Frequently Asked Questions
What are the basic kinds of solar cookers?
There are three basic kinds:
Box cookers This type of cooker has been the advantage of slow, even cooking of large quantities of food. Variations include slanting the face toward the sun and the number of reflectors. You'll find an article discussing solar box cooker designs here.
Panel cookers This recent development was sparked by Roger Bernard in France. In this design, various flat panels concentrate the sun's rays onto a pot inside a plastic bag or under a glass bowl. The advantage of this design is that they can be built in an hour or so for next to nothing. In Kenya, these are being manufactured for the Kakuma Refugee Camp project for US$2 each.
Parabolic cookers These are usually concave disks that focus the light onto the bottom of a pot. The advantage is that foods cook about as fast as on a conventional stove. The disadvantage is that they are complicated to make, they must be focused often to follow the sun, and they can cause burns and eye injury if not used correctly. Some of these concerns have recently been reduced by Dr. Dieter Seifert's design.
Who made the first solar cooker?
The first solar cooker we know of was invented by Horace de Saussure, a Swiss naturalist experimenting as early as 1767. See this article for more info.
Where are solar ovens being used the most?
There are reliable reports that there are over 100,000 cookers in use in both India and China. We are aware of solar cooking projects in most of the countries of the world. Solar Cookers International has recently had a breakthrough in Kenya using the CooKit panel cooker. More than 5000 families are now solar cooking there.
How hot do solar ovens get?
Place an oven thermometer in the sunny part of then oven to get a reading similar to what the cooking pot if "feeling". The temperature reached by box cookers and panel cookers depends primarily on the number and size of the reflectors used. A single-reflector box cooker usually tops out at around 150° C (300° F) as the food approaches being done. High temperatures, however, are not needed for cooking. Your oven will cook just fine as long as it gets up to about 90° C (200° F) or so. Higher temperatures cook larger quantities, cook faster, and allow for cooking on marginal days; However, many people prefer to cook at lower temperatures, since then they can leave the food to cook while they go about their business. With a single-reflector box cooker, once the food is cooked, it just stays warm and doesn't scorch. It's good to keep in mind that no food can go above 100° C (212° F) at sea level anyway, unless a pressurized cooking vessel is used. The high temperatures you see in cookbooks for conventional ovens are just for convenience and for special effects such as quick browning.
How long does it take to cook a meal?
As a rule of thumb, you can figure that food in a single-reflector box cooker will take about twice as long as in a conventional oven . However, since you can't really burn your food, you don't have to watch the cooker or stir any food as it cooks. You can just put in a few pots with different foods and then come back later in the day and each pot will cook to perfection and then stay hot until you take it out.
Panel cookers cook smaller portions, usually only in a single pot, but often they cook slightly faster. Some people have reported the need to stir food every once in a while when using this kind of cooker to assure that the food heats evenly.
Cooking with a parabolic cooker is very similar to cooking on one burner of a conventional stove. Since the concentrated sunlight shines directly on the bottom of a pot, the pot heats up and cooks very quickly. The food will burn though. So you have to stir it and watch it carefully.
Do you have to turn the cooker to follow the sun?
Box cookers with one back reflector don't need to be turned unless you are cooking beans which take up to 5 hours. Panel cookers need to be turned more often than box cookers, since they have side reflectors that can shade the pot. Parabolic cookers are the most difficult to keep in focus. These need to be turned every 10 to 30 minutes, depending on the focal length.
Should I take the time to build a box cooker out of "real" materials like plywood or glass or is cardboard good enough?
Unless you need a cooker that can stay outside even in the rain, you'll do just fine with a cardboard cooker. Cardboard is much easier to work with and holds heat just as well. Some people we know have used the same cardboard box cooker for over 10 years.
Would a mirror make a better reflector?
While mirrors are more reflective than simpler materials such as aluminum foil, but the added gain is probably not worth the increased cost and fragility involved with using a mirror.
Does it help to paint the walls black?
Some people prefer to paint the walls black thinking that the oven will get hotter. It seems, however, that the walls will get hotter, but the food won't necessarily get hotter. We prefer to cover the inner walls with aluminum foil to keep the light bouncing until it hits either the dark pot or the dark bottom tray. Since the bottom tray is in contact with the pot, the heat the tray collects will move into the pot easily.
What type of paint should I use?
In developed countries you can buy flat-black spray paint that says "non-toxic when dry" on the label. Otherwise, black tempera paint works, but you have to be careful not to wash it off when you wash the pot. Solar cookers in Uganda report that they use aluminum pots that have been blackened on the outside by fire.
Is glass better than plastic for the window?
People generally report that glass provides about 10% better performance than plastic. And there is reason to believe that under windy conditions, glass is preferred since it doesn't flap in the wind and dissipate heat from the cooker. Plastic, however, is often recommended since is much less fragile and easier to transport and works plenty well. One excellent, easily-obtained plastic film is oven cooking bags. These are for sale in grocery stores and cost less than US$1 per bag. Other plastics will also work. Plexiglas also works well.
What kind of pots work best?
Ideally, you want to use a dark, light-weight, shallow pot that is slightly larger than the food you will cook in it. Metal pans seem to cook best. Hardware stores in the US usually carry dark, speckled, metal pans called Graniteware. Shiny aluminum pots--so common in developing countries--can be painted black or can be blackened in a fire. Cast iron pots will work, but extra solar energy is used to heat up the pot as well as the food, so they will not work in marginal conditions.
What is the best insulation to use?
If you wish, you can insulate the walls of a box cooker with various substances. Fiberglass or Styrofoam is usually not recommended since they give off ill-smelling gases as they heat up. Natural substances such as cotton, wool, feathers, or even crumpled newspapers work well. Many people, however, leave the walls empty of any stuffing, preferring instead to place a piece of foiled cardboard as a baffle inside the wall airspace. This makes a lighter cooker and seems to be adequate. Most of the heat loss in a box cooker is through the glass or plastic, not through the walls. This is why a few percentage points of efficiency here or there in the walls doesn't effect the overall temperature and cooking power that much.
Could I use high-tech materials to make a more efficient solar cooker?
You may find that creating a high-performance cooker using fancy materials will make solar cooking more attractive to people in developed countries. In these countries, cooking only makes up a small percentage of daily energy use, but this is because people in developed countries consume enormous amounts of energy for other purposes (driving, lighting, air conditioning, etc.). Introducing these people to solar cooking is a good way to show them that they can integrate alternative energy into their lives. Solar cooking and drying clothes outside on a line are the simplest, least expensive ways to use solar energy to offset some of this high energy consumption. This will hopefully open them to the possibility of using alternative energy in other ways.
Millions of poor people around the world, however, still cook over a smoky fire everyday. To find wood for the fire, they have to walk many hours everyday. Other poor city dwellers don't have access to wood, so they have to spend up to half of their income on cooking fuel. These people could never afford an oven made of high-tech materials.
So it's up to you to decide which population you want to serve. You could work on creating the most practical solar cooker for people in developed countries to help lead them into a greener future, or you can investigate how to make cookers out of cheap, locally-available materials for people in poor countries who can't afford more.
Can you sterilize water in a solar oven?
Yes. In all three types, water can be brought to a boil. A little-known fact, however, is that to make water safe to drink, it only has to be pasteurized, not sterilized. Pasteurization takes place at 65° C (150° F) in only 20 minutes. This treatment kills all germs that cause disease in humans, but doesn't waste the energy needed to bring the water to a boil. One reason that people are told to boil their water is that thermometers are not readily available in many places and the boiling action serves as the temperature indicator. Dr. Robert Metcalf has written a very informative piece called Recent Advances in Solar Water Pasteurization. You will find other references in the Water Pasteurization page of the Solar Cooking Archive. Dr. Metcalf has also published a large number of photos from his various African water pasteurization projects.
Can you use a solar box cooker for canning?
Yes, but for fruits only! Do not can vegetables or meat in a solar box cooker, since these foods need to be canned under pressure! You'll find information on canning here.
Can you cook pasta in a solar box cooker?
To keep the pasta from getting pasty, use two pans. Heat the dry pasta with oil in one pan; heat the liquid with herbs in another. Fifteen to 20 minutes before eating, combine the two. If you are going to use a sauce, heat that in a third container.
If solar ovens are so good, why isn't everyone using one?
There are many factors at work here. First and foremost, the vast majority of the world's population does not even know that it is possible to cook with the sun. When they find out about it there is almost universal enthusiasm, especially in regions where the gathering of cooking fuel and the process of cooking over a smoky fire is a great burden. There are many factors that need to be in place to make it possible for poor people to solar cook on an on-going basis. The most successful projects have been ones where the need was the greatest, the weather the most favorable, and where the solar cooking promoters have taken a long-range approach to the transition. An example of this is the work by Solar Cookers International in the Kakuma refugee camp in Kenya.
If you build a box cooker out of cardboard, won't it catch fire?
No. Paper burns at 451° F (233° C) and your cooker won't get that hot.
How much of the year can you cook?
In tropical regions and in the southern US you can cook all year depending on the weather. In areas as far north as Canada you can cook whenever it is clear except during the three coldest months of the year. Click the picture to see a map showing the amount of sunlight each part of the world receives.
What foods should I try first in my new Cooker?
A good first food to try is a small quantity of rice, since it is fairly easy to cook and looks very different cooked than it does raw. Chicken or fish is also very easy to cook. See cooking hints or cooking times.
My cooker only gets up to 250° F (121° C). Is this hot enough to cook when recipes call for 350°F (177° C) or even 450° F (232° C)?
A temperature of 250° F (121° C) is hot enough for all kinds of cooking. Remember that water cannot get hotter than 212° F (100° C). Thus if you are cooking food that contains water, it cannot get hotter than this either. Conventional cookbooks call for high temperatures to shorten the cooking time and for browning. Food just takes longer in most solar cookers, but since the sun is shining directly on the lid of the pot, the food browns just about as well as in a conventional oven.
What happens if the sun goes in front of the clouds while I'm cooking?
Your food will continue to cook as long as you have 20 minutes of sun an hour (using a box cooker). It is not recommended that you cook meats unattended when there is a possibility of substantial cloudiness. More information on food safety, go here. If you can be sure that the sky will stay clear though, you can put in any type of food in the morning, face the oven to the south, and the food will be cooked when you get home at the end of the day.
I'm planning to do a science project on solar cooking. What should I study?
If you're planning a science project, Solar Cooker International wants you to know that your research can help extend the world's knowledge of solar cooking and be of great help to people around the world. You should be aware that it's easy to build a high-performance solar cooker if you have access to modern materials. However, the more than a billion poor people in the world, who could really benefit from having a solar cooker, don't have access to such materials. This means that your research will be most useful if it concentrates on the simplification of cooker design or on the use of low-tech, local materials. For more information, see Topics Needing Research.
What resources are available online?
Solar Cookers International sponsors the Solar Cooking Archive on the World Wide Web at http://solarcooking.org where you will find illustrated construction plans, photographs, documents, and an international directory of solar cooking promoters. Their thrice-yearly newsletter, the Solar Cooker Review, is also available there. An excellent document for further reading is http://solarcooking.org/kerr.htm The Expanding World of Solar Box Cooking], by Barbara Kerr. You'll find a number of audio programs that you can listen to online here. Don't forget to read about eye safety here.
You can also participate in our online forums here.