EDC is becoming increasingly digital these days. Often people go out the door with a multitude of devices; phone, ebook reader, tablet, rechargeable flashlight et al. Having a means to support these devices is a good idea, especially if access to power is unreliable or a while away.
My support kit is built around a Tecknet 15000mAh USB battery. This provides several charges for my USB devices to keep them (and by extension, me) happy over the course of a day or more. It has proven its worth at my former job where the van power socket was broken and I had to run my SatNav for the day off the battery.
Of course, having a battery is useless without the ability to actually charge devices. To that end, I have a micro USB cable and an Apple Lightning cable (iPhone, sue me) which will let me charge the vast majority of anything I come across.
Additionally, I have a set of headphones to avoid being subject to sonic assault from the world at large.
A USB fast charge adapter rounds up the power side of things. Public USB sockets are suspect at best, who knows what is on the other side of the connector. Could be a dumb power supply, could be a full blown computer sucking down all your phone data and sending it off to shady places, possibly giving you malware at the same time. Don't be a fool, protect your tool.
The last few items in the kit aren't strictly power related but they fit in well anyway. I have a physical GPG smart card reader with built in smart card so I can do GPG stuff and have a good degree of confidence that if either my laptop or the kit gets stolen/lost, folk can't impersonate me by email. I have a Kingston 32GB pendrive for quick data storage/transfer and lastly, a lighter. Because fire is a good thing to have on command.
The whole lot fits nicely into a Maxpedition mini pocket organiser which lets me transfer everything between bags quickly and keeps everything modular and neat.