This manual documents how to build an electric resistance heating element that connects directly to a solar panel—without using a battery, charge controller, or voltage regulator. This heating element is used in our insulated solar electric cooker (explained in another manual) and will also be featured in upcoming manuals for a solar-powered coffee maker and footstove.
We also describe how to make a removable heat brick, which we use to replace commercial heating elements in some earlier electric solar cooker prototypes. Our custom-made electric resistance heater is an electric circuit made from nichrome wire, enclosed within a mortar layer.
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### Understanding the Heating Element Design
The length and thickness of the nichrome wire determine its current draw at a specific voltage. This means you must dimension the circuit according to your solar panel’s voltage and power rating to optimize heat generation. The nichrome circuit connects to the solar panel’s electric cables via a short section of heat-resistant electric cable.
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### Why Build Your Own Electric Resistance Heating Element?
Initially, we used commercial heating elements in our first solar oven prototypes but were disappointed with the results. Inspired by a manual from Living Energy Farm, we decided to build our own. Although building your own heating element involves extra work, it is significantly cheaper and worth the effort.
Commercial heating elements usually contain built-in thermostats, complicating temperature regulation inside the oven. They also require voltage inputs that rarely align with solar panel outputs, necessitating extra electronics like buck converters. Additionally, securing commercial elements proved difficult, and moisture issues once caused an electrical fire.
By embedding a self-made heating element inside a mortar base, we solved these problems effectively.
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### What Is Electric Resistance Heating?
Electric resistance refers to the difficulty electric current encounters when flowing through a material, similar to friction in mechanical systems. Resistance generates heat, as explained by Joule’s Law, and is measured in ohms (Ω).
The resistance of a wire depends on the material’s resistivity, length, and thickness. Metals such as copper have low resistance, making them excellent conductors. Insulating materials like plastic, rubber, and ceramics have high resistance, preventing easy electrical flow.
Heating elements—like those in ovens, toasters, and hair dryers—commonly use nichrome wire, an alloy of nickel and chromium. Nichrome has relatively high resistance for a metal, dissipates considerable heat, and glows orange when heated.
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### Components You Need
Below is a list of components, with Amazon links for convenience. You are encouraged to source locally or scavenge parts from old appliances. We do not earn any commission on these purchases.
– Nichrome wire (available in bobbins or spools; can also be salvaged from old ovens, toasters, hair dryers, and other heating devices)
– Heat-resistant electric cable (encased in silicone mesh rather than plastic)
– Thermal switch (optional)
– Thermal fuse (optional)
– Construction mortar for encapsulating the nichrome circuit
– Thick tiles (if building removable heat bricks)
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### Calculating the Resistance Value
The key challenge is determining the proper length of nichrome wire to match your power source’s voltage and current ratings.
Use Ohm’s Law to calculate the required resistance:
**Resistance (Ω) = Voltage (V) / Current (A)**
#### Finding Voltage and Current Values
– Check your solar panel’s label.
– For voltage, use **Maximum Power Voltage (Vmax)** or **Voltage at Pmax** (the voltage when the panel delivers maximum power).
– Ignore **Open Circuit Voltage (VOC)**, which is the voltage when nothing is connected.
For example:
A typical “12V” solar panel has a Vmax of ~18V; a “24V” panel has a Vmax of ~36V.
– For current, use **Maximum Power Current (IMP)** or **Current at Pmax** (current at max power).
– Ignore **Short Circuit Current**.
If the label is missing, measure voltage with a multimeter. Current can be calculated as:
**Current (A) = Power (W) / Voltage (V)**
Example:
For a 100W panel at 18V, maximum current is:
100W / 18V = 5.55A
Therefore, the desired resistance is:
18V / 5.55A ≈ 3.24 Ω
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### Calculating the Length of Nichrome Wire
Nichrome wire has a specified resistance per meter (Ω/m), which varies by wire thickness.
– We purchased thin nichrome wire rated at 8.71 Ω/m.
– To get 3.24 Ω, calculate the length:
(100 cm × 3.24 Ω) / 8.71 Ω/m ≈ 37.2 cm
If you use a different wire thickness, your length will differ.
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### Verifying Resistance: Don’t Trust the Label Blindly
Resistance ratings on packaging may not be precise. To measure accurately:
1. Cut 1 meter of nichrome wire.
2. Connect it to your solar panel or test station through a watt-meter or multimeter.
3. Make a complete circuit.
4. Measure amperage and wattage immediately — do not let the wire overheat.
5. Calculate resistance using Ohm’s Law.
Example measurement:
Measured 18V, 1.76A → Resistance = 18V / 1.76A = 10.2 Ω/m
Recalculate length for 3.24 Ω:
(100 cm × 3.24 Ω) / 10.2 Ω/m ≈ 31.7 cm
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### Extending and Multiplying the Wire
A circuit shorter than your cooking surface will create hot spots and uneven heating.
**Solution:** Connect multiple nichrome wires *in parallel*.
– Doubling the circuit means two parallel wires, each twice as long.
– Tripling means three parallel wires, each three times as long.
Why the length increases?
– Longer wire = higher resistance.
– Parallel wires reduce total resistance.
To maintain the target resistance of 3.24 Ω in parallel wires, lengthen each accordingly.
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### Preparing the Nichrome Wire
– Cut wires to the decided length **plus an extra ~4 cm** to allow for soldering connections.
– Coil the wire by wrapping it around a rod (pen or screwdriver) to make handling easier.
– Gently stretch the coil to loosen it slightly.
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### Adding Safety: Thermal Switch and Fuse
Heat elements can overheat, causing fire hazards or damage.
If your heating element connects directly to a solar panel (which powers off at sundown), safety risk is lower. However, if powering via a battery or grid, add safety controls:
– **Thermal switch:** Opens circuit at a set temperature, closes once cooled.
– **Thermal fuse:** A one-time-use fuse that blows at a higher temperature to prevent catastrophic failure.
Embed the fuse and switch in the mortar surrounding the nichrome wire.
We used a thermal switch rated at 200°C (392°F) and a fuse at 240°C (464°F).
Note: Oven chamber temps are lower (e.g., switch trips at 200°C at heater, chamber around 120°C).
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### Wiring the Circuit
Connect components in this order:
**Positive solar panel cable → ON/OFF switch → heat-resistant cable → (optional) thermal switch → (optional) thermal fuse → nichrome heating circuit**
– The thermal switch and fuse are connected in series.
– Both have no polarity; pins can connect in any direction.
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### Soldering Nichrome Wire to Electric Cables
Nichrome wire does not bond easily with tin solder.
To solder successfully:
1. Tin the stripped end of the heat-resistant cable.
2. Wrap the extra 4 cm of nichrome wire tightly around the cable strand.
3. Apply generous tin over the twisted joint to secure it.
Use appropriately thick cables:
– For 5.555A current, use cables with a minimum core thickness of 1.5 mm² (European standard) or 14–16 AWG (US standard).
– Thicker cables are needed for higher currents or long cable runs.
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### Encapsulating the Heating Element
The nichrome heating element should be embedded in heat-resistant mortar with high thermal inertia.
Two methods:
1. **Encapsulate inside the appliance:**
Embed the element inside the mortar layer of the cooking device (e.g., the bottom of the solar oven chamber).
2. **Encapsulate in a removable heat brick:**
Sandwich the nichrome circuit in mortar between two thick, strong tiles (e.g., terracotta floor or roof tiles). This makes the element replaceable if damaged.
The Living Energy Farm embeds their nichrome circuit in a custom metal shell, which requires advanced tools and skills.
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### Constructing a Removable Heating Brick
**Assembly Steps:**
1. Place one tile back-side up.
2. Spread a layer of mortar nearly to the edges.
3. Lay the nichrome circuit on the mortar.
– Keep wires from touching or crossing.
– Avoid edges to prevent wire exposure.
4. Leave 3–5 cm of heat-resistant cable protruding for connection.
5. Spread mortar on the second tile and press it on top like a sandwich.
6. Let dry for at least 48 hours.
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### Setting Up a Test Station
A test station allows you to test your nichrome heating element using grid power simulating solar panel output.
**Components:**
– DC power supply (12V or 24V, with capacity ≥ your panel wattage).
– Buck or boost converter to adjust voltage output as needed.
**How to use:**
– Connect DC supply output to the buck/boost converter.
– Adjust voltage to match your panel’s Vmax (e.g., 18V).
– Connect the nichrome heating element to this setup.
If you have a 12V or 24V battery with a charge controller, a DC power supply alone suffices for testing.
Budget alternatives:
– Use a laptop adapter (typical output ~19–20V, 70–90W) for testing smaller elements.
– Invest in an adjustable lab DC power supply if possible.
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### Using Other Power Sources
If powering your heating element from a 12V or 24V battery and solar charge controller, design your resistance based on those voltages.
Example current calculation for 100W:
– 12V system: Current = 100W / 12V = 8.33A
– 24V system: Current = 100W / 24V = 4.17A
Adjust your wire length and thickness accordingly.
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This guide helps you build and test efficient, safe nichrome wire heating elements designed to run directly on solar panels without complex electronics. With careful calculation, construction, and safety precautions, you can create a reliable electric heating element for solar cooking and heating applications.
https://solar.lowtechmagazine.com/2025/10/how-to-build-an-electric-heating-element-from-scratch/
