I just saw this "Billyboil" brand "Thermal Cooker" for sale at an energy-efficiency store. Initially I was intrigued by the terrible marketing (what on earth would be a "Non-Thermal Cooker"?) but the concept of using retained heat interests me.

The idea is, you have two pots - an inner one that goes on the stove-top, and an outer one that has extremely high insulation. You heat up the inner one on the stove until it reaches the desired temperature for what you are cooking, then turn off the stove and put the inner pot into the outer one and seal it up. Ideally, the high insulation of the outer one will slow leakage of heat for hours, and so the food in the inner pot can continue to cook from its retained heat with no further energy input required.

That model costs AU$150, and I was thinking that this should be also feasible with other, cheaper containers - you would need to find a pair that complimented each other:

• Inner stockpot with small handles that don't stick out, or better a compact camping pot with folding/removable handles.
• Outer insulator which is large enough to completely enclose the inner pot, made of materials which won't melt or burn when a hot object is put inside, and with insulation effective enough to retain enough heat to keep the food cooking.

Seems like the outer one would be most difficult to find, maybe a large vacuum-insulated metal thermos would work, but it would need to be big enough to contain the inner pot.

Has anyone seen or tried a similar project? I wonder how it would compare to a pressure cooker in terms of input energy required to make what would be 1 or 2 hour stew in a "normal" pot.