Biomass · Thermoelectric
A thermoelectric generator turns a temperature difference straight into direct current with nothing spinning, nothing wearing and nothing to lubricate. On a stove that is already burning, it is the cheapest electricity on this hub.
Most first attempts fail for one of two reasons, and both are decided before the fire is lit: the wrong module was bought, or the cold side was an afterthought.
How it works
Hold one end of a semiconductor hot and the other cold, and the mobile charge carriers inside it drift away from the hot end. They pile up at the cold end, and that imbalance is a voltage. This is the Seebeck effect, and it is the entire operating principle.
A practical module is a grid of small semiconductor pillars, alternating between two types so that the drift produces charge in opposite directions, wired electrically in series so their voltages add and arranged thermally in parallel so heat crosses all of them at once. The whole array is sandwiched between two ceramic plates that conduct heat but not electricity. The material almost always used at stove temperatures is bismuth telluride.
Nothing moves. There is no fluid, no bearing, no seal, no combustion chamber. That is the appeal, and the price of it is efficiency.
Output is set by the gap between hot and cold faces, not by how hot the hot face is. A blazing stove with a hot cold-side produces nothing.
A module's open-circuit voltage scales with the number of semiconductor pairs inside it, which is why modules of the same size differ so much in rated output.
The energy not converted does not disappear. It conducts through the pillars to the cold plate and has to be taken away continuously.
The mistake that ends most attempts
The cheap modules sold everywhere are thermoelectric coolers, usually called Peltier modules. They are designed to move heat when you push current through them. Physically they look almost identical to a generating module, they cost a fraction as much, and run backwards they will genuinely produce a voltage on a bench.
On a wood stove they die, and the reason is in the solder rather than the semiconductor.
Cooling modules are commonly assembled with bismuth-tin solder, which melts at about 138 degrees Celsius, roughly 280 Fahrenheit[1]. Higher-grade modules use antimony-tin at about 235 degrees. A stovetop in normal operation sits between the two and frequently above both.
Above roughly 80 degrees Celsius the tin in that solder diffuses into the semiconductor and forms a film across the element, acting as a resistor that drops the voltage. Devices built with tin-containing solder are described in the patent literature as not serviceable substantially above 80 degrees, losing fifteen percent or more of their performance per year[2].
The numbers side by side
Read that list twice. Even a proper generating module is working near the top of its material's useful range on a wood stove, which is why hot-side temperature control matters as much as cold-side cooling and why a module clamped directly to the hottest plate of a hard-burning stove is not necessarily in the best place.
The real engineering problem
Commercial bismuth telluride modules convert roughly five to six percent of the heat crossing them into electricity. A 2020 laboratory demonstration reached eight percent and was described as forty percent above the commercial state of the art[4], which tells you where ordinary modules sit.
Turn that around. Ninety-four or ninety-five percent of the heat you push into the hot face arrives at the cold face and has to leave. If it does not leave, the cold side warms, the difference across the module shrinks, and output falls away toward nothing while the stove burns exactly as before.
This is why installations that work look like cooling systems with a module attached, rather than modules with a small heatsink stuck on the back.
A finned heatsink, sometimes with a small fan run from the module's own output. Simple, silent when passive, nothing to leak, nothing to freeze.
Adequate for small installations in the single-digit to low double-digit watt range. Air is a poor coolant, and above that the fins stop keeping up.
A water block, a pump and somewhere to dump the heat. Holds the cold side far lower, keeps the temperature difference wide, and lets an array reach the region of 50 to 100 watts.
The heat can go into a hot water loop, which makes it useful rather than wasted. The cost is a pump that must run whenever the stove is hot, and a freeze risk in an unheated space.
The pump dependency deserves a moment. If the pump stops while the stove is burning, the cold side climbs fast, and the module can be carried past the temperature its own construction tolerates. A liquid-cooled installation wants the pump powered from something that does not depend on the module it is cooling.
Honest output
A modest stovetop unit produces enough to run lighting, charge phones and radios, and keep a small battery topped up through a winter evening. A well-executed liquid-cooled array reaches into the tens of watts and occasionally past a hundred. Nothing in this category runs a pump, a compressor, or anything that makes heat electrically.
Judged as a power plant that is a poor showing. Judged correctly, it is a trickle that arrives for free in the darkest months, from fuel already burning for another reason, and it arrives precisely when solar is at its worst. That seasonal complement is the argument for it, and it is a good one.
Getting it into a battery
Module output rises and falls with the stove. A fire lit at five in the evening, roaring at seven and down to coals by midnight presents a voltage that does the same thing, and a battery cannot be charged directly from that with any efficiency.
The answer is a DC-to-DC converter between module and battery, ideally one with maximum power point tracking. It holds a usable charging voltage across the whole range of stove temperatures, and it loads the module near its best operating point instead of wherever the battery voltage happens to drag it.
The same four checks that size any system apply here, and the answer will be a small one. The Power Needs Calculator works out what your loads actually draw, which is the only way to know whether a trickle of this size is worth the installation.
Air gaps between plate and module are insulation. Ordinary silicone thermal compound is not rated for stove temperatures and will bake out.
Modules are ceramic and brittle. Uneven clamping cracks pellets and solder joints, which is the most common mechanical failure in these devices.
Long thin runs from a low-voltage source lose a meaningful share of a small output to the wire itself. Keep the converter near the module.
Safety
Nothing on this page changes anything about operating a wood stove. Every clearance, every chimney inspection, every rule about what may be burned and how the flue is maintained applies exactly as it did before, and a thermoelectric installation does not offset any of it.
Two points are specific to adding a generator.
A thermoelectric unit sits outside the combustion path and does nothing about the products of combustion. Unintentional non-fire carbon monoxide poisoning kills roughly 430 people a year in the United States on the Centers for Disease Control's long-run average[5], and a working alarm on every sleeping level remains the measure that matters.
If anything about the installation changes airflow around the stove or its flue, it has moved into territory that affects combustion, and it needs a different conversation than this page can have.
A stove is a listed appliance, approved as a complete assembly and installed against clearances that assume it stays that way. A free-standing unit resting on the stovetop changes none of that and is the low-risk option by a wide margin.
Drilling, welding, bolting to the body, altering the flue or adding a jacket is a different matter. It may void the listing, it may affect surface temperatures and clearances the installation was approved against, and whether it is acceptable is a question for your local authority having jurisdiction rather than for a supplier. Ask before, not after.
Common mistakes
The single most common failure. They work on the bench, degrade within weeks on a stove, and the falling output is usually blamed on the cooling rather than on the parts.
Ninety-five percent of the heat goes out the back. Whatever removes it is the main component of the system, and it should be specified first rather than last.
Output follows the difference across the module, and bismuth telluride loses performance as the hot side climbs past its useful band. Past a point, harder burning buys nothing and shortens the module's life.
Standard silicone grease is a computer product and bakes out at these temperatures, leaving an insulating gap in the one place the design cannot tolerate one.
Without conditioning, the module spends most of the evening at the wrong operating point and the battery sees a voltage that wanders with the fire. The converter is not an accessory.
A circuit that stops cooling itself the moment output dips has a failure mode built into it, and the failure happens while the stove is at its hottest.
Next
Thermoelectric modules are the least demanding way to take electricity from a fire and the least productive. The next guide covers the machine that takes the same stove heat and returns it in kilowatts rather than watts, at the cost of having moving parts again.
If your assessment pointed at stove heat as your one viable stream, read both before choosing.
Sources
Output figures on this page are ranges for deciding whether the pathway is worth pursuing, not design values for a specific module. Follow your stove manufacturer's stated temperature limits, confirm any physical modification with your local authority having jurisdiction, and take module specifications from datasheets rather than from listings. No figure here is taken from a company selling the equipment it describes.