3 parts to charging: power supply, charger, PV
The PV will have a mAh rating, such as 1000, 650, 180 (for a small, standard, 510)
The PV will also have a nominal charger input voltage requirement -- the vast majority are designed for 4.2V, but I believe at least a few may actually be designed for 5V.
The PV will also have a polarity -- either the center contact is the positive, and the outer
threaded connector is ground, or vice versa.
The charger part (USB on one side, PV threaded connector, or slip on friction connector, on the other) needs to have output voltage and polarity consistent with the PV, and the mA output of the charger should be less than or equal to the mAh rating of the PV. I can use either a 150 mA or 500 mA output charger on a 650 mAh eGo battery, but I cannot use a 500 mA output charger on a little 180 mAh 510 battery.
The charger should have written on it an "Input" section, typically 5V, 0.xxx A (or xxx mA).
The Power Supply needs to have a Voltage and mA output rating greater than or equal to the charger Input requirement. If the Charger says 450 mA input current, then I can use any USB source with at least 450 mA output, but I shouldn't use a Power Supply that is, say, 200 mA output.
The USB spec calls for a port to support at least 500 mA output. This mean that most PC ports are fine with a normal eGo charger.
Personally I avoid using a PC USB port, since a whoops on the generally cheap PV charger, a short circuit, can do harm to the computer. Most of us have multiple USB Power Supply wall warts (that plug into household 120VAC) around, just use one with an output mA >= the Input mA written on the charger.