mAh is the most common label for power bank capacity, but many buyers confuse marked capacity with actual usable power. When selecting a power bank, you need to consider your devices and usage scenarios. 5000mAh models are ultra-light, perfect for daily commuting and emergency top-ups. 10000mAh is the most balanced option, which can fully charge most smartphones 1–2 times and fits easily in backpacks. 20000mAh suits long-distance travel, hiking and multi-device use. You should also learn about conversion loss. Heat and circuit consumption reduce real output. A 20000mAh marked power bank cannot release the full 20000mAh to your phone. If you often travel by air, remember the airline rule: power banks over 100Wh cannot be carried on board. Choose certified, clearly marked products and avoid exaggerated fake capacity labels.