I woke up this morning to the backup alarm on my Android phone; my regular alarm clock had not rung.
There was a simple explanation visible on my alarm clock, which was blinking "12:30"—a power outage.
So I knew when the outage had ended—half an hour ago—but not how long it had lasted. Should I be worried about the food in my fridge?
I remembered that my Android phone, the one that had rung the backup alarm—because it had a battery backup, of course— had been plugged in overnight.
Not wanting to bother launching Eclipse at the moment, I quickly found a great free log viewer called CatLog in the Play Store. One minute later, I had my answer.
The system statusbar logs changes to the signal strength, power state, etc. I just filtered with the keyword "plugged" to track the power state. It was immediately clear that the outage had started around 8:06 AM and was over by 8:10—nothing to worry about.
Of course, this isn't a perfect solution for tracking power outages in general.