If you aren't set up for a generator, the old fashioned way always works too! As long as you have a gas water heater all you need is water bottles and a cooler. I put all my snakes in the incubator and put hot water bottles in there. It keeps it about 90 degrees for 8 hours as long as our house stays in the 60s. I don't know how it does in colder temps, but I would just add more water bottles and warm them up more often. You can also use a cooler or shipping boxes. I prefer to use water bottles as they do not get out of control or give off toxic fumes.
My uncle is coming to install a generator that works with computers this fall, I'm pretty excited about that!