A power bank is a battery in a box, and the box has a number on it. That number is honest but easy to misread. This guide explains what you really get, how to pick the right size and output, and how to keep it safe.
Rated mAh vs real capacity
The capacity printed on the label, say 10,000 mAh, is measured at the battery cell's own voltage of about 3.7 V. Your phone is charged at 5 V or higher through USB. Converting between those voltages, plus heat in the circuitry and the cable, loses energy. In practice you can expect roughly 60 to 70 percent of the rated figure to reach your device, so a 10,000 mAh power bank delivers about 6,000 to 7,000 mAh of useful charge.
Some labels also print the energy in watt-hours (Wh), which is the clearer unit: 10,000 mAh at 3.7 V is 37 Wh. Comparing Wh between products is fairer than comparing mAh.
How many charges will you really get?
Divide the useful capacity by your phone's battery. A phone with a 5,000 mAh battery charged from a 10,000 mAh power bank gets a bit more than one full charge, not two. A 20,000 mAh unit gives that phone roughly two and a half charges. Earbuds and watches have tiny batteries, so even a small power bank refills them many times.
Pick the size by how long you are away from a wall socket:
- 5,000 mAh: a top-up for one phone, fits in a pocket.
- 10,000 mAh: a day out or a long commute, one full phone charge.
- 20,000 mAh: a weekend trip, a tablet, or two people sharing.
- 25,000 mAh and above: laptops, camping and anyone who charges several devices; heavy to carry.
Output watts
Capacity tells you how much energy is stored; watts tell you how fast it comes out. A power bank with a 10 W output will charge a modern phone slowly even if it is huge. Look for a USB-C port with USB Power Delivery and an output of at least 18 to 20 W for phones, 30 W for tablets, and 45 to 65 W or more for laptops. As with wall chargers, a bank with more watts than your device needs is fine; the device takes only what it accepts. If you also charge Android phones that need PPS for full speed, check for PPS support.