Your snakes are fine this time of year without heat as long as you don't feed them. But if you had just fed them when the power went out, the water bottle idea sounds good. In winter, it might be more of an issue, obviously. You can use the water bottle and/or those little hand/foot warmer packs with the wrapping of the viv. You can also get one of those back up power supplies that you would use for camping or starting your car.