The USB port is meant for charging other devices (aka, plug a phone charger into the lamp). As far as I know, it serves only as a power out, not in. Also, the lamp doesn't run on batteries and needs to be plugged into an outlet. Hope that helps :)
According to the label on the lamp, its life span is 40,000 hrs, that gives you about 4.5 years if you leave it on 24/7. So...no, you don't need to replace the bulb, you just have to get a new lamp after 40k hrs.