Fixing Mac Mini that uses 10x normal power in sleep mode
IMHO Your problem could have 2 reasons:
1) Software problem, your sleeping computer was not sleeping, it crashed while trying to sleep
2) Hardware problem, your SMC (System Management Controller) is confused - or worse - your mainboard is partially fried.

This is what I would do in chronological order:

Make sure you have installed the latest firmware updates
https://support.apple.com/en_US/downloads/macmini

Reset the SMC
https://support.apple.com/en-us/HT201295

Reset NVRAM/PRAM (this for sure won't solve your problem but why not try)
https://support.apple.com/en-us/HT204063

Backup files, download El Capitan, make a bootable USB and reinstall OSX from scratch.  Don't do the upgrade, choose for a clean install.

Last resort: get your Mac to the Apple store and ask a Genius to perform hardware analitics.  On a lucky day you might be entitled for a free mainboard replacement if Apple deems that your hardware can be of good use at the investigation department.