Power bank listings lead with a big mAh number because it's the easiest spec to compare, but it's not actually the one most likely to disappoint you. Output speed and port count matter just as much for whether the thing is actually useful day to day.

What mAh actually tells you

mAh (milliamp-hours) is a measure of total stored capacity — roughly, how many phone charges you'll get. As a rough rule of thumb, a modern phone battery holds somewhere around 3,000-5,000 mAh, and real-world efficiency losses mean a 10,000 mAh power bank delivers noticeably less than two full phone charges, not exactly two. Buy based on how many charges you actually need before your next chance to recharge the bank itself, not the biggest number available.

The spec that matters just as much: output wattage

A power bank's output wattage determines how fast it charges your device, independent of total capacity. A high-capacity bank with slow output will recharge your phone frustratingly slowly, even though it has plenty of total charge to give. If you want fast charging, check the bank's output wattage against your phone's supported fast-charging speed — buying a bank that outputs far less than your phone can accept means you're not getting fast charging at all, regardless of what the bank's marketing implies.

USB-C vs. USB-A, and why it matters more now

Most modern phones and laptops have moved to USB-C, and higher-wattage fast charging is generally only available over USB-C, using standards like USB Power Delivery (PD). A power bank with only USB-A ports will work but typically caps out at slower charging speeds — check that a bank's fastest port is USB-C if you want it to actually fast-charge a modern device.

Can it charge a laptop? Check the wattage carefully

Charging a laptop requires meaningfully more power than a phone — many laptops need 45-65W or more to charge at a reasonable rate, sometimes higher for larger machines. A power bank advertised for "phones and tablets" often can't deliver enough wattage to usefully charge a laptop, even if the port is physically the right shape. Check the specific output wattage against your laptop's charging requirement before assuming any USB-C power bank will work for it.

Pass-through charging, if you need it

Some power banks support charging your device and recharging the bank itself at the same time ("pass-through" charging) — useful if you're traveling and want to charge everything overnight from one outlet. Not all banks support this even if they have multiple ports, so check specifically if this matters to your use case.

Airlines generally restrict power banks above roughly 100Wh (watt-hours, a different unit than mAh, though usually listed alongside it) from checked baggage and sometimes require declaration for carry-on — if you travel by air with a power bank, check your airline's specific limit before buying a very high-capacity model.

A simple way to decide