From conversations and personal research, I’ve heard that 250F is the consistent, average temperature that most solar ovens reach and maintain. This is true of both home made and commercially produced ovens. I’ve heard and even experienced higher temperatures with solar ovens, but 250F is the one number I’ve heard the most.
This is adequate heat to bake with, but will at least double the required time to fully cook any batter type of recipe.
Most 'cake' batters require 1 hour of cooking at 350F or higher. With a Solar Oven, you should probably leave that recipe in the oven at least 3 hours (at 250F) and possibly longer.
Unfortunately, if you leave a cake in the sun that long...the top can scorch, but tightly covering a batter won’t allow the steam to escape and will result in gooey, soggy cakes.
Consequentially, I have had better results with pastry baking, when I use Quick Bread recipes such as Coffee Cakes (US term), Biscuits (the fluffy breakfast roll- not the sweet treat eaten at tea time), muffins, ginger bread, brownies or corn bread.
Quick breads usually need only 20-30 minutes of baking time in a traditional oven at 350-400F. In a Solar Oven at 250F, my best results have been to cook Quick Bread recipes 2 hours, not the 3-4 hours that a thin, runny cake batter would need.
Thin, runny batters have a higher water content, and will create steam inside a Solar Oven, that fogs up the glazing glass... which will reduce the temperature inside the oven. In Summer time with intense solar radiation... foggy glazers isn't such a bad thing, since it helps to prevent the tops of cakes from scorching... but it does lengthen the cooking time.
If experience shows that your favorite recipe needs 3 or more hours to fully cook, you will need to cover the top of the recipe about half way through the cook time. I recommend a Black Camping plate, since it will fit loosely over the recipe, and allow the steam to escape; as well as, the black surface radiating IR heat back onto the top of the cake, continuing the cooking process.
All experienced Solar Cooks recommend using shallow tins when you bake. This allows the Conductive Heat to reach the center of the cake quicker, so your cook times are reduced.
Also, elevating your cake tin just a little bit off the floor of the solar oven will allow the Convection heat to reach the bottom of the pan- cooking the bottom of the batter at the same time as the top.
Baking is tricky business in a traditional oven, so don’t feel badly if your first efforts with a solar oven do not turn out perfectly. Experiment with several different types of batters to discover what works best for your area.
FoodNetwork.com has an extensive database of high quality recipes. Do a Search for Quick Bread Recipes, Muffins, Coffee Cakes, Ginger Bread, Muffins, Brownies and Biscuits to find new recipes to try in your solar oven.