The important part to remember is that at their best they charge your thingos at the same rate as it takes from the USB socket in your PC.
That is why I think that the only way to use them is to charge an indipendent USB in/USB out battery from home, than keep charging that up as you transfer that power into your devices.
Even so you can buy now a 12000mAh (not 1200...) battery of that type for less than $30.(about 5 oz)

Still the pot used on a wood burning stove on a long afternoon/winter night or three , particularly if melting snow, could work.
Franco