Power banks come in all shapes, weights and capacities, from emergency credit-card-sized devices that offer a quick boost to keep you going until you get home, to the ultra-high-capacity models which might charge your phone in excess of 10 times - ideal if you’re going camping or have several devices to charge.

Several factors are important when choosing a power bank.

Portability
some power banks are small and light enough to slip into a pocket; others you’ll notice their presence even when slung in a bag. Pay particular attention to their weight and avoid bulky designs if this is a device you'll carry every day, rather than in certain situations only.

Capacity

It’s important to note that a power bank will not deliver its full advertised capacity to your device – some of this energy is lost through heat generated and voltage conversion.

If a power bank manufacturer doesn’t expressly state otherwise, expect to achieve around 70 percent efficiency. However, Pisen power banks give you a 90 percent efficiency. A 10000mAh Pisen power bank therefore, delivers 9000mAh of power.

Check the specification of your phone or tablet’s battery to estimate how many full charges you can expect from this.

Input rating

This is key when it comes to recharging the power bank - the higher 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).

Output rating

This 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 quick charging feature supported by newer phones such as the Samsung Galaxy S6.) 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 recognize the type of device you have connected for charging and deliver the optimum amount of power for that device.


Auto-on and -off functionality

Auto-on allows them to begin charging your device as soon as you plug it in, and auto-off causes them to switch off when them to switch off when the job is done, meaning no power is unnecessarily wasted.

Those that don't support this functionality will have a small power button on their casing somewhere, which is typically also used to light up the LED flashlight (if the power bank has one) or see how much capacity remains. In most cases you’ll find a series of

LEDs used to denote how much capacity remains.

The ideal power bank carefully balances useful capacity with portability, value, fast charging/recharging, multiple outputs and high-end features such as pass-through charging and LCD displays.