Power bank buying guide: real capacity, fast charging and safety

Rated capacity versus what you really get

The mAh number on a power bank is measured at the battery cell's voltage. Converting it to your phone's charging voltage loses energy as heat, so you typically get around 60–70% of the printed capacity. A 10,000 mAh power bank charges a phone with a 5,000 mAh battery about once and a bit; a 20,000 mAh one about two to two-and-a-half times.

10,000 mAh is the most practical size for daily carry. 20,000 mAh suits travel, long power cuts or charging a laptop, but it is heavier.

Fast charging: match your phone

Most current phones fast-charge over USB Power Delivery (PD) through a USB-C cable. Look for a power bank with a USB-C PD output of at least 20 W for phones, and 45–65 W or more if you want to charge a laptop. Some brands use their own fast-charging systems, which only reach full speed with that brand's charger – a PD power bank will still charge them, just more slowly.

Two-way USB-C fast charging matters too: it lets the power bank itself recharge quickly instead of overnight.

Safety and quality

Buy power banks with a BIS registration mark, which is mandatory for them in India, and from brands with clear warranty support. Very cheap, unbranded power banks sometimes overstate their capacity and skip protection against overheating, overcharging and short circuits. Read reviews for reports of swelling or heating.

Flying with a power bank

Power banks must go in your cabin bag, never in checked luggage. Airlines generally allow power banks up to 100 Wh without approval – roughly 27,000 mAh at 3.7 V – and many airlines now restrict using or charging them during the flight. Rules change, so check your airline's current policy before you travel.