Most power banks that you come across will have a capacity rated in milliampere hours (mAh). A 10,000mAh capacity is very common, and it will be enough to charge many phones two times. The charging capacity also generally reflects the size of the power bank itself. You'll be able to carry anything less than 20,000mAh comfortably, but beyond that, things can start to get bulky. On that note, if you're buying a charger for a laptop, you'll want to start at 20,000mAh and go up from there. Just be sure you don't go beyond about 26,500mAh, which comes out close to the maximum 100Wh capacity allowed on commercial flights.