I have a 13000 mAh Anker battery that I originally bought for my phone and iPad. I don't think it generates enough power to USE the Macbook while its charging, but I have used it on multiple occasions to charge the Macbook in my backpack when its closed and it charges quickly when the machine is closed/asleep. You'll need a standard USB to USB-C cable through.
[doublepost=1495064511][/doublepost]You can also slowly charge it (while it's closed) using a USB - USB-C cable with an iPad charger in a pinch. It's good to have for emergencies..
The iPad charger (and power banks with similar power output) takes about twice as long to charge the 12" MacBook than the default charger. It is still able to charge the MacBook even while using it, just much slower.
There may be heavy use cases where the power consumption exceeds the charging speed of the power bank/iPad charger, but I haven't experienced those situations.
Personally, I find it more useful than just "good for emergencies". I usually keep my USB-C charger in my office and my iPad charger at home. That way, I can still charge the macbook overnight with the iPad charger if I need to. This is awesome because I don't need to buy additional chargers for added convenience and I don't have to lug my chargers around.