Yep. Even if your water heater is electric, the water in it will still be hot for many hours. I use 5 gallon jugs for big enclosures and smaller drinking water bottle sizes for smaller enclosures.
Unless the power is off for a very long period (days), I don't see it as a problem. Torts will slow down, but come to no harm ... It would just be a bad weather day.
I'm more anxious about our tropical aquarium in a power outage.
Unless you think your home is going to drop below the 60s for a tropical species or go well over 100 (many tortoises can handle this, too, but I like to build in a safety factor) for almost any species and stay there for an extended period, I would not worry.