Heard a big clunk and the lights flickered. Guessed it was the transfer switch. Actually I suspect that the power had come back and the transfer switch had flipped and then the power glitched while the generator was in its seven minute cool down mode. So it momentarily cycled.
I am thinking this because (1) the transfer switch is usually able to come off the generator fast enough we don't get a glitch and (2) I heard the generator shutdown at 1:58am, which is only five minutes.
From a cold power outage, it takes about six seconds for the generator to come on and stabilize. With the generator already running, the transfer switch has to sense an outage before switching over.
Since I had calculated that the power went out at 5:35am on Sunday and back on at perhaps 1:53am on Monday, we were on the backup power for 20 hours and 18 minutes.
Considering it's 22°F outside right now, the investment in the generator was SO worth it.