iPad

Suppose you have a power bank with a 20,000 mAh capacity and your iPad has a 10,000 mAh capacity. You'd expect to be able to charge your iPad about twice, but that's not entirely true. Every power bank has a certain efficiency and loses some capacity when charging another device. Most power banks have an efficiency of around 80-85%. This means the actual available capacity of a 20,000 mAh power bank is actually 16,000 to 17,000 mAh. Pay close attention to this when purchasing a power bank.

Here's how to calculate how many times you can charge an iPad with a 10,000 mAh battery using a power bank with an 80% efficiency:

16,000 mAh / 10,000 mAh = 1.6 times

If you have a power bank with a capacity of 26,800 mAh, you can charge your iPad 2.1 times, because:

(26,800 * 0.8) / 10,000 mAh = 2.1 times charging.