Which Power Bank Should I Buy?

Phones get faster every year, but the more powerful they become the less their batteries can keep everything going. And, of course, all batteries will degrade over time. If you need extra battery power for your smartphone, tablet or other USB-powered device, you need a power bank. Depending on what features and capacity you need, a power bank can be anything from a few quid to close to £100. See our pick of the Best Power Bank Deals.

You might assume all power banks are much the same thing, but you would be wrong. You can get compact power banks that will charge your phone once, slightly larger portable chargers that might offer two or three charges, or high-capacity banks that can charge your phone in excess of 10 times!

Working out how many times a power bank will charge your phone depends on more than the mAh rating on the packaging. No power bank is 100 percent energy-efficient, with every portable charger losing power through voltage conversion and heat generated. The industry standard for energy efficiency is between 60- and 70 percent, and if it doesn't expressly state otherwise on the packaging this is likely what you'll get. But some of the best power banks can offer as high as 90 percent energy efficiency, which equates to more full charges for your phone.

Power banks can also vary greatly on their inputs and outputs. We're seeing an increasing number of power banks with support for the latest Quick Charge 3.0 and USB-C standards, and in some cases these are supported on the input as well as the output, making them as fast to refill as they are to charge your phone. Some power banks may offer these in place of or alongside full-size USB, Micro-USB and even Lightning ports - which you choose entirely depends on which connection your phone uses, and for how long you intend to keep that phone until you upgrade. (Most power banks are guaranteed for 500 recharges.)

Typically you will need to carry a Micro-USB cable to charge your power bank, and potentially another cable for charging your phone if it doesn't use the Micro-USB connection to charge. Some power banks come with carry cables to ease this requirement; others might feature built-in cables and Micro-USB- to Lightning adaptors so all you need throw in your bag is the power bank itself.

We're starting to see power banks creep on to the market that feature an AC/DC outlet - a plug socket that allows you to plug in much more than USB devices. Unfortunately so far we've seen only power banks with the US two-pin plug, and needing to carry around a UK three-pin adaptor makes them less easily portable and feel less convenient.

Another type of power bank builds in solar panels. You might think living in the UK the grey climate would rule out the use of such a device, but even in cloudy conditions these devices can draw some solar power. However, the sunnier the conditions the better they work, so if this is the kind of device you're after we'd recommend a model with a built in battery that means you will have power for your phone even when it's dark outside.

Design is important when it comes to choosing a power bank, and some batteries are more compact than others at the same capacity. You can also buy those protected from the elements, but be sure to check whether they are probably waterproof or simply rainproof before leaving them out in the cold.

The ideal power bank is one that's small enough to slip into a pocket or bag without it weighing you down, but with enough juice to keep your phone going all day long. Something exactly like the Zendure A2 that sits at the top of our chart: it's fast, it's compact, it's got enough power to charge your phone two- to three times, and it has a sufficiently durable casing that you could even run it over in your car.

Nearly all power banks use a series of (typically four) LEDs to show you how much power remains in the device. This is fine for smaller-capacity devices, but when each LED represents two- to three full charges for your phone it can be difficult to see exactly where you are. We favour power banks with built-in LCD screens, though you will usually pay a little more for the privilege. No power bank we've tested has a more informative and useful LCD screen than the Omnicharge.