Necroing to post the solution:
You need the UPower package. OMV just so happens to use Debian as its backbone too, so it's easy to install:
sudo apt-get install -y upower
From there, just configure /etc/UPower/UPower.conf using nano. I set it at 95%, or a little after the machine loses AC power.
Also don't forget to enable (so it runs at boot) and start the service:
sudo systemctl enable upower.service
sudo systemctl start upower.service
The downside is that it will not power up automatically when AC power comes back, but you can just set up Wake-on-LAN from the router and use that instead.