My Dad had turtles for over 10 years. Red-eared sliders. Raised them from when they were babies and one summer it was EXTREMELY hot and he uses to have them in the sun because they would always come out whenever they were near. That day the turtles died because he forgot to put them in the shade. To say he deviated is putting it very lightly. Sorry to hear that. Just a side note, I highly doubt that the pump being off for a few hours would kill them. As a fish and pond owner living in South Africa we have daily power outages up to 2x a day sometimes and never had this issue. Most likely a spike in the water quality or the tank was way overcrowded.
On my uncle’s chicken farms, there is a generator that automatically fires up within about ~2 seconds of a power failure. When the power goes out, there are only 2 12V batteries to operate the vents, which are opened to 100% (or less/zero, if the chickies are young). The problem is when they’re older and ventilation is key. It’s a similar issue here! I’ve lost thanks to this sort of thing. Power outage, I’m gone for the weekend, come home to dead tanks. It hurts in the deep down inside place.