Originally Posted by brandob9
Don't forget that the efficiency of the transfer will not be anything close to 100%. A more likely factor is 50%.
Its probably much worse than that. Unless that USB port solution has an external low voltage AC/DC converter You have to first convert a battery to 120vac using an inverter which depending on the load and the inverter is probably 5-70% efficient. Then you tack on the efficiency losses to convert back to 5v USB voltage
You will be much better off with a goal zero type battery at 6v with a USB output to quick charge your batteries
Top that off with a 6v compatible solar panel. Although if you get the right capacity battery you probably won't need to recharge it on the trip. The newer lithium fe phosphate batteries are safe, lightweight, and can be recharged many times in its lifetime
Of course if you are taking along a 12V marine starting battery and don't care about weight, then disregard the abovementioned rant