How to choose a power bank that actually lasts the day

Almost every power bank is sold on one number: capacity in mAh. It is the largest text on the box and the least useful thing printed on it. A 20,000mAh brick that charges at 10W will take longer to refill your phone than a 5,000mAh one that pushes 22.5W — and you will carry three times the weight for the privilege.

Capacity tells you how many times it can refill a phone. Output tells you how long each refill takes. Most people care far more about the second.

Start with output, not capacity

Output is quoted in watts. Anything under 10W is slow enough that you will notice — plugging in for twenty minutes will buy you very little. 18W is the sensible floor. 22.5W and above will take a typical phone to roughly half charge in half an hour.

Look for Power Delivery (PD) or Quick Charge (QC) on the spec. Without one of those standards, a power bank falls back to a trickle no matter what the advertised wattage says.

Then work out the capacity you need

Real-world capacity is roughly 60–70% of the printed figure. Some energy is always lost converting voltage, so a 10,000mAh bank delivers around 6,500mAh to your phone.

• 5,000mAh — about one full phone charge. Pocketable, fine for a day out.
• 10,000mAh — two charges. The best balance for most people, and still cabin-bag friendly.
• 20,000mAh — three or four charges, or a laptop top-up. Heavy; worth it for travel, overkill for a commute.

Ports matter more than you expect

A USB-C port that does both input and output means one cable charges the bank and your phone. Two output ports let you charge a phone and earbuds together. If the bank only has micro-USB for input, skip it — you will be carrying a second cable purely to feed the battery.

The flying rule

Indian airlines allow power banks up to 100Wh in cabin baggage, which covers everything up to about 27,000mAh. They are not allowed in checked luggage at all. If the capacity is not printed on the casing, expect questions at security.

What to ignore

Digital percentage displays are pleasant but not a reason to choose one bank over another. Solar panels on power banks are close to useless — the panel is far too small to meaningfully charge the cell. And a bank that is unusually cheap for its claimed capacity is almost always overstating it.