By Dave Miller · off-grid sparky · 22 years wiring solar, batteries and sheds in central QLD
“After blowing three fuses in my shed, I learned wood gas generators need a proper charcoal bed or they're just expensive firewood burners.”
By Dave Miller · off-grid sparky · 22 years wiring solar, batteries and sheds in central QLD
“After blowing three fuses in my shed, I learned wood gas generators need a proper charcoal bed or they're just expensive firewood burners.”
Look, if you're off-grid in Australia and want reliable power without diesel, a wood gas generator is the real deal. No fancy tech, just wood and a bit of sweat. Here's how it actually works.
A wood gas generator burns biomass in a restricted-oxygen environment to produce a combustible gas that drives an internal combustion engine. It can supplement or replace diesel/petrol gensets where fuel logistics are painful or fuel costs bite. This page covers what a working system looks like, the maintenance you actually inherit, and where it makes sense at a small Australian household scale.
Wood gasification is a two-step process. First, biomass is heated with limited oxygen (pyrolysis), driving off volatile gases and leaving behind charcoal. Second, a small, controlled amount of air passes through that hot charcoal, partially combusting it to produce a mixture of carbon monoxide (CO), hydrogen (H₂), methane (CH₄), and other gases. This producer gas is then filtered, cooled, and fed to an engine.
Filtration is essential to remove tar and particulates before the gas reaches the engine. Standard petrol engines will not run on wood gas without conversion. Common conversions include a gas mixer/air-fuel adapter, reduced compression for some configurations, and ignition timing adjustments. Hardwoods are generally better than softwoods for gasification due to lower resin content, and wet wood produces more tar and is less efficient.
The end result is an engine-and-alternator package that can charge batteries or feed AC loads, with fuel that grows on the property or costs less per kilowatt-hour than bought diesel in many parts of regional Australia.
A typical small stationary wood gas genset has these building blocks:
A layer of coal/ash is typically left on the bottom of the gasifier for heat insulation, preserving core temperature during operation.
A small wood gas engine system generally needs to operate at at least 5 hp to ensure enough airflow through the gasifier without resorting to a very small gasifier requiring pellets or highly processed particulate biomass. Plan around that floor.
For fuel sizing: one bucket of fuel chunks weighs approximately 3.5–4 kg and lasts about one hour. The hopper holds three buckets when fully loaded, though typically two buckets are added when refilling a running engine. It usually takes about half a hay bale to refill the filter when filter maintenance rolls around.
The generator does not require constant babysitting; if everything is running well, it can run for about an hour without intervention, even without automation such as a timer for grate shaking or automatic gas mixing. For longer unattended runs you would want at least basic automation.
| Item | Typical Value / Note |
|---|---|
| Engine power floor | ~5 hp minimum for passive airflow |
| Fuel consumption | ~3.5–4 kg of dry hardwood chunks per hour |
| Hopper capacity | 3 buckets at full load; 2 buckets per top-up while running |
| Filter media refill | ~½ hay bale per filter change |
| Output power for charging | ~2500–3000 W into batteries |
| Battery bank sizing | Large bank required to accept full charge rate |
| Gas cooling requirement | Forced or large passive cooler; condensate will form |
| Gasket material (heat exchanger) | Silicone rubber (regular rubber hardens) |
| Insulation strategy | Coal/ash bed retained on grate |
| Wood preference | Dry hardwood (lower resin than softwood) |
These figures reflect small stationary systems as reported by working builders; treat them as ballpark, not catalogue numbers.
The electrical output of a wood gas genset is not a clean sine wave you can feed directly to household loads. In practice the engine drives an alternator, the alternator charges a battery bank, and a separate inverter supplies 230 V/50 Hz AC to the house.
Battery charging from such a system would be on the order of 2500–3000 watts, which only very large battery banks can accept. Smaller banks either need a charge controller to throttle the field, or the genset runs at reduced throttle.
One workable configuration runs a small wood gas engine system for 3–5 hours per day, using engine power to drive both an alternator (for battery charging) and an automotive A/C compressor to chill water stored in an insulated tank. Automotive A/C compressors are inexpensive and durable, and can deliver impressive cooling performance when paired with larger heat exchangers and good cooling; their stock performance is limited because OEM heat exchangers are compact and poorly cooled in an engine bay. Operating the A/C and battery charging at night (when ambient temperatures are lower for condenser cooling) reduces net battery discharge because a modest solar array can charge during the day.
Roughly two-thirds of the fuel's energy leaves the engine as heat. Capture it rather than letting it cook the shed. Engine waste heat can be used for:
Without an A/C compressor load, the excess shaft energy could be diverted to electricity generation for direct electric heat at roughly 1500–2000 watts, or used to power a modified sine wave inverter running conventional space heaters to draw down the batteries during charging and reduce net charge rate. This is a useful trick for keeping charge current inside the bank's comfort zone.
In cooler months, an A/C system can be reconfigured for heating by placing the condenser inside the home, transferring outdoor heat indoors; this works less well when outside temperatures are very low.
Expect to do these tasks regularly:
1. Filter changes. Half a hay bale per service, drum-swapped in place of the dirty filter.
2. Cooler cleaning. The soot-plus-condensate mix in the cooler is the worst job in the routine. Plan access and drainage before you build.
3. Ash and char handling. A layer of ash/char is left on the grate for insulation; you are removing excess, not all of it.
4. Gasket inspection. Silicone rubber survives in the heat exchanger; other elastomers harden and leak.
5. Charge monitoring. Charging current is commonly monitored at home using a Bluetooth BMS application; if amps decrease, the operator goes to the workshop to check on the system.
One specific battery management note: a Jikong 200A with 1A balancing current works well except for an intermittent current sensor issue that occurs with simultaneous large and nearly equal charge and discharge currents. If your house load is close to your charge rate, expect the BMS to occasionally misreport and verify with a clamp meter before acting on it.
Wood gas is toxic and contains carbon monoxide. Treat the entire gas path — gasifier, filter, cooler, mixer, exhaust — as a hazard zone.
How much dry wood does a wood gas generator use per hour?
For a small stationary unit running hardwood chunks, roughly 3.5–4 kg per hour at the 5 hp floor. Softwoods burn faster but produce more tar and clog filters quicker.
Can a standard petrol genset run on wood gas?
Standard petrol engines will not run on wood gas without conversion. Conversions include a gas mixer, modified air intake, and timing changes; some builders also lower compression.
Is hardwood or softwood better?
Hardwoods are generally better than softwoods for gasification due to lower resin content. Resins condense into tars that foul filters and coke on cool surfaces inside the cooler.
Does wet wood work?
Wet wood produces more tar and is less efficient. Kiln-dry or air-dry to under ~20% moisture for clean running.
How much electricity can a small wood gas genset make?
Battery charging from such a system would be on the order of 2500–3000 watts, so the battery bank needs to be large enough to accept that without tripping BMS limits.
How often do you have to babysit it?
The generator does not require constant babysitting; if everything is running well, it can run for about an hour without intervention, even without automation such as a timer for grate shaking or automatic gas mixing. Add automation if you want longer unattended runs.
Can the engine waste heat be used?
Yes. Engine waste heat can be used for fuel drying, water pasteurisation/processing, space heating, or regenerating a desiccant to reduce latent cooling load in humid climates.
Who has built working heat recovery systems like this?
Ken Boak is cited as an example of someone who built a heat-exchange system capturing heat from cylinders, the gasifier, and engine exhaust.
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