Which Personal Hotspot mode uses least battery? Both iPad and iPhone's Personal Hotspot mode allows 3 ways to connect:


WiFi
USB cable
Bluetooth


I want to use Personal Hotspot on my MacBook Pro, this means that I can use either of the three. Therefore I was wondering:

Which connection type uses the least battery of the device providing the Personal Hotspot service?
USB tethering will use the least battery because the phone will charge while it acts as a hotspot. Bluetooth should be next (especially if both devices go into Bluetooth 4.0's low power mode), with WiFi last.