There are many solutions that provide the fuel savings of a thermal cooker without the expense. The concept of retained heat cooking has been around for at least 200 years that we know of as a product or appliance using a bucket and hay or even a hole in the ground. The Fireless cookers of a 100 years ago were quite a big deal and old working antiques can still be found available that work the same today as they did then.
I've collected a number of different methods of thermal cooking and retained heat cooking on the
www.thermalcook.com blog and they range from a laundry basket with blankets and towels with your own cook pot placed inside to the $200+ dollar vacuum insulated commercially made thermal cookers. The methods all work, it's just the difference in size/convenience and the performance that separates the different methods.
Performance wise a vacuum insulated thermal cooker is one of the best retainers of heat and the most compact and even portable and will keep 4 quarts of food hot (above 140F) for 16 hours or more letting you cook all your days meals once in the morning and then eat them anytime needed throughout the day. This was one of the reasons I really like thermal cookers for emergency preparedness. I live in an area where an earthquake is a big potential and if anything major happened I wanted to be able to conserve the fuel stored and also the time needed to make and prepare meals. To be able to cook only once a day but still have a hot meal anytime during the day is a big plus in an emergency and it wouldn't be such a bad thing for homesteading in general as the time savings would be as valuable as any fuel savings.
Other retained heat cooking methods aren't as efficient at keeping things hot for the same length of time as a vacuum insulated thermal cooker but they still will cook food and conserve fuel in the process. Most meals can be cooked in a few hours of thermal cooking and as long as the retained heat method can keep the food above about 165F for those few hours (after having brought the food to a boil) you have a method that works even if it's a sleeping bag stuffed in a laundry basket with the pot inside. You can optimize the performance of retained heat cooking methods by using better insulation and pots such as cast iron that hold onto the heat more readily.
For freeze dried or dehydrated foods most thermal cooking methods would work just with the addition of boiling water or bringing the contents to a simmer for a minute before putting the pot into the thermal cooker. For dry good such as pinto beans, you will want the most efficient cooker available as it takes 4-6 hours to cook even soaked beans. Cooking beans is a really good example of how well a thermal cooker can be used to save in fuel consumption as the amount of fuel needed is to bring the beans to a boil (about 20 minutes) and then you need to keep the beans simmering for about 25 minutes before placing the pot in the thermal cooker to finish for the next 4-6 hours. Less than an hour of fuel use instead of 4-6 hours to cook the same staple food.
Thermal cooking doesn't loose much moisture so recipes cooked using this method will need to take that into account. When i've used a crock pot to cook beans I'm counting on the water to evaporate to help thicken the beans by the time they are done. With a thermal cooker you don't get that effect. Rice for example cooked in a thermal cooker uses a bit less water than cooking it on the stove or rice cooker as the water isn't lost in the cooking process and if you use the same amount of water called for in most recipes you will have soggy rice. Less water use is also a good thing in emergency preparedness needs.
Thermal cooking also needs a good amount of food mass to hold onto the heat otherwise the food cools too fast even in well insulated containers to complete the cooking process. If you are cooking for more than two people it's not hard to have plenty of food mass in the recipe to make it work but for smaller recipes of less than a couple quarts you may have to reheat things half way through the cooking time. It is very dependent on the type of food being cooked as some foods cook fast no matter what (like oatmeal) but larger chunks of meats and vegetables might take a little effort to get the hang of it for those smaller meal sizes.
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