|The amount of energy you will save from powering down an otherwise inactive hard disk will be tiny. The power is used when the hard disk is reading and writing, writing more than reading.|
I wont do a lot for wear ether. either. Most of the wear on the drive spindle takes place during start up when it is cold and is using a lot of energy to bring it up to speed. Once the disk is up to speed the wear and energy consumption is minimal.
If you only use the drive once a month for backups you would be better taking it out and putting it in an external enclosure.