Some computers actually allow power through USB whilst the computer is off
But generally when the computer is off, its off
You need an external power adaptor that you can just plug into the wall outlet (just like the laptop has)
i will travel about 2 days and it will be nice to have my phone or PSP on but i dont think that its if the laptop is on with a off screen it would last more that .... 8 hours or maybe less is it possible to leave the usb on some how throw the BIOS ?