Perhaps they've incorporated a micro-cold fusion generator in the back of the unit and the USB is purely for the inbuilt light??
Chicken got it spot on with 2.5w ain't gonna do a great deal.. That's like running your modern home nuker at 1/400th of the power...it take a 30secs to heat up a pot of beans in 1000W's so in this it should take a minimum of an hour (taking into account the smaller size so less wave loss)