last winter we had a major power cut lasting for about 15 hours. The fish tank was surviving happily but I was watching a couple of grand worth of specialist reptile snakes get colder and colder and colder. I eventually managed to clobber together a portable gas heater & tank of gas as the room had dropped to about 4 degrees by then. I now keep those two items spare, ready for action!
Edit: Should mention, dont have mains gas so normal heating was also out.
Edit: Should mention, dont have mains gas so normal heating was also out.