Let’s say, as rules of thumb (definition: wrong but tends to work anyway) that
1.) There are about 250 BTUs in a square foot of sunshine per hour.
2.) There are approximately 8,600 BTUs in a pound of wood. Wood is stored solar energy!
3.) It takes a solar oven with about 34 square feet of intercepted sunlight to equal the cooking power of one pound of wood burned in one hour.
4.) In a solar oven with the firepower of one pound of wood (burned in an hour) the intercepted sunlight would be about 6′ by 6′. About 1/3 of the energy would cook the food and about 2/3rds would do other less useful things.
5.) In our 6’ by 6’ solar oven (give or take) 3,000 BTU would boil 2 gallons of water in about an hour. The pot is black, it has a tight lid, the box is well insulated, and airtight. The glass is double glazed. The losses are minimized and the solar gain is optimized in a solar oven that efficiently cooks food.
With this kind of solar oven, large amounts of food can be made every sunny day without using up any earthly resource! And it’s a convenient way to cook. In our experience, solar cookers are great when they are big enough to do the cooking task in a reasonable amount of time. ARC cooks used them in the summer to cook lunch and dinner for twenty people and it was nice to have a no-fuss oven that needed little tending and did not increase entropy. It was certainly more comfortable not to deal with a hot fire on already hot summer days. The staff at the Aprovecho farm used stored solar energy in the winter and direct solar energy in the summer for cooking, heating water, etc. Everything is dependent on sunlight.
Aprovecho Research Center
PO Box 1175
Cottage Grove, OR 97424, USA