The most power-efficient external drive

[vmilanv88]

I would like to use and external drive with my 2014 rMBP and the main goal should be the least power consumption as possible.
I have thunderbolt 2 ports and USB 3.0. I tried to google about different drives power consumption but I barely found anything useful.
Basically I don't know if HDDs or SATA SSDs, or even NVMe SSDs (both in an external case for sure) would be the best in general?
I really appreciate any help.

 

[mj_]

A SATA SSD should be your best bet. NVMe SSDs tend to get really hot, and as we all know heat equals energy. And spinners have moving parts, which means higher power consumption when idle and when spinning up and/or down.