mAh power bank is roughly twice the weight and size of a 10,000 mAh one, so think about how you will carry it every day.
Output and fast charging
Output is measured in watts (W). Older and cheaper power banks offer 5 V at 1 to 2.4 A, which is 5 to 12 W. Modern phones can charge much faster when the power bank and phone agree on a fast-charging standard.
- USB Power Delivery (USB PD) works over USB-C and is used by iPhones, most Android phones, tablets and many laptops. A power bank with 18 to 30 W PD output charges most phones quickly; laptops usually need 45 W or more.
- Qualcomm Quick Charge is found on many Android phones and some chargers with USB-A ports.
- Wireless output lets you lay a Qi-compatible phone on top, as on the Anker PowerCore Wireless 10K. It is convenient but slower and less efficient than a cable.
Check the input speed too. A large power bank with a slow input can take a whole night to refill. Power banks with USB-C PD input recharge much faster from a suitable wall charger, which you will find among the chargers.
Ports, size and extras
Look at what you need to plug in:
- USB-C for modern phones, tablets and laptops, ideally a port that works both in and out.
- USB-A for older cables, earbuds and gadgets.
- Number of ports if you charge two devices at once. The total output is often shared, so each port gets less when both are used.
- Built-in cable or plug. Some models combine a power bank and wall charger, like the Anker PowerCore Fusion, which saves carrying a separate plug. More models, cables and plugs are in the Anker range.
- Solar panel. A small solar panel, as on the Bear Grylls solar power bank, can add a trickle of charge outdoors, but it is slow. Charge it from the mains before a trip and treat solar as a backup.
Flying with a power bank
Power banks must travel in your hand luggage, never in checked bags, because a battery that overheats in the cabin can be spotted and dealt with quickly. The UK Civil Aviation Authority sets the baseline limits:
- Up to 100 Wh: allowed without the airline’s approval.
- Over 100 Wh and up to 160 Wh: only with the airline’s approval.
- Over 160 Wh: not allowed as ordinary passenger baggage.
The CAA also limits passengers to two power banks each, and airlines can set stricter rules of their own, including on whether you may use a power bank during the flight. Check your airline’s page before you travel, and do not take a power bank that is swollen, damaged or recalled. Most power banks up to about 26,800 mAh fall under 100 Wh, but check the Wh figure printed on the case.
Using and storing a power bank safely
Lithium batteries are safe when they are made and treated properly. The same habits apply to rechargeable batteries for tools and gadgets:
- Buy from a reputable retailer. Electrical Safety First warns that counterfeit and substandard chargers carry a higher risk of fire.
- Charge on a hard surface in a ventilated spot, not under pillows or in a bag.
- Do not charge unattended overnight, which is also Electrical Safety First’s advice for USB chargers.
- Keep it out of hot cars and direct sun.
- Stop using it if it swells, gets very hot, smells or is damaged, and take it to a battery recycling point.
- For storage over months, leave it partly charged and top it up now and then.
Retailers that sell larger quantities of batteries must offer free take-back of portable batteries, and many supermarkets have collection bins. Do not put lithium batteries in your household bin. For more on wall chargers and cables, read the guide to USB and wireless chargers.
Quick checklist
- 10,000 mAh for daily carry, 20,000 mAh for tablets or trips.
- Check the Wh figure, especially if you fly.
- USB-C Power Delivery for fast charging of modern phones.
- 45 W or more if you want to charge a laptop.
- Enough ports for the devices you carry.
- Fast USB-C input so it refills quickly.
- Hand luggage only on flights.
Common questions
How many times will a 10,000 mAh power bank charge my phone?
Usually about twice for a typical smartphone, sometimes less for phones with very large batteries. Conversion losses mean you never get the full rated capacity into the phone.
Can I take a 20,000 mAh power bank on a plane?
Generally yes. At about 74 Wh it is under the 100 Wh limit, so most airlines allow it in hand luggage without approval. It must not go in checked baggage, and some airlines set their own rules.
Is it bad to leave my power bank plugged in?
Most have charge protection, but it is better to unplug once full and not to charge overnight or while you are out. Heat is the main enemy of lithium batteries.
