Along around 11PM the power indeed cut out, and the window fans that maintained a tolerable temperature in the house at night quickly wound down to a stop. And sometime thereafter I could see martes navigating around the house with a flashlight.
Sometime even later, around midnight I think, I woke up all hot & sweaty as the fans were off and it was now all hot and muggy in the house. I lay there and wished I had some kind of back up power source for such occasions. Then I remembered I actually DID, something I had bought two years ago for earthquake related power outages.
At that time we were going through our "Rolling Blackout" period, and the local grocery store had been carrying an item then called the "X-Power" which was essentially a car battery and a power converter in an attractive carrying case. I had only tested it the day I bought it and had not charged it in over a year, but I decided to give it a shot. I got it from the garage, took it to the bedroom and pushed the test button. 25% or less power remaining. Oh well, I'd gone this far, so I plugged the power strip supplying the window fans into it and turned it on.
Both fans came on full blast, and in a couple of minutes the room was comfortable again. Not knowing how long the power would last, I turned off one of the fans and in addition decided to stick with the wind up clock and not try to run the electrical one off the battery. The clock would not have been much of any drain at all on the battery in the X-Power, but if the fan ran it down the alarm would not go off, so I decided not to risk it. Now nice and comfy, I went back to sleep.
About 2:20 Am the wind up clock went off (too early, but it is nowhere near as accurate as the digital clock) and I noted the fan was still running full speed and the X-Power still had 25% power remaining after over two hours. Nice little device, I wish I had remembered it earlier.
In the event of an extended outage, there is another power inverter in the van that can be used with any car battery.