This is not about which one will fit in your pocket or your purse. The size of the battery is about the power inside. Most power banks will feature a number on the front that tells you the number of mAh or “milliamp-hour”.

Common sizes range from 2,000mAh up to 10,000mAh or even 26,000mAh. Bigger numbers mean more power and more recharges for your smartphone, or charging for bigger devices that need more power, like a tablet. 

We’ve used smartphones and tablets to compare battery sizes, but most power banks will charge any device that charges via USB, such as a GoPro camera, Kindle reader or Bluetooth headset. Bigger devices that draw more power will need bigger batteries – more mAh – to charge them fully.