You can get a big package of 7 hour heat packs pretty cheap at D ick's sporting goods that are made to keep your hands or feet warm. I have some of those just in case. You can buy 40 hour heat packs online from reptile supply businesses. Nothing beats having a generator for a power outage but that isn't feasible for everyone.
edit: I can't believe that the store name was censored but it is the nickname for Richard.