Hi and welcome to the forum
As the others have mentioned, insulate, insulate and insulate.
You can tape 1 inch thick sheets of polystyrene foam on the back, sides and front of the tank.
Have a coverglass on top. Get 4, 5 or 6mm thick coverglass because it is less likely to chip compared to thinner glass.
If you have an external power filter, insulate that too.
Raise the tank's temperature to 28C before the power failure.
Close the door for the room the tank is in and try to warm the room up before the power failure.
When the power goes off, cover the tank with towels or blankets for more insulation. Make sure they don't come in contact with the water because they could drain the tank.
Remove covers when power comes back on so lights don't cause towels/ blankets to burn.
Get a battery powered air pump and have it bubbling away a bit during the power failure. However, if the room gets cold, turn the air pump off so it doesn't pump cold air into the tank and drop the water temperature.