Power Banks Explained: What mAh Actually Gets You

Power bank listings lead with one number, capacity in mAh, and it is the number that misleads people most. A 10,000 mAh bank charging a 5,000 mAh phone does not give you two charges. It gives you closer to one and a half.

Here is why, and what else to look at.

Why you lose capacity

The mAh figure is measured at the voltage of the cells inside, typically around 3.6 to 3.7 volts. Your phone charges over USB at 5 volts or higher. Converting between those voltages loses energy as heat, and there are losses again inside the phone.

In practice, expect roughly 60 to 70 percent of the advertised capacity to reach your device. So a 10,000 mAh bank delivers something like 6,000 to 7,000 mAh of usable charge.

Against a phone with a 4,500 mAh battery, that is one full charge plus a good part of a second. Which is genuinely useful, and it is not the two charges the box implies.

This is not a scandal or a sign of a bad product. It is physics, and it applies to every brand equally.

Rough sizing

• 5,000 mAh: about one phone charge. Pocketable. Good for a long day out.
• 10,000 mAh: one and a half charges. The best balance of size and usefulness for most people, and generally still allowed in carry on luggage.
• 20,000 mAh: three charges or so, or a tablet. Heavy, and worth it for camping or several days without power.
• Above 20,000 mAh: check airline rules before flying. Lithium batteries above certain capacities are restricted in cabin baggage.

The number that matters more than capacity

Capacity tells you how much. Wattage tells you how fast, and for most people speed matters more day to day.

Look for the output rating in watts. A bank that outputs 18W or more will fast charge most modern phones. A cheap bank at 5 or 10W will take hours, and that is the actual difference you feel.

Check the input rating too, meaning how fast the bank itself recharges. A 20,000 mAh bank with slow input can take most of a day to fill, which defeats the point.

Ports and cables

USB-C with Power Delivery is what you want, in both directions. It is faster, reversible, and it is what almost all modern devices use.

Check what is in the box. Many banks ship with a short charging cable and no wall plug at all, and some ship with an older cable that will not carry the fast charging the bank is capable of. The cable is often the bottleneck.

If you want to charge a laptop, you need Power Delivery and a considerably higher wattage, usually 45W or more. Check the laptop requirement first.

Looking after it

• Do not store it fully flat. Lithium cells degrade when left empty for months. Around half charged is kindest for storage.
• Keep it out of heat. A hot car is the fastest way to age a battery.
• Top it up every few months even if unused.
• Retire it if it swells. A visibly puffed battery should be recycled properly, not used and not thrown in household waste.

Before you buy

Check four things: capacity, output wattage, port type, and what cable is included. A bank with a big number and slow output is worse in daily use than a smaller, faster one.

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