How to choose a power bank — and why 10,000mAh isn't 10,000mAh

A power bank is one of the simplest gadgets to buy and one of the easiest to get wrong. The capacity printed on the box isn't what reaches your phone, "fast charging" means different things on different devices, and a cheap unit with poor protection circuitry isn't worth the risk. Here's what to check.

The short answer

Choose a capacity that matches how you'll use it, check that it supports the fast-charging standard your phone uses, buy from a brand with proper safety certification and warranty, and stay under 100Wh if you fly.

Why you get less than the rated capacity

A power bank's rating describes its internal battery cells. Converting that energy to the voltage your phone needs, plus heat and your phone's own charging losses, means you typically get around 60–70% of the rated capacity into your phone. So a 10,000mAh power bank charges a phone with a 5,000mAh battery roughly once and a bit — not twice.

As a rough guide:

10,000mAh: about one full phone charge, pocketable. Right for most people.
20,000mAh: two or so phone charges, or charging a tablet. Heavier.
Larger: for laptops or several days away from power.

Fast charging: match the standard

Fast charging only works when the power bank and your device support the same standard:

USB Power Delivery (USB-PD) over USB-C is used by iPhones, Pixels, most recent Android phones and laptops. Around 20W fast-charges most phones.
Qualcomm Quick Charge and brand-specific standards are used by some Android phones. A power bank without your brand's proprietary standard will still charge the phone, just more slowly.

Check the output wattage per port, and what happens when two devices charge at once — total output is often shared.

Charging a laptop

Many laptops charge over USB-C, but they need much more power than a phone. Look for USB-PD output of 45W or more for thin laptops, and 65W or more for larger ones. A lower-wattage power bank may slow the laptop's drain rather than charge it.

How fast does the power bank itself recharge?

A 20,000mAh power bank on a slow input can take most of a night to refill. Look for USB-C input with fast-charging support, which can cut recharge time dramatically — provided you use a suitable charger.

Safety and certification

Lithium batteries store a lot of energy, and protection circuitry is what prevents overcharging, overheating and short circuits. In India, power banks fall under the BIS compulsory registration scheme, so genuine products should carry a BIS registration mark and number on the body or packaging. Avoid unbranded units with no certification, and stop using any power bank that swells, gets unusually hot, or is damaged.

Flying with a power bank

Airline rules worldwide, including in India, generally require power banks to be carried in cabin baggage only, never in checked luggage. Power banks up to 100Wh are generally allowed; those between 100Wh and 160Wh usually need airline approval. To convert: Wh = mAh × cell voltage ÷ 1000 — at the common 3.7V, a 20,000mAh power bank is about 74Wh. The rated Wh figure is often printed on the power bank. Check your airline's current rules before travelling.

Size, weight and ports

A 20,000mAh unit weighs considerably more than a 10,000mAh one — check the weight if you'll carry it every day. Make sure the ports match your cables: a USB-C port that works both in and out is the most flexible. A display showing the exact remaining percentage is more useful than four small LEDs.

Warranty

Battery capacity drops with age and charge cycles. A brand offering a clear warranty and a working service process is worth a small premium over an unknown one.

Checklist before you buy

What capacity do you actually need — one charge or several?
Does it support your phone's fast-charging standard (USB-PD for most)?
Enough wattage for your laptop, if that matters?
Fast USB-C input so it recharges quickly?
A BIS registration mark and a known brand?
Under 100Wh if you fly?
Weight, ports and warranty acceptable?