Build and use a solar oven

Solar cooking has a reputation in Britain as something that works in Arizona and not here. That reputation is wrong, and the real limitation is different and more interesting than "not enough sun".

The honest British assessment

A well-built box cooker in the UK reaches oven temperatures of around 250°C on a good day, which is hotter than most domestic ovens ever run. Practitioners here achieve this routinely.

More specifically:

• It works through the British summer, on clear days and often on lightly clouded ones if the insulation is good.
• It works from an ambient temperature of 0°C, given direct sun. Air temperature matters far less than cloud does. A crisp bright February day can cook; a warm overcast July day cannot.
• Britain gets roughly 150 sunny days a year. That is the real constraint, and it is a constraint on reliability rather than on capability.
• Box cookers do not work through the colder months, mainly because the sun is too low and the days too short. Evacuated tube designs do, and will cook with snow on the ground provided the sun is out.

So the correct summary is not "solar cooking does not work in Britain". It is "solar cooking works well in Britain about 40% of days, mostly between April and September, and you cannot choose which days."

The seasonal mismatch

This is the part worth thinking about before building anything.

Solar cooking is at its best in June, July and August. Fuel is at its scarcest and most valuable in December, January and February. The method peaks exactly when you need it least and fails exactly when you need it most.

That does not make it useless. It makes it a specific tool for a specific job:

• It saves your fuel stocks during the months you would otherwise burn them anyway, leaving more for winter. In a long emergency this is the real argument, and it is a good one: every summer meal cooked on sunlight is wood still in the pile in January.
• It cooks without smoke, flame or fuel, which matters for concealment, for fire risk, and for not signalling that your household has resources.
• It cannot be left burning, so it needs no watching and cannot start a fire.
• It pairs perfectly with a haybox: solar oven to bring it up to temperature, haybox to finish it after the sun goes off. That combination cooks a full meal on no fuel at all.

Do not plan winter cooking around it. Plan a rocket stove and a wood pile for winter, and use the solar oven from spring to autumn to keep that pile intact.

The three designs

The box cooker is the one worth building. A panel cooker is a good afternoon's experiment to prove the principle to yourself, and to the children, before committing to the larger build.

Building a box cooker

The principle: sunlight enters through glazing, is absorbed by black surfaces inside, becomes heat, and cannot easily escape because the box is insulated and the glazing traps infrared. Reflectors bounce extra light in.

Materials: two cardboard boxes (one fitting inside the other with a 5 cm gap all round), aluminium foil, black paint or black paper, a sheet of glass or a double layer of clear plastic, insulation (newspaper, wool, straw), tape and glue.

1. Nest the boxes with roughly 5 cm of gap on all sides and underneath. Pack that gap with insulation. Crumpled newspaper works; wool is better.
2. Line the inner box with black. Matt black paint, black paper or black card on the bottom and the lower part of the sides. Black absorbs; shiny reflects. This surface is what converts light to heat.
3. Line the upper inner walls with foil, shiny side in, to bounce stray light down onto the black base.
4. Make a lid with glazing. A sheet of glass, an old picture frame, a double-glazed unit if you can find one, or two layers of clear plastic with an air gap. Two layers beat one substantially, because the trapped air is insulation.
5. Seal the lid so it sits tight. Heat escapes through gaps faster than through any material.
6. Add reflectors. Foil-covered card flaps around the opening, angled to bounce extra sunlight in. Four flaps roughly double the light entering. This is the single cheapest performance improvement.
7. Add a trivet or rack so the pot sits slightly off the base, letting hot air circulate underneath.

Pots matter more than people expect. Use thin, dark, matt cookware. A black-painted thin steel pot with a lid absorbs heat far better than a shiny thick-based pan, which reflects it away. Enamel roasting tins are excellent. Putting the pot inside a clear oven bag adds a further insulating layer and gives a noticeable temperature gain.

Using it

1. Start early. Solar cooking wants the hours either side of solar noon, roughly 11am to 3pm in British summer time. Food that goes in at 4pm will not cook.
2. Preheat the empty oven for 20 to 30 minutes while you prepare the food.
3. Aim it at the sun, and re-aim every hour or two. A box cooker is forgiving; a parabolic is not.
4. Do not open it. Every opening costs 20 minutes of recovery. There is no browning to check on and nothing needs stirring.
5. Allow roughly twice the normal cooking time, and know that it is very hard to burn anything.
6. If cloud comes over, close it up and move it into the haybox, or finish it on a stove.

What it cooks well: stews, casseroles, rice, beans, root vegetables, bread, cakes, baked potatoes, eggs, and pasteurising water. It bakes bread genuinely well.

What it does not do: anything needing high heat quickly, frying, searing, or anything requiring a precise temperature. It is an oven and a slow one.

Water pasteurisation is a use worth knowing separately: a solar oven will hold water above 65°C for long enough to make it microbiologically safe, which is a fuel-free way to treat drinking water on a sunny day. It will not remove chemicals or algal toxins. See judging raw water.

Food safety

Vegetables, grains, beans and bread are far more forgiving and are where a solar oven earns its keep. Save the meat for the fire.