The max of USB is 2,5W or 500mAmps at 5V.
It probably takes even a little less but still a lot more than you think.
You have a 60Wh to achieve 7h your notebook must use only 8.5W on average.
Now add just 1W. With 80% charge you end up with 5h.
Simply pluging in a USB stick keeps the notebook from saving energy by shutting down all the USB ports as well as putting to sleep all the responsible controllers and stuff and again it is CDMA which needs more energy than Wifi for the simple reason that it needs to transmit over much greater distance.
Also if you don't need the backlight keyboard turn it off entirely. While half/min or max backlit keyboard makes little difference. Between on and off is almost around 0.5-0.8W. It is a farily useless feature anyway. Turn on some lights that is much healthier for your eyes every eye doctor will tell you that.