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Part VIII: Wind, Water, and BiofuelsChapter 13. Biofuels, Wood, and Solar Cooking

Build a Box Solar Cooker

An insulated box with a glass lid and a reflector that cooks on a clear day.

Difficulty
Beginner
Cost
$20–60
published

A box cooker is an insulated oven with a glass lid and a reflector that aims extra sun through that lid. On a clear, calm day it can cook food in a dark pot. On a cloudy or windy day it can leave the same food sitting in a warm box, which is the dangerous outcome. The thermometer in the food decides.

This is a cooker, not a canner. Pressure canning and boiling-water canning belong to a tested process from the National Center for Home Food Preservation, not to a cardboard box. Their site is nchfp.uga.edu. Use it for what "done" means for a given food. Cooking write-ups in the links below are other people's. This box is a generic one.

Difficulty

Level
Beginner
Time
An afternoon to build, and a clear day to learn its pace
Skills
Using a food thermometer and quitting when the clouds win

Cost estimate

$20–60

Cardboard or a scrap wood box, foil, and insulation you already have can land in the range if you also already have a black pot and a tempered glass lid. Buying the glass and the pot can pass the range. Do not save money by using a pane of ordinary window glass overhead.

Materials

ItemQtyNotes
A rigid box you can insulate1Cardboard or wood. Line it so insulation cannot touch the pot or the food. Do not use foam that will melt onto dinner.
A tempered glass lid1Large enough to see the pot, tight enough to hold heat, arranged so it cannot fall onto anyone. Annealed window glass is the wrong pane.
A foil-covered reflector1A hinged lid of cardboard. Aim it into the window. It is a mirror. Do not stare into it.
A dark pot with a lid, and a food thermometer1Smaller pots heat faster. The thermometer goes in the food, not only in the air of the box.

Build the box

Keep the window on the sun side and the walls thick. Several layers of corrugated cardboard, or a wood box with dry crumpled paper or cotton held behind a clean inner liner, is enough insulation for a first cooker. The liner should be something you can wipe, dark if you can make it dark without a paint that will reek when hot. A rack on the floor of the box lets air move under the pot. A pot sitting on cold cardboard loses heat downward.

The glass sits in a lid you can lift without dumping the pane. Two layers with a small air gap lose less heat than one, and one layer of tempered glass is still a cooker, just a leakier one. Tape and wood stops should make it tedious for the glass to slide out. The reflector hinges above or in front of the glass and is covered with aluminum foil glued flat enough to bounce light in, not to focus a tight spot on a single point of the pot.

Set the box on a level place out of the wind, on ground that is not dry grass. Turn it every hour or so as the sun moves, or accept that the afternoon will be weaker. Wind strips heat off the glass. A clear plastic bag around the pot is a trick some people add. It is also a melt and a food-contact question. Skip it until the plain box has worked. Do not cook in the box and also store raw meat above the pot "for later." One food, one pot, a lid on the pot.

Insulated box, glass, reflectorA box with a glass lid, a tilted reflector, and a dark pot with a thermometer.PotThermometer in the food. Clouds are a reason to finish on a stove.
An insulated box, a glass lid, a reflector, and a dark pot. The thermometer is in the food.

When it counts as cooking

Perishable food left warm for hours can grow bacteria even if it looked hot once. If clouds arrive and the food stalls, finish it on a stove. Do not "let it ride" through a dim afternoon and call it solar. Leftovers get the same rule: heat them hot, or keep them cold, rather than warming them gently all day. Eggs, meat, dairy, rice, and beans are the foods that punish a slow box. A pot of water you only wanted warm for washing is a kinder first test, and even that water is not sterile because it sat in the sun.

The box can get hot enough to burn skin and to shatter a cold splash on hot glass. Use dry towels. Set the pot down before you admire it. Children and pets are not the temperature control.

Other box designs exist in the further-reading list. They are not this one. If you build from those pages, you are following those writers, including their food-safety choices. Keep the thermometer habit either way.

  1. Insulate a box and glaze it with tempered glass

    Liner between the insulation and the food. A rack under the pot. Glass that cannot fall out.

  2. Add one reflector and a dark lidded pot

    Foil aimed into the window. Small pot, lid on. Thermometer in the food.

  3. Cook on a clear, calm day and aim it

    Level ground, no dry grass under the box. Turn it as the sun moves. Wind is a reason to wait.

  4. Finish on a stove if the food stalls

    Warm and hopeful is not done. Canning is a different process from the food-preservation center, not from this box.

Further reading

These links go to outside sources. This book cites them by link and does not copy their words or pictures.

General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.