Keep Mac ON for background work with minimum power Say I am doing a long download.  How can I keep the Mac stays sufficiently awake but with minimum power usage?

I would also like minimum power when:


I am using it as a surveilance camera.
I want it to be available for requests from the Wi-Fi network.
Picasa/Google photo backup


OS X Yosemite

Also how do I avoid that some passerby put it to sleep (to sleepy)
While newer Mac OSX offers something called PowerNap, the feature does not support the uses you listed. Therefore, the only options are within System Preferences. 

In this case, you should set your Energy Saver settings to Computer Sleep- Never, and Display sleep - 1 min. Check the box for 'put hard disks to sleep when possible'. Wake for network isn't helpful, since your computer will not sleep.