Some USB outputs are better than others. Earliest ones put out 500 milliamps, newer ones put out 1000, (i.e 1amp) still not enough for a Dot , but enough to charge a cell phone, and some newer PCs have enough juice to charge an iPad (2amps) which is also what the Dot requires. One needs to know the output spec of a PC's USB, and not all are the same on a single PC, either.
2amp battery banks can serve up 2A as well, so they can power a Dot until the battery goes dry.