I just purchased a USB carcharger pack with the tips for my phone and N810. Cost was less than the tips were elsewhere. Tested the power output with the thing plugged into a USB desktop port briefly, was less than the standard charger. Left it plugged in overnight, never came to full charge but does read 7 hours remaining,, versus 9 or 10, which I normally get from the wall wart.
I'd suspect this unit is slightly underpowered but would work.