How to choose a portable charger (power bank)

Portable chargers look simple - a rectangle with a number on the box and a USB port on the side. The number on the box does not tell you what you actually get out of it, though, and bigger is not always better when it means carrying a brick around all day.

Pick capacity based on what you actually need, check the output wattage for charging speed, and make sure the ports match your devices. That covers most of it.

Capacity: how many mAh you actually need

Power banks are rated in milliamp-hours (mAh), but the rated number is not what your phone receives. Voltage conversion - from the bank's internal 3.7 volts down to the 5 volts a USB port uses, plus heat loss - eats roughly 30 to 40 percent. A 10,000 mAh bank delivers around 6,000 to 7,000 mAh of usable charge. That is physics, not a labeling trick.

Most phones have batteries between 3,000 and 5,000 mAh, so a 10,000 mAh bank realistically covers one to two full charges.

A 5,000 mAh bank handles a single topup - small, pocketable, good for a midday dead-battery rescue. A 10,000 mAh bank is the better choice if you are out all day without access to an outlet. For multi-day trips, tablets, or charging more than one device at a time, 20,000 mAh gives you room. Go above that and you are approaching the airline carry-on watt-hour limit, on top of carrying more weight.

Buy for typical use, not worst case. A 20,000 mAh bank weighs about twice as much as a 10,000 mAh one. If you carry it daily and rarely use more than a third of it, the extra capacity is just ballast.

Output: wattage matters more than mAh for speed

Capacity is how much the bank stores. Wattage is how fast it moves that energy into your device. These are different, and mixing them up is probably the most common mistake when buying.

A bank with solid capacity but low wattage charges slowly - three or four hours where a faster bank does it in one. Speed comes from the charging protocol: USB-C Power Delivery (PD) is what most phones and laptops use. For a phone, look for at least 18W PD; for a laptop, 30W or more. Some Android phones also support PPS (Programmable Power Supply), which lets voltage step in finer increments for more efficient delivery - not all banks support it, so check if yours does.

Look up the maximum input wattage on your phone's spec page. A 45W bank is no faster than a 25W one for a phone that tops out at 25W. Match or slightly exceed your device's spec.

Ports and cables

USB-C in and out is the right setup to aim for. It carries more power than USB-A and works with modern phones, earbuds, tablets, and laptops. If the bank also charges via USB-C, it recharges itself faster than one stuck on micro-USB - useful when you need to top it off overnight before an early flight.

How many ports depends on how many things you charge at once. One or two is fine for one person. If you charge a phone and earbuds at the same time, or travel with someone, get at least two. Some banks come with built-in cables, which solves the "I forgot the cable" problem but locks you into that one connector. A port with your own cable is usually more flexible.

USB-A ports appear on most banks and work fine for older devices. If you only use USB-C, USB-A is neither useful nor a problem - just do not pay extra for it.

Size and weight tradeoff

Every extra mAh is more grams. That is obvious until you are on a trip and realize you have been carrying 450 grams of battery that never dropped below 70 percent.

Slim 5,000 to 10,000 mAh banks fit in a pocket or lie flat in a bag. They weigh about as much as a small phone. A 20,000 mAh bank is closer to a thick paperback - fine in a bag, not in a jacket pocket. Above 20,000 mAh you are near half a kilogram and getting close to airline capacity limits.

Think about how you actually carry it. Daily commute, one phone - a slim 10,000 mAh bank is probably enough. Weekend trip, two people, uncertain outlet access - 20,000 mAh is worth the weight. If it has to live in your bag every day, buy for daily use. If it is for a specific trip only, a heavier pack makes more sense.

Features worth paying for

Pass-through charging - charging the bank and a device at the same time from a single wall outlet - is useful when outlets are scarce. Airport gates, hotels with one socket behind the nightstand, coffee shops with one power strip. Not every bank supports it.

Fast input charging for the bank itself is something most people overlook. A bank that needs eight hours to recharge is genuinely annoying on a real travel schedule. Look for 18W or higher USB-C input. Some banks support 45W or 65W input and are close to full in two hours.

A battery percentage display is more useful than four LED dots. Knowing you have 47 percent left beats guessing whether you are above or below half.

Buy from a brand with a real track record. No-name banks sometimes fall short of their rated capacity and can present safety concerns. A few extra dollars for a reputable product is worth it.

Built-in AC outlets and wireless charging pads are lower priority. The AC inverter adds bulk and most people do not need wall-outlet power from a portable bank. Wireless charging is convenient but lossy - it puts out less per hour and generates more heat than wired. Fine as a bonus feature, not something to pay a premium for.

Safety and airline rules

Power banks go in carry-on bags, not checked luggage. Airlines and the FAA prohibit lithium batteries in cargo holds because a thermal event in a sealed hold is much harder to contain than one in the cabin. Every major airline follows this.

The carry-on limit without advance approval is 100 watt-hours (Wh) per bank. To convert from mAh: multiply by 3.7 and divide by 1,000. A 27,000 mAh bank at 3.7V is almost exactly 100 Wh - right at the line. Most airlines allow up to two banks under 100 Wh per passenger. Between 100 and 160 Wh requires advance airline approval and is not guaranteed. Over 160 Wh is generally not allowed.

In practice: a 20,000 mAh bank works out to about 74 Wh - comfortably under the limit. A 26,800 mAh bank is around 99 Wh - under, barely. If you are buying specifically for air travel, verify the stated Wh rating on the label before purchasing. Reputable brands print it; that is what the TSA agent will check.

No-name budget banks sometimes use lower-quality cells. The failure rate per unit is low, but lithium battery failures are fast and do not go well in a small enclosed space like an airplane cabin.