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Box Cooker

4 min read

A box cooker is an insulated box with a black floor, a glass top and a reflective lid that props open to bounce extra light in, and it reaches 150-180°C, enough to bake, roast and cook stews. It is steadier than a panel cooker because the glass and insulation hold the heat, but its net power is about 60 W, so 2 kg of water needs about 3 hours to reach 100°C: put the food in early and leave it.

Part of Solar Cooking & Pasteurization

A box cooker is a small insulated, glazed oven that uses sunlight alone, a collector and a cooking chamber in one.

How Much Heat Reaches The Pot?

Inputs, assumed: the inner box is 40 × 40 cm, so the glass top is 0.16 m²; the reflector lid raises the effective collecting area to about 0.25 m²; direct sun at 1,000 W/m²; about 25% of the incident power reaches the pot.

  1. Incident power = 0.25 × 1,000 = 250 W.
  2. Net power into the pot = 250 × 0.25 = 62 W, about 60 W.
  3. Heat to bring 2 kg of water from 20°C to 100°C = 2 × 4.18 × 80 = 669 kJ.
  4. Time = 669,000 J ÷ 60 W = 11,150 s = 3.1 hours.

This agrees with the parent page’s advice to start the meal 2-3 hours early. The box’s advantage is not speed but holding temperature: with a stove, the pot stops cooking when the fire dies; in a box it stays near simmer as long as the sun stays on it. Re-aim the box every 30-45 minutes.

What You Need

  • Two nested boxes — outer 60 × 60 cm, inner 40 × 40 cm, with insulation between (parent page). Dry insulation that tolerates heat: crumpled paper and straw can scorch at the box’s hotter end, so keep it clear of the floor
  • Reflective lining for the inner box
  • A black floor or a black metal tray
  • A glass or clear plastic top, clamped or hinged
  • A hinged lid with reflective inner face and a prop
  • Black pots with lids
  • Dark glasses

Method: Build And Cook

Step 1: Build the box with insulation between the outer and inner shells, a snug fit for the glass, and no gaps.

Step 2: Line the inside with foil; paint the floor black.

Step 3: Set the glass top. Add the reflector lid and a prop so it throws light into the box (parent page).

Step 4: Place black pots with lids, close the glass, and aim the box so the lid’s shadow is as small as possible.

Step 5: Open the box as little as possible. Each opening releases the stored heat.

Common Mistakes

  • Leaving gaps around the glass. Heat escapes; the box stays lukewarm.
  • Cooking with the lid closed in shade. A cloud takes the heat away within minutes.
  • Assuming 150-180°C means fast. The temperature is the ceiling; power sets the time.

Safety

Glare, hot glass and food held lukewarm

The lid throws bright light at the cook; wear dark glasses, and never leave the lid aimed where passers-by or animals can be dazzled. The box and glass can burn on contact, and a lid opened at the end releases steam. Food left in a box that cools slowly spends hours between 4°C and 60°C where bacteria multiply: bring it up to temperature in the sun, and eat it or cool it promptly (Minimum Safe Temperatures). The parent page mentions sterilizing; do not rely on a cooker to sterilize anything unless you have measured the temperature and time.

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