If your phone is usually limping into the evening, a 10,000 mAh power bank is the sweet spot for most people: enough energy for useful top-ups without turning your bag into luggage. A 20,000 mAh model suits long trips, several devices or sharing, but you will feel the extra weight. This power bank capacity guide also explains why the number on the case is not the amount that reaches your phone. Budget shoppers can start with our best power banks under $20.
01
Why advertised mAh is not delivered mAh
A 10,000 mAh bank will not deliver a full 10,000 mAh at the phone's battery voltage. The label describes the internal cells, commonly around 3.7V. USB output uses a different voltage and the conversion wastes some energy, which is why dividing one mAh figure by another always promises too many charges.
Watt-hours are more honest for comparing energy. Ten thousand milliamp-hours is 10 amp-hours; at 3.7V that is about 37Wh before losses. The real result changes with the electronics, cable, temperature and device, so leave some margin instead of planning around a perfect calculation.
02
Choose capacity for the time away
For an emergency boost, 5,000 mAh can be enough and barely takes up space. Around 10,000 mAh works well for everyday carry. Around 20,000 mAh is better for several days, a tablet or more than one person. Beyond that, ask whether you will genuinely carry the thing. The biggest battery left in a drawer has no capacity at all when you need it.
Integrated cables reduce clutter, and a combined wall charger can make a neat travel kit. The trade-off is repairability: when a built-in lead fails, part of the power bank's appeal goes with it.
03
Check output wattage and ports
Capacity tells you how long; wattage tells you how fast. A phone has modest needs, but a working laptop may require 45W, 65W or more. Check that the headline output is available from one port and see how it changes when two devices charge together. A large bank that takes all day to refill can also be surprisingly inconvenient.
If you need household AC sockets rather than USB, you are shopping in a different category. Use our portable power station size calculator instead of buying an enormous USB battery and hoping for the best.
04
Account for travel rules and safety
Airlines normally express battery limits in watt-hours and require spare lithium batteries in carry-on baggage. Rules and approval thresholds can change, so check the operating airline before flying. Our carry-on luggage size guide covers the bag itself.
Choose a bank with clearly printed capacity and credible safety information. Stop using it if the case swells, cracks or becomes unusually hot. For overseas charging, remember that a travel adapter does not convert voltage.
Small details make a bank nicer to own: a percentage display, a low-power mode for earbuds and cables that support the promised speed. A USB-C hub or dock may pass charging, but every link in the chain must handle the wattage. Buy enough energy for the day, enough output for the device and no more weight than you are happy to carry.
Sources and further reading
We use primary and specialist sources to check the principles in this article. Product recommendations are researched separately.
