Hi,
As long as they don't freeze ( heating system loss ), you could leave them for at least a day without cares (no light, no misting).
You can relax :)
You could use a battery powered ups that some use for computers (unplug your apple ... ho no, you could no longer read the forum :( )... if really the situation was to/would cause heavy stress to you.
Being prepared for what could happen, is the key (don't drive your car in the middle of the night to fix the situation) ! Good luck !