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?

Solution 1:

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.

Solution 2:

USB uses the least power by far, only one radio working, plus the phone may run cooler, some people have had heat issues with extensive WiFi/Bluetooth use.

I did an more complete comparison of all three types here: WiFi or Bluetooth (or USB) for iPad Tethering

"Bottom Line: USB is the best if range isn't an issue, If distance is needed use Wifi with WPA2"

Sources for heat issues:

https://discussions.apple.com/thread/2046464

http://forums.macrumors.com/archive/index.php/t-593613.html