The input rating is key when it comes to recharging the power bank - the higher is this figure the more quickly it will charge....
The input rating is key when it comes to recharging the power bank - the higher is this figure the more quickly it will charge. You'll usually see a figure in Amps, for instance 1A. You multiply this number by the voltage (5V for USB) to find the rating in Watts. A 1A input can charge at 5W, therefore, but you'd do better to look for a device that can charge at 2A (10W).
Don't expect to get a USB charger in the box - you can use that which was supplied with your phone or tablet. Do note, however, that a power bank with a 2A (10W) input will not recharge its own battery any faster than one with a 1A (5W) input when used with an underspecified USB charger. The reverse is also true when it comes to charging your devices - a phone that supports only a 1A (5W) input won't charge faster from a 2A (10W) output.
The output rating refers to how quickly a power bank will charge your devices. In most cases you'll find 1A (5W), 2A (10W) and often even 2.5A (12.5W) outputs, the former intended for smartphones and the latter two for tablets or for fast-charging phones that support the feature. (This is not the same as the Qualcomm Quick Charge feature supported by several flagship phones.) You can use either output to charge any USB device - it will draw only the power it needs. However, you might find some tablets - usually iPads - will refuse to charge from lower-specified outputs.
Increasingly power banks feature clever technology which is often referred to as PowerIQ or similar. This allows the power bank to recognise the type of device you have connected for charging and deliver the optimum amount of power for that device.
If a power bank has several outputs then the maximum total output capacity is key. For example, the LimeFuel Blast Pro L240X has four 2.4A (12W) outputs, but delivers only 4.2A (21W) total output. With four USB devices attached you will find that each charges much more slowly.