You can use power banks with 30,000mAh capacity to charge smartphones, tablets, and laptops. Most of these power banks have a capacity of 27,000mAh. Power banks with more capacity aren't allowed on the plane. With 27,000mAh capacity, you can fully charge most smartphones about 5 times. You can fully charge tablets and laptops 2 times, and smaller models even 3 times. The actual capacity of power banks is about 2/3 of the given capacity. This has to do with energy loss and voltage conversion. From 27,000mAh, approximately 18,100mAh remains. Compare the battery capacity of your device with that number to see how often you charge your device. Keep in mind that power banks with this capacity are quite large and heavy.