A portable power bank will stop you from being caught short of battery power while you're out and about. Just make a deposit into the bank by charging it up - it is then ready for that desperate time when your low-battery mobile device will need to make a power withdrawal. There are a lot of power banks on the market, and many of them look very alike, so this buying guide will point out the different features to consider when buying one.

A numbers game

Check the milliamp hour (mAh) number of the power bank to give you an indication of how much charging power it has. The higher the number, the more power it has in the bank. For example, a 3,000 mAh power bank should give you just one charge of a smartphone (most smartphone batteries generally have around 3,000 mAh). Obviously, a power bank with 10,000 mAH will give you more charging capability. As a rule of thumb, you get what you pay for when it comes to mAh - the more you pay, the more mAh you can expect.

It's meant to be portable....but

A power bank is meant to be portable; that's the whole point of it! But if it becomes too big for the average pocket or bag, much of that portability is compromised. This can happen when power banks get into the 20,000 mAh and above range - but all that extra grunt means the power bank is capable of doing great big things, like jump-starting a vehicle. Consider what you want your power bank to charge and expect reduced portability if those charging responsibilities go beyond just breathing life into your smartphone.

How many devices do you want to charge?

Some power banks have just one USB port to charge one device at a time. But if you'll need to charge more than one device, search out power banks offering more than one port.

Know your bank balance

In the case of a power bank, your balance is all about how much power you have remaining in the device. An indicator that gives you a clear and accurate picture of how much power you have at your disposal is a very handy feature.

A comfortable shape

If you want to carry your power bank in your pocket, look for designs that are shaped to look and feel just right. Some power banks are shaped in such a way that they may feel uncomfortable in your pocket - they're better suited for bags. Give some thought how you want to carry your power bank, and look for designs that best fit your thinking.

Amp up the charging

To keep your other devices juiced up while you're out, your power bank will need to be fully charged. To ensure that charge is quick and easy, look for the input range on the power bank. A higher input number e.g. 2.4 Amps means a faster charge.