I just had this idea pop up If you have a MBP and an iPhone there are two ways to tether - over USB or over Wifi. Oftentimes when Im tethering Im on the road as a passenger or in a vehicle of some kind. Scenario A: Ive got the wifi on my Mac and my iPhone and Im tether to the Mac wirelessly and the Phone is handling its own power source - cigarette lighter or battery power. Mac isn't supplying any power to iPhone but it is powering its wifi chip. Scenario B: Mac is connected via USB to the iPhone which is tethering over USB and unavoidably drawing current from the Mac over the same USB cable. Wifi chip is off on the Mac. Which of these two scenarios will drain my Mac's battery fastest?