You can use a power bank with 20,000mAh capacity to charge your smartphone, but also your tablet or laptop. You can fully charge the average smartphone about 4 times. That's useful if you're going away for a weekend and want to charge multiple phones. You can fully charge tablets about 2 times. Due to voltage conversion and energy loss during charging, the actual capacity of a power bank is two-thirds of the given capacity. So, you have about 13,400mAh from 20,000mAh. Compare that to the battery capacity of your device to see how often you can fully charge your device. Do you want to charge a laptop? The power bank must have a USB-C port that provides at least 45W of power.