How to know if (and when) a shutdown occurred due to power outage I have a Mac in a remote location that operates as a server.

It's configured (from Energy Management) to automatically power up in case of blackout as the power comes back.

How can I tell if a power outage has happened (and the mac self-restarted later) and when ?

I guess I can inspect the console, but what should I search for?
If you can get to Terminal on your remote machine, just type uptime
This won't, of course, tell you why it was last restarted, but when.