When we lost power earlier in the year here in New Hampshire, I went to a Rite-Aid Pharmacy and picked up some hand warmer pads and stuck them to the bottom of the tanks & tubs and piled a good amount of aspen. Worked great.

They were good for 8 hours or so, but I got home around the 6 hour mark and the heaters were dead! Poor little guys were freezing. I ended up getting my hands on a little indoor heater that connects to those little propane grills. It is called the " Mr. Buddy ", pick one up sometime.