LOW←TECH MAGAZINE English

Winter is Coming: Build a Solar Powered Foot Stove

Sun, 16 Nov 2025 00:00:00 +0000

Image: The electric foot stove that we build in this manual. Photo by Marie Verdeil.

We built an electric cube heater, powered by a 100-watt solar PV panel. During the day, the solar panel slowly heats the cube, which radiates heat to its surroundings. Due to its high thermal mass, the object continues to radiate heat for hours after sunset.

Electric Foot Stove

The heat cube can serve multiple purposes. You can use it as a modern variant of a preindustrial foot stove. Put your feet on the cube and throw a blanket over your lap to trap the heat.

Historically, foot stoves contained glowing sintels from the fireplace, but an electric version is safer and healthier. There is no risk for carbon monoxide poisoning or fire. The heat cube contains no flammable materials.

Image: A nineteenth century foot stove. Source: Museum Rotterdam (CC BY-SA 3.0).

You can also put the cube under a table that has a blanket on top, and that is another method to build an electrically heated table. The heat cube can also quickly dry a pair of socks or keep a prepared dish warm in the kitchen or on the table.

Rather than storing electricity from a solar panel in a battery to operate an electric heater at night, the solar panel stores heat in the thermal mass of the heat cube itself. That’s cheaper and more sustainable, because batteries account for 70-90% of the money and energy invested in an off-grid solar PV system.

Heat can be stored for even later in the night by covering the cube with one or more wool blankets.

Image: The heat cube used as a hand and foot warmer. Photo by Marie Verdeil.

Bricks, Mortar, and Tiles

We created the heat cube using inexpensive and simple materials: bricks, mortar, and tiles. The solar panel supplies power to an electric resistance heater, which we made ourselves and which we laid between several layers of bricks. The electric resistance heater connects directly to the solar panel, without a solar charge controller or voltage regulator in between. If you add more solar panels, the heat cube also works in cloudy weather.

Electric Tile Stove

Our heat cube is relatively small (20x20x25cm), but this manual can serve to build a much larger version, which could take the form of an electric tile stove, which could include a heated bench or sleeping platform.

Tile stoves date back to the Middle Ages and accumulate heat from a biomass fire within a stone or brick mass. They are fired only once or twice per day and continue to radiate heat for approximately 12 to 24 hours. However, a tile stove can also work electrically. In that case, there is no need to add a labyrinth of smoke channels that delays the release of heat through the chimney; instead, only layers of bricks with electric heating elements in between are required. Neither do you need a chimney.

Image: A 1920 tile stove in the "Tautes Heim". Source: www.tautshome.com (CC BY 3.0).

Tile stoves are very heavy, and powering them electrically doesn’t make that any better. Once built, they remain in place. Even our small cube heater is not precisely a portable device: it weighs 26 kg. If you want to move it around, it’s a good idea to put it on a wooden board with wheels mounted underneath. Make sure it can’t slide down a slope.

Solar Oven vs. Foot Stove

This solar-powered cube heater forms part of a larger collection of devices that we built, including a solar powered oven and a solar powered coffee maker. They are all based on direct solar power and heat storage rather than electric energy storage.

All these devices are inspired by tile stoves. However, while the solar oven and coffee maker are designed to keep the heat inside, the cube heater is designed to radiate heat outwards. Therefore, it is the only appliance that has no thermal insulation. That also makes it easier, quicker, and cheaper to build than the others.

Thermostat

The heat cube we built works without a thermostat. Because it has no thermal insulation, there is little danger of overheating and damage to the electric resistance circuit. It’s the room that gets warmer: the cube continues to radiate heat into its environment and maintains a steady temperature. Furthermore, the sun goes down every evening, cutting off the power supply.

However, overheating could occur when you charge the heater with one or more blankets on top, effectively adding insulation, or when you operate it on grid power using an AC/DC converter. To avoid this, you can add a thermostat, which turns off the power source when the surface temperature exceeds a preset limit.

A thermostat can also be helpful when the heat cube surface becomes too hot to touch with bare skin. However, you can also wrap a blanket around it or make a fitting cover, similar to the ones used for hot water bottles. Installing an on/off button provides you with a manually operated thermostat.

What You Need

Six bricks. We used refractory bricks (also known as fire bricks), which are used in high-temperature environments. Refractory bricks are made of silica and store thermal energy very well. However, regular bricks work as well, because they withstand temperatures much higher than those reached in the cube heater. Just make sure the bricks have a 1:2 size ratio, allowing you to create a pile with alternating brick layers.
Mortar. We used regular construction mortar, which can withstand temperatures up to 300°C. Refractory cement combined with sand and water could also work. However, it’s not a requirement because the heater doesn’t reach a temperature of 300°C.
Electric resistance heating element. We made a 100-watt electric resistance heater, which we fixed in the mortar between the bricks. It’s made from nichrome wire and heat-resistant electric cables. See our separate manual.
Tiles. We used ceramic tiles to cover the sides and the top of the cube, and a thicker floor tile at the base. The tiles radiate heat, waterproof the structure, and help to keep it together.
Adhesive mortar. To paste the tiles to the bricks.
A fuse. You add this in the positive wire between the solar panel and the heat cube. Its Amps rating should be slightly higher than that of the heating element. Read more about fuses.
A thermostat (optional). In contrast to the other devices in this collection, the heat cube does not have a thermal switch and thermal fuse. You could add a thermostat to regulate the temperature of the heat cube (see our manual on building an electrically heated table).
An on/off switch (optional). Add this when you don’t have a thermostat.

Image: Step by step instructions to build the heat cube. Illustration by Marie Verdeil.

Building steps

Cut a large and thick floor tile into a square. Its dimensions should be bigger than two bricks lying side by side. We have more information about cutting tiles in the solar oven manual.
Prepare the heat-resistant circuit of 100W. Since we have three layers of bricks, we prepare two separate strands of nichrome wire to evenly spread the heat. One strand goes between the first and second layers of bricks, and the other between the second and third layers. Solder both strands to heat-resistant electric cables. Next, connect these heat-resistant electric cables in parallel. To decide the length and the thickness of the nichrome wire, and other building steps, consult our separate manual.
Prepare some construction mortar following the package instructions. Apply some mortar to the bottom tile and place your first brick on top of it. Add some mortar on the inner side of the second brick before pressing it next to the first brick.
Add a generous amount of mortar on top of this first level and place the first part of the resistance circuit, ensuring the nichrome wires are spread evenly, don’t cross, and remain in the center. Take care that the nichrome wire doesn’t come closer than 5 cm to the sides of the cube, otherwise the surface becomes too hot.
Cover with more mortar and layer a second level of two bricks, perpendicular to the first one.
Place some mortar on top of the second level before laying down the second part of the circuit. Make sure the ends of both circuits are on the same side of the cube and that the heat-resistant cable ends extend at least 5 cm beyond the cube. We will connect them in parallel later.
Place the last layer of bricks, perpendicular to the second level. Use the leftover mortar to fill the holes on the edges of the bricks.
Wait 24 hours for the mortar to cure.
Solder the two separate circuits together in parallel, ensuring there are a few centimeters of space left to extend the circuit with regular cables later.
Prepare some adhesive mortar and cut the tiles for the five sides of the heating cube (except the bottom).
Drill two holes for the cables through the tile on the side where they stick out.
Apply generous amounts of adhesive mortar on all sides and cover the surface. Let it dry according to the mixing instructions.
Once dry, apply grout in between the tiles’ edges.
Connect regular electric cables to each strand to create a power supply cable. We made it 1m long. Optionally add connectors at the end of each cable. The simple resistance circuit has no polarity.

Image: Six bricks form the main mass of the heat cube. Photo by Marie Verdeil.

Image: All bricks are fixed to each other with a mortar layer that also has the electric resistance heating elements inside. Photo by Marie Verdeil.

Image on the left: Tiling the heat cube. All tiles are provisionally taped around the bricks. Image on the right: Grouting the cube. Two holes in the side made to let the cables pass through. Photos by Marie Verdeil.

Credits:

Concept: Kris De Decker
Design: Marie Verdeil
Construction and documentation: Marie Verdeil, with assistance from Hugo Lopez.

How to Brew Solar Powered Coffee

Sun, 09 Nov 2025 00:00:00 +0000

Image: The solar powered coffee maker that we build in this manual. Photo by Marie Verdeil.

There are many different methods for making coffee, some more energy-efficient than others. However, there are no coffee makers that you can power with a small solar PV panel. For example, a commercially available 12V DC drip coffee maker requires a solar panel of 300 watts to brew coffee and keep it warm.

The key to making a more energy-efficient coffee maker is insulation. Regardless of which conventional coffee maker you purchase, it will typically have little to no heat insulation, and most of the heat generated by the energy source will be wasted into the environment. Therefore, we made an insulated solar electric coffee maker ourselves.

Our coffee maker operates on the same principles as our solar-powered oven and runs on a 100W solar panel. We embedded an Italian coffee maker—a moka pot—in a mortar slab, surrounded by cork insulation and a layer of ceramic tiles.

We embedded an Italian coffee maker—a moka pot—in a mortar slab, surrounded by cork insulation and a layer of ceramic tiles.

Image: Connecting the coffee maker to a solar panel. Photo by Hugo Lopez.

Image: Cross-section of the coffee maker. 1. Tiles, 2. Cork, 3. Mortar. Illustration by Marie Verdeil.

The cooker has an electric resistance heating element integrated inside, which is directly connected to the solar panel without a battery, solar charge controller, or voltage regulator in between. Although it’s solar-powered, the coffee maker can be located inside your kitchen or next to your bed—only the solar panel needs to be outside.

Although it’s solar-powered, the coffee maker can be located inside your kitchen or next to your bed—only the solar panel needs to be outside.

The moka pot was invented in 1933 and uses pressure rather than gravity or a pump to brew coffee. It consists of a bottom chamber (a boiler that acts as a base), a funnel filter with a plate and a rubber joint, and an upper chamber where the coffee is collected. Just before the water boils, the steam increases the pressure inside the heating vessel, pushing the water through the filter and the ground coffee. The moka pot is an energy-efficient appliance, comparable to a pressure cooker for food.

How to brew solar-powered coffee

Once the solar panel receives sunlight in the morning, the coffee maker will start heating up. Consequently, if you put water and coffee in the machine in the evening, the sun will brew your coffee in the morning. Coffee is often a collective beverage and our coffee maker is most practical when several people use it. The first coffee takes roughly one and a quarter hours to make. However, once the mortar slab is warm, subsequent brews take only 20-25 minutes.

Preparing the coffee works similarly to a regular moka pot. You unscrew the top part and remove the funnel, fill the bottom part with water until the valve, place the funnel back, fill it with ground coffee, and then screw the top part back on. To serve the coffee, you unscrew the top part using the longer handle we attached to the moka pot. The handle was inspired by the Arabic raqweh.

Image: Unscrewing the top part of the moka pot. Photo by Marie Verdeil.

Image: Preparing coffee. Photos by Marie Verdeil.

Image: Serving solar-powered coffee. Photo by Marie Verdeil.

Our coffee maker is the first insulated cooking device we built, and there is room for improvement. It doesn’t produce the characteristic gurgling noise when the coffee is ready, probably because the temperature isn’t high enough.

Also, not all water evaporates. Therefore, cleaning involves placing the base under the tap, letting water run in, shaking, and then turning the coffee maker upside down to empty the base. That’s a bit impractical, because the appliance weighs 10 kg.

Because the coffee collects in the upper chamber, it is not kept warm by the mortar slab and the cork insulation. We solve this by wrapping one or more towels around the top part to keep it warm. Alternatively, you could incorporate heat insulation into the design, for example, with a tea cozy.

What you need

Image: The moka pot that we started with. Photo by Marie Verdeil.

Moka pot. Choose the size you need. If you get a second-hand device, make sure that the rubber seal is still in good condition. Also, ensure you can easily take off the handle to replace it with a longer, straight handle.
Mortar. Any construction mortar will work. Make sure it’s not too coarse. Read more about mortar in our solar oven manual.
Metal reinforcement. We pour quite a big chunk of mortar, so we reinforce it with a metal mesh to prevent it from breaking. We use a frying sieve. Chicken wire mesh shaped in a cylinder will work as well.
Cork. For thermal insulation, we use cork sheets available at home decor shops.
Tiles. Read more about tiles and tiling in our solar oven manual.
Chimney. Because the water chamber of the coffee pot is sunk in mortar, you need a way to maintain an opening around the valve to release pressure if the coffee maker overheats. We use a metal pastry bag tip, but any metal tube will work. You can also cut a piece out of an aluminum can and roll it up.
Nichrome wire, heat-resistant electric cable. These are the components for building an electric resistance heater (see our separate manual).

Step 1: Thermal mass and electric resistance heating

Image: Step-by-step instructions (figures 1 to 9). Illustration by Marie Verdeil.

fig. 1 — Start by making a mold for the mortar base. Since the moka pot is circular, we decided to shape our cooker’s heating element into a larger cylinder. Get a wooden board. Use a plastic sheet to form a cylinder and tape it to the board with duct tape. We use an old plastic office folder and shape it around a round cork pad that we place on the wooden board.
fig. 2 — Prepare some mortar and pour a layer of about 2 cm at the bottom of the mold. Let it cure for a couple of hours.
In the meantime, prepare your electric resistance circuit based on the instructions in our manual. We made a circuit with two nichrome wires in parallel (2 x 64 cm) for a resistance of 3.2 Ohms at 18V. It draws around 5.5 Amps.
fig. 3 — Make two holes in the plastic mold to weave the heat-resistant electric cables through. Distribute the resistance wires evenly on the surface of the mortar layer. Make sure they do not touch each other.
fig. 4 — Prepare some more mortar and pour it on top of the first layer so that the electric heating circuit is completely submerged.
Add the metal mesh. Gently press it into the mortar. Ensure it doesn’t touch the nichrome wire and that there is sufficient space in the center for the pot to fit. Add a bit more mortar if necessary so that the mesh is trapped at the bottom. Wait for a couple of hours for it to set.
fig. 5 — Cut the chimney tube at an angle to fit snugly around the coffee pot valve while pointing up. Attach it to the moka pot body with hot glue or clay to ensure it stays in place while pouring mortar. It will later be submerged in mortar.
fig. 6 — Place the coffee pot on top of the mortar layer, in the center. Remove the handle; otherwise, its tip would get stuck in the next mortar layer. We’ll replace it with another handle later in the building process.
fig. 7 — Prepare more mortar and pour it into the mold, around the coffee maker. Fill the mold up to the screw rim of the coffee pot. Let the mortar cure for at least 24 hours.
fig. 8 — Once hardened, remove the plastic mold sheet.
fig. 9 — Let the base dry for an additional 48 hours. Position the base on top of a grid to allow the bottom to dry as well. The mortar should feel completely dry to the touch.

Image on the left: The mold for the mortar slab. Image on the right: The electric resistance heating next to the mold in which the mortar is cast. A metal mesh sieve acts as a reinforcement inside. Photos by Marie Verdeil.

Image on the left: The electric resistance is embedded in the mortar. Image on the right: The moka pot, with the original handle removed, in the mortar slab. The chimney is glued to the base. Photos by Marie Verdeil.

Step 2: Insulation

Image: Step-by-step instructions (figures 10 to 15). Illustration by Marie Verdeil.

We insulate the mortar slab with a 2 cm thick cork layer on all sides (including the top and bottom). We use rectangular and circular cork sheets, 4 mm thick, which means we use five layers.

fig. 10 — Layer 5 circular cork pads for the bottom part. Use wood glue to secure them together.
fig. 11 — Use another five cork pads for the top. Cut out a circle in the middle, about 2 cm wider than the coffee pot, so that there is room to add a plaster protection to the cork. Glue them together.
fig. 12 — Start cutting and layering cork sheets around the cylinder, using painter’s tape to keep them in place. In each layer, make two holes to weave the electric wires through.
fig. 13-15 — Before adding the last sheet, decide where you want to route the cables from the electric heating resistance. You can cut away a path into the last but one layer of cork to allow the wires to pass. We let them out at the bottom. Add the last sheet of cork (fig 15.). Use some tape to secure it.
At this stage, you can run a test. We used water only, in order not to stain the cork with coffee. Test how long it takes for the water to reach the top chamber.

Image on the left: Cork sheets on top of the finished mortar slab. Image on the right: The device is covered in cork and ready to test with a wattmeter. Photos by Marie Verdeil.

Step 3: Tiling

Image: Step-by-step instructions (figures 16 to 21). Illustration by Marie Verdeil.

We chose to cover our coffee maker in glazed tiles. They are waterproof, easy to clean, and commonly used in kitchen furniture, walls and counter tops. Tiles are easy to adapt to various shapes and are aesthetically pleasing. Furthermore, they are easy to obtain and tiling doesn’t require expensive tools.

fig. 16 — First, prepare the surface for tiling. Adhesive mortar doesn’t stick well on cork, so start by covering the cork with plaster. We used plaster bands, similar to those used for medical casts. The bands also help to keep the cork sheets together and fill the air gaps. At this stage, protect the coffee maker with painter’s tape to prevent it from getting plastered.
Leave the plaster to dry for 24 hours.
fig. 17 — Once the plaster has dried, you can tile the surface. We found vintage glazed tiles on the street that we cut into strips and sanded off their sharp edges. For the top part, we use tile fragments from tiles that shattered during the cutting process. Mix approximately a cup of adhesive mortar for walls (which is stronger than adhesive mortar for floors). Using a spatula, spread some mortar at the back of each tile before pressing it against the cylinder. Leave a gap of a few millimeters between each tile for grouting. Repeat this process for the top, and ensure the chimney entrance remains clear. Leave everything to dry as indicated on the mortar packaging.
fig. 18 — Now it’s time to grout the tiles. The aim of grouting is to seal the joints between the tiles to avoid moisture and dirt getting in. Mix the grout with water following the instructions on the package. With a scraper, or an old plastic card, press the paste in between the tiles. After about 20 minutes, clean up the excess grout with a damp sponge. Smooth out each grout line with a wet finger or sponge. Leave everything until it’s dry.

Image: Tiling the plastered surface. Photos by Marie Verdeil.

Step 4: Finishing touches

fig. 19 — Add a handle. Remove the original handle of the moka pot, as it has a vertical design that would get stuck in the mortar. Replace it with a horizontal handle, which gives leverage to screw and unscrew the top chamber and helps to serve the coffee. We made a handle out of wood and attached it with a bolt and nut to the coffee pot using the metal attachment welded on the cylinder. A length of about 20 cm is ideal. Use strong wood because the handle needs to withstand significant stress.
fig. 20 — Wiring. Extend the short heat-resistant cable with a longer 1.5 mm² regular electric cable. Use shrink tape to prevent water or coffee from coming in.
Base. Add a 20 mm thick wood base and cork layer to the underside of the structure. It creates room to lift the coffee maker more easily, and protects the tiles from damaging the countertop surface.
fig. 21 — Done! Brew your first solar-powered coffee.

Image: The coffee maker is finished. Photo by Marie Verdeil.

Other ideas

You could also fit the coffee pot in mortar in an existing box, like an old wooden crate or a scavenged metal drum. Instead of tiles, you could use other kinds of coating with a cement, lime or plaster base. You could also decide to integrate the pot in the kitchen furniture counter.

Credits

Concept: Kris De Decker, Marie Verdeil.
Design: Marie Verdeil, with input from Anna Mareschal de Charentenay.
Construction & documentation: Marie Verdeil, with assistance from Hugo Lopez.
Thanks to AkashaHub Barcelona for the workspace. Living Energy Farm & Cal Poly for their pioneering work on insulated solar electric cookers.

How to Build a Solar Powered Electric Oven

Mon, 13 Oct 2025 00:00:00 +0000

Image: The insulated solar electric cooker that we build in this manual. Photo by Marie Verdeil.

ARTICLE

STEP BY STEP BUILDING PROCESS

Cooking’s high power use

Electric cooking devices are challenging to operate on an off-grid solar PV system. For example, an electric oven requires between 1,000 and 5,000 watts of power, while electric stove burners have an average power consumption of 1,000 to 3,000 watts per burner. If you want to use an oven and one electric stove burner simultaneously, you need a solar array of at least 32 square meters in optimal weather conditions - just to cook. ¹

You can overcome this problem by storing solar power in lead-acid or lithium-ion batteries. If these batteries are powerful enough, they can temporarily provide you with a higher power supply than your solar array can deliver. Batteries are also necessary if you want to cook after sunset or in bad weather, which is likely the case. Unfortunately, batteries account for 70-90% of the costs and the energy invested in a solar PV system. ²

Cooking thus makes it difficult to completely disconnect a household from the power grid and switch to autonomous, smoke-free, small-scale power production. ³ That is especially so when you have a small budget and limited space for solar PV panels. For example, when I attempted to go off the grid in my apartment in Barcelona by using solar panels on the balcony and window sills, it was mainly the electric cookstove that thwarted my efforts. ⁴

Image: The insulated solar electric cooker that we build in this manual. Photo by Marie Verdeil.

How to adapt an electric cooking device to solar power?

Many of the devices we take for granted nowadays were designed for an era of abundant electricity generated by fossil fuels. However, the electric oven we build in this manual demonstrates that modern appliances can be redesigned for an era of intermittent, less concentrated power sources, such as wind and solar energy. Our oven is powered by a 100-watt solar panel, small enough (50x90cm) to fit on a balcony. Furthermore, it can cook after sunset, without the use of batteries.

The key to significantly reducing the power consumption of a cooking device is thermal insulation. Our electric solar oven has 5 cm of insulation on all six sides. Its power use is further reduced by a lower cooking temperature of about 120°C (248°F). You can cook all food safely at much lower temperatures than those typical in modern cooking devices - it just takes longer.

Our oven is powered by a 100-watt solar panel, small enough (50x90cm) to fit on a balcony.

The key to cooking after sunset without batteries is thermal mass. Rather than storing electricity from solar panels in a battery for operating the cooker at night, the heat supplied by the solar panel during the day is stored in the appliance itself. Because the oven retains a high temperature at sunrise, it’s quickly ready for cooking again in the morning. Connected to a solar panel, it’s almost always preheated and ready to use.

Thermal mass also allows the cooker to continue operating after periods of clouds and rain. Likewise, opening the oven door hardly affects the temperature inside. The heat is stored in the mortar and tiles, and the air temperature quickly returns to normal when the door is closed again.

Our oven is heated by a self-made electric resistance that connects directly to the solar panel, without any intervening battery, solar charge controller, or voltage regulator. To maximize energy efficiency, the oven chamber is dimensioned around a metal oven tray and features a side door. Its shape and weight resemble those of a regular oven.

Image: A drawing of the solar electric tiled cooker. Illustration by Marie Verdeil.

The advantages of solar electric cooking

The device we build in this manual is known as an “insulated solar electric cooker” or “ISEC”. The ISEC is a more recent and more sophisticated version of the “solar box cooker”, which is an insulated wooden box with one or more transparant glass plates on top. When a solar box cooker is put in the sun, its interior reaches temperatures that are high enough to boil water and cook food. ⁵

While an insulated solar electric cooker also consists of a well-insulated box, it does not have a glass plate on top. It’s powered by a solar PV panel instead, which is connected to an electric heating element inside the cooker. One could also describe the ISEC as a fireless cooker with an electric heater inside. ⁶

Image: Two conventional, non-electric solar box cookers. Solar energy enters the glass plate and heats up the interior. Built by Audrey Belliot (Slowlab) and Marie Verdeil. Photo by Marie Verdeil.

Image: An insulated solar electric cooker is powered by a solar PV panel, which is connected to an electric heating element inside the cooker. Photo by Marie Verdeil.

The conventional solar box cooker is a very simple device that works without electricity and is cheap and easy to build. ⁷ By comparison, the ISEC is a bit more complex to build and requires a high-tech solar panel. However, solar electric cooking has several important advantages that can make the extra effort worthwhile:

An electric solar oven can be located inside your kitchen. Conventional solar box cookers only work when they are outside in the sun. That is great for events and picnics, or if you have a garden. However, for many people, it would be more practical to have their cooking appliance in their kitchen. The ISEC makes this possible because only the solar panel needs to be outside. In winter, having the cooker inside will also increase its energy efficiency. It will lose less heat to the environment due to cold and wind.
An electric solar oven can be insulated on all sides. Solar box cookers cannot be insulated on the top side; otherwise, solar radiation cannot enter the appliance. ⁸ In contrast, an ISEC can be insulated on all sides, making it more energy efficient than a non-electric solar box cooker. You can further increase the oven’s insulation by draping one or more wool blankets or carpets over it, which is not possible with a non-electric solar box cooker.
An electric solar oven works well in cloudy weather. Conventional solar cookers require full sun to function effectively. That is especially true for parabolic cookers, which concentrate solar rays at a focal point; however, solar box cookers also exhibit low performance during cloudy weather. In contrast, the electric solar cooker can get around that problem by using more or larger solar PV panels. During sunny days, you can use the excess solar PV capacity for other purposes. ²
An electric solar oven needs no attention. Solar box cookers need to be turned towards the sun at least every 15-30 minutes. Parabolic solar cookers require even more frequent movement. In contrast, an ISEC requires no attention. Of course, you could turn the solar panels towards the sun every 15 minutes, which will speed up the heating of the ISEC’s interior. However, solar panels are less sensitive to solar orientation than solar box cookers. Once you have placed food in the insulated solar electric cooker, you can leave it alone.
An electric solar oven allows you to cook after sunset. By embedding the electric heating element in a material with a high thermal mass, an ISEC can remain at high temperature for many hours after sunset. While several methods exist to add heat storage to a conventional solar cooker, they are complex and don’t work very well. For example, adding thermal mass to a conventional solar box cooker would make it too heavy to move around and follow the sun. An ISEC with thermal energy storage is also heavy, but it can remain stationary.

Our choice of building materials

Low-tech Magazine did not invent the ISEC. Our experiments with insulated solar electric cookers, which began in the summer of 2024, are inspired by the work done at Cal Poly University and Living Energy Farm ⁹¹⁰, which we described in an earlier article on direct solar power. ² We borrowed ideas and knowledge from the manuals made by these pioneers, but we also saw some room for improvements, mostly in the choice of building materials. We also applied the concept of insulated solar cooking to a DIY coffeemaker.

Rather than glass wool, sheet metal, and plastic buckets, we have chosen to build our cooking device with tiles, cork, plaster, wood, and mortar.

Rather than glass wool, sheet metal, and plastic buckets, we have chosen to build our cooking device with tiles, cork, plaster, wood, and mortar. These materials are easier to obtain and to work with, and they are more aesthetically pleasing. We aimed to design an appliance that people would actually want to see in their kitchen, and which can be built and repaired with only a few standard tools. It remains to be seen how durable our material choices will be in the long term. Still, for now, the device has been operated extensively for several months without any significant signs of damage.

Image: Some of the materials we have used. Photo by Marie Verdeil.

Our solar electric cooker consists of several key components: the structure (a wooden box), the electric heating element (nichrome wire), the insulation (cork), the thermal mass (mortar and tiles), and a solar PV panel.

Ceramic & terracotta tiles

The use of tiles is the distinctive feature of our design. Most ISECs built to date use aluminum for the inside oven compartment and metal or plastic (for example, a bucket) for the outside. However, making a water-tight aluminium box is not easy and requires specialist tools. Plastic works well as an outer shell, but it looks rather bad and may become brittle over time.

The use of tiles is the distinctive feature of our design.

In our oven, thick terracotta tiles form the cooking chamber, providing a waterproof, fireproof, and easy-to-clean interior surface. The tiles prevent water from entering the insulation layer or the electrical system, and they ensure that the insulation does not become damaged by heat. ¹¹ We also put tiles on the outside of the cooker, where they please the eye, protect the device against water damage from the outside, and make it easy to clean.

Image: The oven chamber, made from thick terracotta roof tiles. Photo by Marie Verdeil.

Tiles are easy and cheap to obtain: we collected all of them on the streets of Barcelona. Tiling can be accomplished with minimal skills and tools. You need a tile cutter if you want to cut tiles shorter, but you can avoid this by choosing the right size of tiles or by applying a “trencadís” technique, made famous by architect Antoni Gaudí. This technique involves dropping tiles so that they break and rearranging them altogether as a mosaic.

Using a tile cutter requires some practice, and you will likely break a few tiles as you learn. Make a groove in the tile by passing the sharp tungsten wheel several times across it. Then apply some pressure on both sides with your hands: the tile should break along the line. Some tiles are more challenging to cut than others. To fix tiles to wood, use a cement-based adhesive mortar. To attach tiles to cork, we applied a layer of plaster bands in some cases, as mortar doesn’t adhere well to cork. Use grout to seal the joints between tiles.

Cork or wool for insulation

Insulation is key to the workings of all solar cookers, including the ISEC. It’s the insulation that allows cooking with a very small solar panel. The heat accumulates over time because the insulation slows down the release of heat from the cooker to the outside environment. For insulation, we use a 5 cm layer of expanded cork on all six sides of our device. Expanded cork is a natural insulation material that provides excellent insulation. It’s made from cork waste, bound using steam. Standard cork sheets also work well.

For insulation, we use a 5 cm layer of expanded cork on all six sides of our device.

Cork is expensive, and it’s not a material you easily find on the streets. You can obtain a cheaper but more labour-intensive insulation material by cutting up discarded and second-hand wool clothes and blankets. Apart from cork and wool, there exist many other insulating materials. However, many are toxic or unpleasant to work with, and unlike cork and wool, they are often flammable. Cotton and cellulose are cheap and sustainable (waste) materials, but they do not insulate as well as cork or wool.

Image: The expanded cork insulation. Photo by Marie Verdeil.

Mortar for heat storage

We made the base of our cooking device out of mortar, a material that retains a lot of heat. Thermal mass is the key to cooking after sunset. The solar panel stores heat in the oven rather than electricity in a battery. The mortar serves a dual function: it also safely encapsulates the electric heating element (see below). Every ISEC with a self-built heating element will have some thermal mass; however, we have made a thicker slab to enhance heat storage. The tiles of the oven chamber provide extra thermal mass.

The solar panel stores heat in the oven rather than electricity in a battery.

Mortar is composed of cement, sand, and water. Sold as a powder in bags, it must be mixed with water before application. You can also buy a bag of cement and mix it with sand and water to obtain mortar. Follow the instructions on the packaging for the powder-to-water ratio. Once cured, which takes several days, mortar becomes (and remains) hard. There’s no need to use refractory cement, which is made to withstand high temperatures in fireplaces and pizza ovens, because the temperature in our oven is not that high. Sand is an alternative material with a high thermal mass.

Heating element and electric system

Our oven is heated by an electric resistance element, which is connected directly to the solar panel. We initially used commercial heating elements in our prototypes, which yielded disappointing results. Therefore, we decided to build our own, based on the manual provided by the Living Energy Farm. ¹² Building your own heating element involves extra work, but it’s worth the effort.

Many commercial heating elements have built-in thermostats, which can complicate temperature regulation inside the oven. They also require a voltage input that does not align with the voltage output of most solar panels, which introduces the need for an extra electronic component (a buck converter). Securely fixing commercial heating elements proved to be difficult as well, and we had trouble keeping moisture away from the electrical system; at one point, this resulted in an electrical fire.

Building your own heating element involves extra work, but it’s worth the effort.

By embedding a self-made heating element in a mortar base, we solved all these problems. A custom-made electric resistance consists of a circuit made of nichrome wire, which is an alloy of nickel and chrome. The length and thickness of the nichrome wire determine its heat dissipation and power consumption, allowing you to precisely scale the circuit according to the voltage and current produced by your solar panel. You connect the nichrome circuit to the electric cables of the solar panel, with a short section of heat-resistant electric cable in between (see our manual).

Our solar electric cooker features a thermal fuse and an internal thermostat, both of which are embedded in the mortar layer. However, these components are not necessary when operating the oven on a solar panel without a battery. Nature already provides the thermostat: once the sun goes down, the heating element stops working, making it unlikely that the oven overheats.

Image: Building your own heating element involves extra work, but it’s worth the effort. Photo by Marie Verdeil.

Image: The thermal switch and fuse are seen on the first layer of mortar. Photo by Marie Verdeil.

How to use the solar electric oven

Our cooking appliance can be used to cook raw food (vegetables, grains, meat, fish). It can also work as a (slow) microwave oven, warming up leftovers or a ready-made meal.

Food safety

Our oven reaches a maximum cooking temperature of about 120°C (248°F). To avoid food poisoning from potentially dangerous bacteria, food should either be refrigerated or heated to a minimum temperature of between 58 °C and 74°C (136°F-165°F), depending on the type of food, for at least 15 seconds. Cooked vegetables and fruits must reach a temperature of 58°C (136°F). Most meats and seafoods can be safely cooked at 63°C (145°F). Ground meats require a temperature of 71°C (160°F), and leftovers and poultry should reach 74°C (165°F). We monitor the temperature of the food using a food thermometer, which we inserted through the chimney hole at the top.

Image: Food made in the electric solar oven. Photo by Marie Verdeil.

Keep in mind that the temperature inside the oven will drop once you put the food inside. You should not put frozen food inside, because the temperature will drop spectacularly, and it may take many hours before a safe cooking temperature is restored. For the same reason, the oven should be preheated before the food is placed inside. However, because it’s connected to a solar panel, our oven will be at a sufficiently high temperature for most of the time. You should not keep food in the solar cooker overnight, unless you are sure it maintains a safe temperature until the morning (our cooker does not).

Image: Slow-cooked food (before and after cooking). Photo by Marie Verdeil.

Cooking time

It’s perfectly possible to build an insulated solar electric cooker that cooks food just as fast as a normal oven (see further). However, it makes a lot of sense to build a low-temperature, insulated “slow cooker” instead. First, it allows you to cook with a smaller solar panel. Second, slowly cooked food tastes better and retains more of its nutrients. Third, at lower cooking temperatures, food cannot burn and for that reason you don’t need to stir it either. The inconvenience of a longer cooking time is thus compensated for by a more relaxed and easier cooking process.

On average, it takes about twice as long to cook food compared to using a conventional oven. Most meals we make, cooking raw food, take between two and four hours. Heating leftovers or a ready-made meal takes about one hour. These time spans were measured in optimal weather conditions and with a preheated oven.

Cooking after sunset

Due to the cooker’s high thermal mass, it will take several hours to heat it when you first connect it to a solar panel. However, from the second day onwards, the solar panel will keep the cooker at a continuously high temperature, even for many hours after sunset. When fully charged at sunset, reaching a temperature of approximately 120°C (248°F), our electric cooker maintains a sufficiently high temperature to cook for 4-5 hours. Once the food gets in, the temperature drops but remains high enough (above 80°C/176°F) to cook food safely. The stored heat that remains at the end of the night allows us to restart the cooking process quickly in the morning - our oven is still above 40 or 50°C (104-122°F) at sunrise.

The stored heat that remains at the end of the night allows us to restart the cooking process quickly in the morning.

Draping one or more wool blankets over the oven at sunset further increases the heat storage, allowing for cooking a meal even later in the evening, or starting cooking even earlier the next day. You can also use blankets to raise the energy efficiency of the oven during the day, resulting in a higher cooking temperature.

Moisture

Depending on the food you prepare, excessive moisture in the cooking chamber can be a problem. The water in the food may evaporate and collect in the oven space. Therefore, our oven has a small chimney through which the moisture can escape. It can be closed with a cork cap if you want to keep the moisture inside. It is a good idea to leave the oven door open occasionally so that any moisture in the insulation layer can evaporate.

Alternative cooker designs

Because we shaped our cooker around an oven tray, it’s mostly suited for oven dishes. However, other designs are possible. An earlier prototype we built has a heat chamber the size of a soup pot, and so that one is better suited to prepare stews and soups. Whatever form you choose, it’s always a good idea to dimension your cooker around a specific cooking utensil. If you put a small pot into a large oven chamber, you will waste a significant amount of energy heating empty space.

Image: An earlier prototype we built has a heat chamber the size of a soup pot, and so that one is better suited to prepare stews and soups. Photo by Marie Verdeil.

Thicker insulation

The thicker the insulation layer, the more energy efficient the oven will be. A thicker insulation layer allows you to use a smaller solar panel for the same cooking time, or a faster cooking time using the same solar panel. A thicker insulation will also improve the heat storage. However, keep in mind that the device’s volume will increase exponentially. Another 5 cm of insulation on all six sides would have made our oven’s size unpractical for most kitchens.

Higher cooking temperature

If you want a solar electric cooker that cooks faster at a higher temperature, you should choose a larger solar panel and a more powerful heating element, and you should raise the setting of the thermostat and thermal fuse. Adding extra insulation also accelerates the cooking time. However, please note that we did not test our building materials at higher temperatures; therefore, proceed at your own risk.

Much depends on the local customs surrounding eating times, especially dinner. For example, European dinner times vary from about 17:00 to 19:00 in northern countries to between 21:00 and 23:00 in southern countries. The early dinner times in the north align with solar cooking. However, the late dinner times in the south would require more powerful cookers with higher temperatures and more thermal storage to cook after sunset, or to safely store a warm dish prepared in the morning.

More or less heat storage

While a solar electric cooker always needs insulation, you can build it with little or no thermal mass. The choice depends on how you want to use the device. With little to no thermal mass, the cooking appliance will heat up and cool down relatively quickly, and it will be somewhat lighter. But it won’t be able to cook after sunset. Many of the ISECs built by others are of this type.

On the other hand, it’s also possible to create a larger version of our solar cooker that can be used to cook for 24 hours a day. Add more thermal mass, insulation, and consider a higher oven temperature, as well as using a larger solar panel and a more powerful electric resistance. Such a cooking device would always be ready to use immediately, without any need for electricity storage, and it could work in industral kitchens or as a community cooking appliance.

You could also build a solar electric cooker with a heat storage consisting of metal rather than mortar. It does not allow you to cook after sunset, but it does enable you to reach higher cooking temperatures for a short time. That makes it possible to bake and fry food.

Step by step buiding guide

Image: Exploded isometric drawing of our insulated solar electric oven. Illustration by Marie Verdeil.

What you need

Cooking utensil

Oven tray. To hold the food that you are cooking in the oven. This tray, which can be made of metal, ceramics, or heat-resistant glass, is the first thing to obtain, as you will dimension the oven around it.

Electric heating element & electrical system (see our separate manual)

100W solar panel.
Nichrome wire.
Heat-resistant electric cable.
Thermal switch.
Thermal fuse.

Structural materials

Wood boards. The oven is built around a wooden structure. You can reuse an existing box or make it from scratch. Reclaimed wood or chipboard is fine, since none of it will be visible.
Tiles. We use tiles for both the interior of the cooking chamber and the exterior of the oven.
Wood screws.
Hinges and hooks. To attach the oven door.
Feet for the oven. We made these out of wood. Feet make it easier to lift and move the oven, and they protect the oven against water damage from below.
Handle for the oven door. We made one out of wood.

Insulation materials

Expanded cork boards. We used 5 cm thick expanded cork boards as insulation on all sides. We used roughly 1 m2 of expanded cork. You can also use regular cork or wool insulation. Avoid flammable materials such as cotton, wood chips, or any oil-based insulation material. Cork and wool are fire-resistant materials.
Thin cork sheets (4 mm). You place them as a sealant between the oven door and body. You also use them to fill in the height differences between the expanded cork layers above the oven chamber.

Heat storage material

Construction mortar. We use mortar to provide thermal mass for heat storage and to embed the electric resistance heater.

Fixing & filling materials

Adhesive mortar. To fix the tiles to wood and cork surfaces.
Grout. To fill up the space between the tiles.

Extra components

Food thermometer. You need one with a long sensor, as it will have to travel through the thick insulation layer at the top to reach the food.
Chimney pipe. Roll a tube of thin aluminum sheet, which you can cut out of a soda can. Alternatively, buy a metal tube of the correct size.

Tools

Screwdriver
Wood saw
Drill (you need a concrete drill bit for the chimney opening)
Soldering iron and tin
Tile cutter (optional if you find the right size of tiles)
Measuring tools
Utility knife
Mortar trowel and mixing container

Step 1: Build the structure

Image: Step-by-step instructions (fig 1. to 4.). Illustration by Marie Verdeil.

Obtain an oven tray and measure it. We dimensioned our cooking device around a stainless steel tray approximately the size of an A4 sheet: 20 cm x 27 cm.
fig 1. — Using tiles, create a box around the tray with enough room to slide in and out easily. The box will become the inner chamber of the solar cooker. For now, keep the structure together with tape. When determining the dimensions of the heating chamber, leave some space at the top of the oven tray to allow for heat circulation. Ideally, you find tiles that have the correct dimensions. Otherwise, cut the tiles to the correct dimensions using a tile cutter.
Measure the exterior dimensions of the tiled oven chamber to calculate the dimensions of the wooden box that will surround it. Add 5 cm of space on all six sides to fit the expanded cork layer. At the bottom, add about 2-3 cm extra to account for the mortar, which will embed the heating element. Add 5 mm to the dimensions on all sides to ensure everything fits.
fig 2. — Build the wooden box according to the calculated dimensions (don’t forget to add the wood thickness). Screw the wood together in a way that allows for the removal of the upper part later in the building process (see step 3). Measure everything a few times before you start cutting the wood, as it’s easy to make mistakes.
fig 2-3. — To make sure that the door neatly aligns with the rest of the wood box, build the structure as a whole and then cut away (saw) the door part off the box. The door part needs to be 6 cm deep to fit in the insulating cork layer and the oven chamber tile on that side.
fig 4. — Once you have cut the wood, unscrew the top board to gain better access inside.

Image: The oven chamber inside the wood structure. The space in between will be filled up with cork insulation and a mortar layer at the bottom. Photo by Marie Verdeil.

Step 2: Make the electric heat resistance

Create a resistive heating element using Nichrome wire. See our separate manual for instructions.

Step 3: Add insulation, create the heat storage and add the electric heat resistance

Image: Step-by-step instructions (fig 5. to 8.). Illustration by Marie Verdeil.

fig 5. — Using a thin saw or utility knife, cut and glue the expanded cork insulation boards to cover all sides of the area. You can use wood glue or hot glue. Keep the top board apart to add it later.
fig 6. — Mix some construction mortar with water and create a layer of about 10-15 mm on the bottom cork layer. Leave it to set for a couple of hours.
fig 6. — At the back of the box, about 10mm above the mortar layer, pierce a hole through the cork and wood to channel the electric cables through.
fig 7. — Place the resistance circuit on top of the mortar bed and drive the heat-resistant cable endings through the hole in the back of the box. Make sure the nichrome wires don’t cross or touch, and that the (optional) fuse and thermal switch are also lying on top of the mortar.
fig 8. — Pour another 10-15 mm of mortar to cover the circuit.

Image: The first layer of mortar with the nichrome circuit, thermal switch, and thermal fuse on top. All these components will be hidden in the second layer of mortar. Photo by Marie Verdeil.

Phase 4: Fix the cooking chamber in place and complete the insulation

Image: Step-by-step instructions (fig 9. to 16.). Illustration by Marie Verdeil.

fig 9. — Take the tiles you prepared for the inner chamber. Place some mortar at the back of the bottom tiles and press them onto the mortar bed.
fig 10. — Using adhesive mortar, fix the remaining tiles to the sides and back of the cork boards, recreating the oven chamber that you taped together in step 1.
fig 11. — Using a drill with a concrete drill bit, make a 10-12 mm hole in the center top tile to fit an air vent chimney.
fig 11. — Lay the remaining top tiles to rest on the edges of the side tiles with a bit more mortar. It is a good idea to tilt the top tiles slightly to one side to guide condensation moisture away from the food tray.
fig 12. — Before placing back the top cork board on top of the inner tile chamber, mark the position of the chimney and drill through the cork and wooden boards.
fig 12-13. — Place the top corkboard on the top tiles with a bit of adhesive mortar and screw back the top wooden board to close the box. If there are some air gaps, fill them in with cork scraps or sheets to prevent heat leakage.
fig 14. — Glue 4 mm thick cork sheets to the box insulation that surrounds the oven chamber on the front. Use wood glue. This extra layer helps to close the door tightly and prevents any heat from escaping.
fig 15. — Insulate the door by fitting a 5 cm expanded cork board inside with wood glue. Using a bit more adhesive mortar, place the last tile on the door, making sure it aligns and closes the inner chamber when the wood box is closed.
fig 16. — Glue another 4mm corksheet to mirror the chamber’s edge.

The box is now mostly finished. Let everything dry/cure for at least 48 hours.

Image: Fixing the cooking chamber. Photo by Marie Verdeil.

Step 5: Finishing touches

Image: Step-by-step instructions (fig 17. to 19.). Illustration by Marie Verdeil.

fig 17. — Tile the box top to make it waterproof and heat-resistant (you can put the oven tray there when it comes out of the oven). Ensure that you leave a hole for the chimney.
Make and insert the chimney. Fit it into the hole you made.
Grouting. Seal the inner tile chamber with grout to prevent moisture from entering the cork insulation. Do the same for the exterior tiles and for the joint with the chimney. We also added plaster on the sides to protect the wood.
fig 18. — Add a handle to the door.
fig 19. — Add hinges to attach the door to the oven body using screws. Place a metal latch on each side to tightly lock the door during operation.
Add small feet to the oven to make it easier to lift and to protect it against water damage.
The oven is finished!
Connect the heat-resistant cables sticking out of the oven to the wires of the solar panel. Insert an on/off switch between them (on the positive wire).

Image: Some assembly steps for the oven (from left to right): Fig.1: Drilling a hole for the chimney, with all insulation in place. Fig.2: Extra layer of cork to cover the door and box insulation. Fig.3: Tiling the exterior of the solar electric oven. Fig.4: Grouting. Photos by Marie Verdeil.

Credits

Concept: Kris De Decker, with input from Marie Verdeil.
Design: Marie Verdeil, with input from Anna Mareschal de Charentenay.
Construction & documentation: Marie Verdeil, with assistance from Hugo Lopez.
Design & construction of two earlier prototypes: Vaiva Vinskaité, with input from Kris De Decker and Marie Verdeil.
Thanks to: Samira Allaouat & Alexandra Tollefsrud for the tiles. AkashaHub Barcelona for the workspace. Living Energy Farm & Cal Poly for their pioneering work on insulated solar electric cookers.

In affluent, industrialised societies, cooking is rarely seen as a problem when it comes to resource use and carbon emissions. For example, in the US, only around 5% of a household’s energy use is attributed to cooking devices (such as stovetops, ovens, microwaves, and water kettles). However, while cooking requires relatively little energy, it requires a lot of power. Energy consumption equals power consumption multiplied by time. Since cooking devices are only used for a short time during the day, their energy consumption is relatively low. However, their high power use makes it challenging to operate them on an off-grid solar PV system. ↩︎
Direct Solar Power: Off-Grid Without Batteries, Kris De Decker, Low-tech Magazine, August 2023. https://qelnixcor.cloud/2023/08/direct-solar-power-off-grid-without-batteries/ ↩︎ ↩︎ ↩︎
Too Much Combustion, Too Little Fire, Kris De Decker, Low-tech Magazine, December 2019. https://qelnixcor.cloud/2019/12/too-much-combustion-too-little-fire/ ↩︎
How to Get Your Apartment Off the Grid, Kris De Decker, Low-tech Magazine, May 2016. https://qelnixcor.cloud/2016/05/how-to-get-your-apartment-off-the-grid/ ↩︎
The first recorded use of the solar box cooker goes back to the eighteenth century when the increased use of glass made people aware of its ability to trap solar heat. For more information, see Hirst, Eric. “A golden thread: 2500 years of solar architecture and technology: by Ken Butti and John Perlin Cheshire Books, distributed by Van Nostrand Reinhold Company, New York and London, 1980, 304 pp,£ 11.95.” (1981): 167. /// Daniels, Farrington. Direct use of the sun’s energy. Yale University Press, 1964. /// Telkes, Maria. “Solar cooking ovens.” Solar Energy 3.1 (1959): 1-11]. ↩︎
If We Insulate Our Houses, Why Not Our Cooking Pots?, Kris De Decker, Low-tech Magazine, July 2014. https://qelnixcor.cloud/2014/07/if-we-insulate-our-houses-why-not-our-cooking-pots/ ↩︎
Find an example of a manual here: https://reclaimdesign.org/diy-solar-oven ↩︎
Although glass plates reduce the heat losses from the oven interior somewhat, they cannot offer the same level of thermal resistance as a thick layer of wool or cork. ↩︎
See: https://sharedcurriculum.peteschwartz.net/isecooker-construction/ ↩︎
See: https://livingenergyfarm.org/insulated-solar-electric-cooker/ ↩︎
It may seem unusual to build an oven from materials such as wood, cork, or wool. However, the temperature reached inside our oven does not pose any risk for these materials. Cork and wool are thermally stable up to approximately 200°C and start to degrade above that temperature. They are fire-resistant materials: they don’t ignite and won’t spread fire. Wood does not ignite at temperatures below 250ºC. Furthermore, all these materials are separated from the heating element and the interior oven chamber by mortar and tiles, which resist much higher temperatures. When we had an electrical fire in one of our first prototypes, the fire did not spread. ↩︎
See: https://conev.org/ISECmanual14.pdf ↩︎