Thank you both for the answers. Effectively the problem was that my ups was setted by default to no switches itself off after a power outage. I resolved the problem by connect the ups to my PC and by modify this option via the software provided by the manufacturer. Then I set the restore power policy on the BIOS to "Always ON" and everything work as I want.
Furthermore, after doing some test and inspecting the parameters visible by the OMV-Web-UI I found that the behaviour of the system is regulated by 3 parameters (see attachments below) and is like this:
1) Power Outage occures --> the UPS safely shutdown the server after "ShutdownTimer" seconds (in my case 30) and switches itself of after "ups.delay.shutdown" seconds (in my case 30) after the shutdown of the server.
2) After Power is Restored the UPS come back on (only if a minumum of "ups.delay.start" seconds (in my case 180) have passed) and the omv-server restart.
The only editable parameter seems the ShutdownTimer . The other two (visible via the services tab of the OMV-Web-UI) seems not editable
So, problem Solved. Thank's again.