10,000 mah doesn't exactly mean 10,000 mah... - Nitecore NB10000 Gen 3 Ultra-Slim USB-C Power Bank

When picking a power bank for all of your on-the-go charging needs, make sure to buy one that has 2x the capacity of whatever you need to charge. Why? Small-device power banks like this one are victims of inefficiencies/losses that can make their USABLE capacity 30-40% lower than the number on the box. For example.. this Nitecore will contain 10k mAh worth of energy, but it's only going to be able to GIVE something else 6,200 mAh... at best. A minimum loss of 38%.