I have a propane heater that is designed for indoor use that I use when the temps really start to drop (I'm in North/Central NC, and most of my power outages are ice-related so it gets cold pretty quickly). The thing is, you have to be there to check on it every half hour or so to make sure things are well ventilated and it's working properly. It can keep a poorly insulated room at about 70 until you run out of fuel.
I don't have a huge amount of snakes (and most of my herps are temperate species that can range into Nova Scotia, so a temp drop doesn't really bother them that much) and I've put them into their travel containers and placed them near several gallon jugs filled with hot water and put a pretty heavy blanket over them... kind of creating a water jug fort. It works for several hours keeping the temps inside the containers at around 70-75 (the last place I lived had HORRIBLE insulation and would end up close to outside temps inside a couple hours, so you might not need that).
I also keep a supply of the hunting chest warmers that are designed to last for like 12 hours. My power is usually back on pretty quickly, so the only time I've used those was putting one on myself and basically cuddling my turtle till the power came back on.