I have some open-sided racks downstairs. During the winter the daytime ambient temp drops to 70-72*F so I cover 3 sides with blankets at all times, and I cover the fronts at night since the air temp can go even lower.
Keeping the racks covered won't do much during a power outage. You're better off just heating a small room for all of your snakes rather than trying to run thermostats and heat tape off of a generator. Generator power is pretty "dirty" and a lot of sensitive electronics like digital t-stats won't like it much. A space heater should be fine though.