UPDATE (May 2019): PolyMaths no longer uses Amazon affiliate links, but feel free to browse previous recommendations below.
I spent a lot of time researching the perfect power bank. I wanted large capacity, compact size, flexibility, and USB-C compatibility. This one perfectly fit the bill: it has a capacity of 20,000mAh, enough to charge an iPhone six or seven times. You can charge it in three different ways: USB-C, micro-USB, and even lightning. Charging via lightning is actually a pretty rare feature among power banks as far as I could tell, but it’s probably the way I most often charge it. I liked the idea of having the other two options as well because I know I’ll always be able to find some way to charge it when travelling.
It has three USB-A outputs, and one USB-C output. The USB-C is great for charging my Nintendo Switch. One weird exception to its flexibility is that it can’t accept input from a USB-C to lightning cable, though it can output through one (from its USB-C port), and you can also charge it from an Apple power brick with USB-C output if you have a separate USB-C to USB-C cable.
The size is good for its capacity: it’s a little under 8” by 4”. I don’t notice it in my backpack, but it’s certainly not something you’re going to one to carry in a pocket.
The only feature it lacks that I would ideally have liked is compatibility with the USB-C Power Delivery spec. This is the fast charging spec supported by the iPhone 8, 8 Plus, and X. But with a portable charger, the amount of time it takes to charge is not really that important because you can charge your device while carrying it. There are power banks out there that support this kind of fast charging, but for me there would have been too many trade-offs in terms of flexibility and size.
Avoid the analysis paralysis of trying to find the perfect backpack, and just buy this $30 one from the AmazonBasics range. It has lots of pockets for all your gear, and it’s pretty rugged and hard-wearing. It covers a lot of use cases: I use it to carry my iPad and books back and forth to school each day, but I’ve also used it for camping and travelling. It can fit a surprisingly large amount of stuff if you need it to: I managed to pack everything I needed for a week long trip to the States in just this bag, with no checked bag!
If you struggle with how many things you have to charge, especially when travelling, this is a great product. It has four USB-A ports for charging iPhones, iPads, or Apple watches, and it also has one USB-C port for charging your MacBook, Nintendo Switch. The USB-C port also supports the Power Delivery spec, so you can fast charge your iPhone 8, 8 Plus, or X if you have a USB-C to lightning cable.
Top tip for super efficient travel: buy yourself a cheap cable from Amazon with a two-prong (C7) connector on one end, and the plug of the country you are going to on the other end, and then you can just swap out that part depending on where you are going. Here are cables for Europe, the UK, and the US.