Baking a chocolate cake in the solar oven
I finally wrote up instructions on how to build a solar oven!
It’s the most magical toy! I love cooking things off the grid, using only the power of the sun. And solar ovens are fantastic for summer cooking when you want to cook (or bake!) without heating up your kitchen.
This design produces a powerful solar cooker — 350° or more. It can be made for only a few dollars, using ordinary household materials and tools, and is also a great project for older children.
Although my solar cooker is based on Joe Radabaugh’s original “Heaven’s Flame” (a.k.a. “SunStar”) design, I’ve simplified and made some modifications, which are reflected in my instructions below.
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Materials:
– Small Box. This is the inner part of the oven, where you put the food. Ideally square in shape, and measuring about 9-12” wide and deep. (Square makes a more powerful oven, but rectangular would work.)
– Large Box. This is the outer box, and needs to be 2-3” larger (or more) in all directions than the Small Box.
– Insulation cardboard. Gather lots of boxes of any size, since you’ll be cutting them up to stuff in between the Small Box and Large Box. Try the liquor store or grocery store for boxes.
– Cardboard for Collectors (4 pieces). Find four large, flat pieces of regular (not double strength) cardboard. Each piece should measure about 2’ x 3’. Appliance stores or bike shops usually have big boxes you can cut up.
– White Elmer’s Glue. (1 part glue to 2 parts water so that it’s more easily spreadable)
– Aluminum Foil, one roll. Any type will work, but an extra-wide roll of heavy duty foil is ideal.
– Glass. About ½” larger than both the length & width of the Small Box. Double strength glass will insulate better than single strength. It’ll only cost a couple dollars at a hardware store, and they can cut to size. Be sure to sand down the sharp edges of the glass with sandpaper or a rock.
– Duct tape
– 5 Pipe cleaners, or some twine
Making the Oven:
1. With a mat knife, begin cutting up your insulation boxes to fit into the bottom of the Large Box. Build up the cardboard layers so that when you place the Small Box inside, its top edge rests one inch below the top edge of the Large Box.
2. If your Large Box still has its flaps attached, leave two opposite flaps sticking out, and fold in the other two so that they’re inside the box. For the Small Box, bend all 4 flaps all the way back and tuck in between the Small and Large Box, or else just cut them off altogether. If you cut them off, take care to leave a smooth edge around the top rim.
3. Cut up the rest of the insulation boxes and stuff them into the space between the side walls of the Small Box and Large Box. Try not to leave big gaps in the insulation, and use enough insulation so that the Small Box is wedged tightly inside.
Solar Oven Cross Section
4. When your solar oven is in use, it’ll be tipped toward the sun. Therefore, you’ll need to leave a piece of insulation cardboard sticking up a bit to keep the glass in position. If you have a rectangular oven, it should be tipped on its wider side for stability. Therefore, the cardboard should stick up on one of the two wider sides.
Also, when you rest the glass on top of the Small Box, be sure there are no gaps between the box rim and the glass where hot air can escape.
5. Line the inside of the Small Box with aluminum foil, shiny side out. Glue it down if you want. (10/3/2012, Edited to add: Instructions for many solar ovens will tell you to paint the inside of your oven black. In fact, underneath the foil in my own oven is black paint! I use foil because I discovered that it makes for a hotter oven — hotter by about 25° to 50°. This is actually fairly significant especially if you’re baking things in your oven. Although that’s reason enough for me, another advantage to foil is that the black paint will off-gas when it’s heated in direct sun. Even after 8 years, when I took the foil off to re-test my conclusion, I could smell the black paint. And also, foil is more likely to be found in a typical household cupboard than black paint is.)
Making the Collectors:
6. On your four flat pieces of cardboard, draw the collectors according to the pattern below. Note that if your Small Box is rectangular, your collectors will be two different sizes (based on the length & width of your glass), and if it’s square, the collectors will all be the same size. The 67 degree angle can be found by using a protractor, or by folding paper as shown in the second diagram (don’t worry about the 67.5 degrees — it’s close enough to 67 degrees, and pinpoint accuracy is not crucial here).
7. Cut out all four collectors with your mat knife. With a blunt-pointed tool, draw a crease along the dotted lines, and then fold the cardboard along the crease lines.
8. On three of the four collectors, bend the top and bottom flaps all the way over and tape them down with duct tape. On the fourth collector (which should be one of the wide collectors if your oven is rectangular), bend and tape the top flap, but don’t bend or tape the bottom flap because you’ll be attaching the Slip-In Piece to that bottom flap later.
9. Now, flip all four collectors over so that the taped flaps are underneath. You will now cover the smooth surface of your collectors with aluminum foil. With the shiny side up, roll the foil out over the collectors and cut so that it almost reaches the creased edges of the cardboard. (Don’t cover the side flaps with foil.) Thinly spread some of the Elmer’s Glue mixture onto the collectors and lay the foil in place (again, shiny side up), smoothing it outward with a clean cloth to minimize wrinkles.
Making the Slip-In Piece
10. The Slip-In Piece is a piece of cardboard that attaches to the bottom flap from Step 8 that you didn’t tape down. It slips in amongst the pieces of insulation cardboard, allowing easy attachment of the collectors to the solar oven base. To make the Slip-In Piece, cut a piece of cardboard that’s roughly equal in dimension to (or a little smaller than) the height and width of your Small Box. Punch two sets of two small holes along the narrow end of the Slip-In Piece (as in the diagram on the right), and then punch corresponding holes into the bottom flap of your collector. Attach them with one of the pipe cleaners which has been cut in half (or, use twine).
Connecting the Collectors
11. Punch three small, evenly-spaced holes into the side flaps of each collector, as in the diagram on the right. Place the holes in the exact same spot on all of the collectors’ side flaps so that they’ll line up when you’re ready to connect the collectors. And try to punch the holes as close to the crease in the cardboard as you can.
12. If you’re using pipe cleaners, cut four pipe cleaners into three pieces each. If you’re using twine, cut twelve 4-inch-length pieces. You’ll attach the collectors so that the foil-covered surfaces are facing each other, as in the diagram below. Thread the pipe cleaners or twine through the holes in the side flaps, and tie tightly.
If possible, get a cat to inspect your handiwork.
Setting Up & Cooking In Your Solar Oven
Tipped toward the sun, resting on the edge of the raised garden bed.
13. Take your solar oven to a spot in your yard that receives unobstructed sunshine. Attach the collectors to the oven by sliding the Slip-In Piece in between pieces of cardboard insulation at the top of the oven. Tilt your solar oven so that it’s pointed at the sun, and support it using bricks, rocks, overturned clay pots, or other sturdy things. Wearing sunglasses, fine-tune your oven’s position by observing the shadows inside the Small Box. Place your food inside the Small Box and set the glass into place. Again, there shouldn’t be any gaps between the glass and the top rim of the Small Box, and the glass should be supported by the piece of cardboard insulation that you left sticking up in Step 4.
– For best results, you’ll want to reposition your oven approx. every 30 minutes to keep it aligned with the sun. However, you can also cook while you’re away by pointing the oven toward where the sun will be at mid-day; you’ll then return home to hot food!
– My oven reaches a maximum of 350°. If I used double strength glass, it would probably be higher. For a lower temperature, keep the oven slightly misaligned with the sun.
– Your solar oven can be used to cook anything: rice, beans, grains, vegetables, meat, eggs, bread, cakes, cookies, pies, fruit cobbler, etc. I’ve noticed, though, that food doesn’t tend to brown in the same way that it would in a normal oven, so it may not look done when it actually is.
– I do most of my cooking in a large, wide-mouth glass jar with the lid screwed on very loosely. You can also cook (or bake!) in normal pans if you rig up a flat cooking rack.
– Don’t put anything into your solar oven that you wouldn’t put into your regular oven (plastics, etc.). Use an oven mitt or tea towel to lift off the glass — it gets very hot! Also, don’t forget to wear sunglasses when working around your solar oven (try without and you’ll see why!).
– It’s fun to keep an oven thermometer inside your solar oven to gauge the temperature.
– On a windy day, poke holes in both flaps that were left sticking out of the Large Box, and then poke some holes into the collectors. Tie the collectors to the Large Box flaps with twine.
– After using your oven a few times, the insulation cardboard might shrink a little. Add more cardboard so that it’s packed snugly.
– To keep your food warm after cooking, cover your oven with its collector panels.
Cooking brown rice
Solar Baking: Herbed Eggs & Apple-Blueberry Crisp
Thank you for very clear directions for this oven. It is exactly what I was wishing for.
Really brilliant idea. I don’t know that we have any place around here that gets consistent, direct sunlight (it’s generally blocked out by the buildings, trees, and a railroad bridge), but I would love to try this. Maybe on the roof?
Ryan,
I know. Our apartment is definitely not solar-oven friendly so I use it at my parents’ house. If you have access to your roof, that seems like the perfect solution!
Built our second solar cooker yesterday to your design, got our cat to inspect it and can’t wait to try it out. Our first cooker had chili boil over in it and had to be thrown out.
Darick,
Excellent! Hope it cooks up a storm for you. And glad you got your cat to sign off on it.
I like the idea, I work at a silver mine and we have to dry about 100 mineral samples a day. We use a gas oven which sucks the gas and is not big enough for all of the samples. A new oven costs $45,000. I think I am going to make a long narrow oven (about 60 feet long) and bake the samples first in the sun and then use the gas oven for the last 20% of the drying.
Shaun,
What an awesome idea! Having a solar oven really drives home the power of the sun! They get HOT!!! Such a long oven will be quite the project, but worth it if it saves $45K! If you build it, I’d love to know how it goes!
Beautiful! I’m off to get the cardboard! Just the thing for the Solar Cook-off! And we have an A#1 Feline Project Inspector just laying around with nothing to do.. thanks amillion!
Hi Leon! A solar cook-off?! FUN!!!! Let me know how it goes!
And yes, I’ve noticed that most Felines tend to be very good project inspectors! 😀
Does the cat scan insure that the food is OK as it does in the medical field? 😉 I have a dog, so I get a lab report… Great instruct-able.
Hi Woody, thanks for your comment. A lab report!! How cute. 😀 I think your dog would have lots of fun reporting on the food that you cook out in the backyard!
Thanks so much!! Very clear instruction and great for my solar oven project! I better get an A
How long does it take to heat up?
Hi Jack,
Hmm, I haven’t timed it. Depends if you have food in it. If you do, it’ll heat up more slowly. If you don’t have food in it, it doesn’t take long; 15 or 20 minutes to maximum temp maybe?
What is the purpose of the glass above the small box? Can I do without it?
Hi Monica,
The glass is to provide further heat retention within the oven. You can do without it but the oven won’t be nearly as hot.
What a great project! I can’t wait to try it. I’ve made smaller solar cookers before, but they just never seemed to get hot enough. I think the larger solar collectors on yours will really do the trick. And I love your step-by-step instructions. I do have one question about the design: in other solar cooker designs I’ve seen, they always paint the inside of the cooker black (they say this converts the sunlight to heat). Have you ever tried it this way, and does it make a difference?
Hi Desiree! Oh goody, I hope you do try this cooker. I use it daily! Hah, funny you mentioned the black. Underneath my foil is, in fact, black paint! I did originally do black, and then decided to try foil. For me, the foil actually seemed to make for a hotter cooker. My foil is coming off at the moment, so I’m gonna leave it off for a while and see if I still come to that same conclusion 🙂
Thanks for asking!!!
Blessings,
Lindsey
(See edit to Step #5 for follow-up)
Awesome! I’ll definitely try it!
But Lindsey, am I supposed to attach all the four collectors together? Can I unattach it after I’m done cooking?
Shall I use a slip in piece for all the collector panels?
How am I supposed to cover the oven with the collector panels?
And, and, what’s the point of the top flap in the collector panels if it’s going to be folded over anyways?
Thank you, you awesome woman you! You go girl!! I am going to make an oven, give it a whirl, and then get a whole group of neighbors together for a solar oven making day:) I’m wondering how it cooks when it is only 20 degrees outside? We have lots of sun, but gets real cold. Do you have any thoughts?
Tessa,
Hehe! I totally loved your comment!! The thing cooks great even in cold weather. It’s more the sun’s angle and the clarity of the day (haziness, or lack of) than the weather that influences the temperature. Although the outside temp does play a small role, it’s a small one. 🙂 HAVE FUN!!!!!!!!!! I am in love with my solar oven. The darn thing is just the coolest toy ever.
Hey thanks fOr such a detailed explaination im gonna use a version of this in my schools solar cook off and ill tell you how it does after
Do you by any chance have an idea on how this solar energy produced by the oven could be converted to electricity
Addo – Nope, I have no idea. Someone else would though, I’m sure. It would be a great thing, wouldn’t it….
I LOVE IT! SOOOO COOL! I WANNA MAKE MY OWN. DOES IT NEED GLASS TO WORK? HOW LONG DOES IT TAKE TO COOK RICE? DID IT TAKE LONG TO BUILD? CAN I MAKE MY OWN EASILY? DO YOU RECOMMEND IT? CAN I BOIL WATER ON A SLIGHTLY CLOUDY DAY? DOES IT NEED TO BE HOT OUTSIDE TO USE? CAN I KEEP IT INSIDE IN A SUNNY SPOT? WILL IT BURN FOOD?
Narwhals,
The solar oven is the coolest thing you will ever make because it is so versatile and useful! I love mine.
It does need glass to keep the heat in so that stuff will cook. (Think greenhouse effect/hot car on a sunny day.) I’ve done brown rice in it. Once the water boils (or cheat a little and use hot water off the stove — I often do this because I’ve made tea anyway and have the hot water to spare), it’ll probably cook at about the same rate as stovetop, perhaps a little longer.
Didn’t take TOO long to build, no, half a day maybe? I don’t recall.
You can definitely build your own easily. The materials are cheap or free, and readily available.
Yes I totally recommend it!
Boiling water on a slightly cloudy day will take a while. Depends how much water, too. Having a slightly cloudy day does impact cooking time and temperature.
Best cooking is done in Summertime, when sun’s at it’s highest angle in the sky.
It doesn’t need to be hot outside to cook; I cook in the winter all the time. Tricky part is the low angle of the sun makes cooking time longer and the oven doesn’t get quiet as hot. But if you can find a nice good sunny winter cooking spot, unobstructed by trees, go for it. You will have success! Outside temperature impacts in-oven temperature much less than you’d expect.
Using it inside the house is my next experiment. I’m sure you can. The quality of the sun is what matters. Clean windows would help too.
Food doesn’t burn per se, but it can overly dried out and turn brown and have a not-so-pleasant taste.
Build one! You’ll be glad you did!
I love this oven. its hot. I cant wait to make my own and eat some wonderful tasting cookies. can anyone give some more info on how to build it?
Hi! My son wanted to make a solar oven as an experiment, so we are going to try this one!! I’m going to share this on my business page facebook.com/peonyandpoppy thank you!! http://www.peonyandpoppy.com
Hi! My son wanted to make a solar oven as an experiment, so we are going to try this one!! I’m going to share this on my business page facebook.com/peonyandpoppy thank you!!
I hope this works well because I am going to use it as a scenice exsprement I hope it gos well thanks a bunch
what will be its temperature and is it able to cook any kind of food and what will be its time duration???
Hassan, it all depends. There’s no cut and dried easy answer to this — it depends on how well insulated your oven is, and what time of year, how clear the air is (basically how much is, or isn’t, in the way of the sun rays). Things take longer to cook in a solar oven, sometimes much longer. Again, it depends on the factors above.
Is it 350 deegres celsius or farenheith?
Nacho, it’s 350 degrees Fahrenheit!
Will Plexiglass work? It would be safer than glass I would think. Any thoughts? What do you think about Styrofoam packing material? There is the peanut style and the flat strips. I would like to reuse the Styrofoam as opposed to putting it in a land fill!
Dawn these are great ideas. I would think as long as the plexi didn’t melt or discolor or off-gas it would work just fine. I don’t know plexiglass’s properties at all but no harm in just trying it out.
I’ve been meaning to try straw as insulation instead of cardboard, but I like your styrofoam packing peanuts or flat sheets idea even better. The flat sheets would be easier to use, though packing peanuts contained in HDPE plastic trashbags would probably be nearly as easy. Do report back here if you try it.
It’s awesome
Thanks for the post, it is exactly what we are looking for. We are making a solar oven for a class project and we live in a very sunny, hot and humid place (the amazon jungle) and would love if this works to cook a plate that is favorite among the population here. It is a curious technique: fish or chicken and yucca wrapped in banana leaves. It is delicious but people use carbon to make it.
I do have a couple of questions: does it matter if we use wood as opposed to cardboard for the boxes? I would rather make something that last a little longer. Does the lid have to be made of glass or can it be made of metal?
Thanks again for the instructions. I am sure it is going to be a fun project.
Jorge,
Thanks for your comment and questions!
Answers: Wood can be used for the solar oven but it will make it much heavier, so as not to be as portable as cardboard. I would still suggest insulating with cardboard, even if you use wood for the outer box. The lid does have to be glass; the greenhouse effect of the glass is what traps the solar energy that is funneled into the small box via the collectors, and therefore cooks the meal. So yeah, glass is a must. Good luck – I absolutely love my solar oven and it’s a fun project.
Hi Lindsey, years ago I made a big box style solar oven. It had a big glass lid and the whole thing was too big and heavy. But I cooked great meals all the time. My insturctions that I used were just lots of balled up newspaper as the insulation. It worked great and was soooo easy. I just packed it in…..
You can always just add that one piece that has to stick up for the glass I guess….
What a great idea about the balled-up newspaper Sherryruby. Thank you for adding this tip! Seems like that would be a lot lighter than cardboard. ? Maybe?
Is this technique more effective than the solar box oven? I’m doing a project and it’s a huge competition to see which group has the best one, would this be the most effective?
Can it heat water another 3 degrees celsius in half an hour? Would it heat faster if the installation was painted black? Would the cardboard installation work better than the newspaper?
this is for a school competition in Georgia
Hi Maggie,
I’m not sure what the solar box oven is. Also not sure what you mean by heating another 3 deg. celsius in half an hour… but I have tested the inner box painted black vs. lined with foil. The foil worked better.
I haven’t tried newspaper insulation but cardboard, on principle, would work better as it has more air space (air is a good insulator) and also it would be lighter than newspaper. Unless you’re using crumpled-up newspaper… in that case, I’m not sure which one would insulate better. Try it and see; good luck!
All i have to say is awesome OMG