A clear, repeatable method for working out the right wattage class for your setup, with worked examples drawn from real Australian touring loads, plus a frank look at where most caravanners get the maths wrong.
Every Australian caravan park, free camp, and bush block in the country has at least one tourer who has just discovered that their new inverter generator cannot start the air conditioner. The unit was sold at 2.4 kilowatts. The air conditioner compliance plate said 1.5 kilowatts. On paper, the maths worked. In practice, the generator tripped on overload protection within two seconds of pressing the thermostat.
That conversation, repeated weekly across the Outbax phone line and the Gentrax service inbox, is what this article exists to prevent. It is not a wattage table. It is a four-step sizing methodology used by Outbax product specialists to match a generator to a tourer's actual load profile, with worked examples and the specific Gentrax wattage classes they map to.
Gentrax GS2-7500IE 7.0kW Inverter Generator
Why Sizing Is a Methodology, Not a Guess
Most generator buying advice treats sizing as basic arithmetic. Add up the wattage of everything that might run, add a bit on top, and pick a generator that exceeds the total. The trouble is that running wattage is only one of four numbers that matter, and on its own, it produces the wrong answer for most caravanners.
A reliable method accounts for four variables. The first is rated running wattage, which is what the generator can sustain continuously. The second is surge wattage, the brief spike a generator can handle during compressor startup and motor inrush. The third is your simultaneity factor, the realistic share of appliances actually running at the same time. The fourth is a headroom multiplier for Australian operating conditions: heat, altitude, and engine wear across a five-year ownership window.
Apply those four numbers in order, and the right wattage class becomes obvious, often a class smaller than the buyer first assumed.
Gentrax GT2000 Inverter Generator
The Four Numbers That Actually Matter
Step 1: Build Your Load List
Walk through the caravan with a notebook and write down every appliance you might run on generator power. Reverse cycle air conditioner, fridge in 240 volt mode, induction cooktop, microwave, kettle, hair dryer, water pump, CPAP machine, TV, laptop chargers, and the LED lighting circuit if it runs off 240 rather than 12 volts. Beside each, note the plate rated wattage. The compliance plate is the only source that matters, not the box and not the brochure.
Step 2: Identify Your Surge Loads
Anything with a compressor, motor, or pump has an inrush of three to five times its running wattage for one to three seconds at startup. A 1.5-kilowatt air conditioner can pull four to six kilowatts on compressor startup. A 200-watt fridge compressor can pull eight hundred. Mark each surge appliance and write the surge figure beside it. The largest single surge becomes the floor for your generator's maximum output rating.
Step 3: Apply a Simultaneity Factor
Caravan touring almost never involves every appliance running at once. A realistic figure sits between 0.6 and 0.8: you might run the air conditioner, the fridge, and the lights together, but the kettle, microwave, and induction cooktop tend to be used one at a time. Add up the running wattage of appliances genuinely running together, not your total connected load.
Step 4: Add a 25% Headroom
Petrol engines lose roughly three per cent of rated output for every 300 metres above sea level. Above 30 degrees Celsius, output drops further. Across five years of ownership, expect a small efficiency decline as the engine ages. Twenty-five per cent headroom covers all three with margin to spare.
VoltX VX6600 2-Wire Inverter Generator
A Worked Example: A 19-Foot Caravan with Reverse Cycle Air Conditioning
Take a typical 19-foot van running a 1.5-kilowatt reverse-cycle air conditioning, a 150-watt fridge in 240-volt mode, sixty watts of LED lighting, a 700-watt kettle, an 800-watt microwave, a 30-watt CPAP, and a 50-watt TV.
The largest surge is the air conditioner at roughly 4.5 kilowatts on compressor startup. That sets the generator's maximum output floor at 4.5 kilowatts, and nothing less will do, regardless of how generous the rated figure looks.
The realistic simultaneous load while touring is the air conditioner plus the fridge plus the lights, totalling 1.71 kilowatts. Apply the 25 per cent headroom multiplier, and the required rated continuous output is 2.14 kilowatts.
In plain language, this tourer needs a generator that can run continuously at just over 2.1 kilowatts but spike comfortably to 4.5 kilowatts at startup. A 2.4-kilowatt-rated unit cannot do that. A 3.5-kilowatt-rated unit can. The Gentrax GTX4200 Pro, rated 3.5 kilowatts continuous with a 4.2 kilowatt maximum, is sized exactly for this profile, with 700 watts of surge headroom above the air con's inrush and rated output that covers the working load with room to spare.
Here's what one of our customers said:
“Fantastic product especially if you are a woman who just needs the basics. This is a top quality easy to operate machine. And its pretty. I love how it looks.”
How the Gentrax Range Maps to Caravan Types
The Gentrax range covers four practical wattage classes, and each is sized for a different style of touring.
The 800-watt unit, called the Gentrax GT800 Inverter Generator, at 8.5 kilograms, suits swag campers and small camper trailers, running lights, phone charging, a 12-volt fridge in mains mode, and the occasional laptop. It is too small for any reverse-cycle air conditioning and is honest about that.
Here’s what one of our customers said about this unit:
“I have had this little fella for several years now, and its performance has been faultless. I live in FNQ and my power supply is appalling. For example, the genny has been running for about 25 hours in the last week. I kid you not. So, it has done a lot of hours. I give it regular oil changes and it starts first pull every time. It is all that I need to run the essentials - lights, computer, fridge (inverter motor), and occasionally, the washing machine. This is starting to sound like a paid ad., but its not. At this price, it is an absolute bargain.”
The 2-kilowatt class fits small camper trailers and tent-based tourers who want to run one appliance at a time: a fridge, a kettle, or a small inverter air conditioner built specifically for camper trailer fitment. For example, the Gentrax GT2000 Inverter Generator.
The 3.5-kilowatt class, including the Gentrax GT3500, GTX3500, and TrekGen 3500, is the workhorse for mid-size caravans up to roughly nineteen feet with a single reverse cycle unit and a moderate appliance list.
The 4.2-kilowatt class, represented by the GTX4200 Pro at 41.5 kilograms with wheels and an ergonomic handle, is the right pick for full-size caravans, dual air conditioning setups, food vendor stalls at weekend markets, and tourers who want parallel capability as future insurance. Its specification sheet is honest about the weight trade-off, and the wheel and handle combination means most owners only have to lift it in and out of the tunnel boot, not carry it any distance.
Gentrax G3500 Inverter Generator
Pure Sine Wave and Your Sensitive Electronics
Sensitive electronics are the second variable most caravanners underestimate, after surge wattage. Pure sine wave output is a genuine safety consideration for anyone who sleeps with a CPAP machine, runs a BiPAP, or owns a caravan fitted with a modern variable-speed compressor fridge.
A modified sine wave generator produces a stepped, blocky waveform that mains appliances were never designed to receive. Cheap units sold on classifieds and online marketplaces often hide this in the fine print. The waveform damages CPAP humidifier circuits, makes BiPAP variable pressure motors run hot, causes laptop power supplies to whine or fail prematurely, and can confuse the inverter circuits in modern caravan fridges into reporting false fault codes.
Pure sine wave units, by contrast, reproduce a smooth oscillating waveform almost indistinguishable from grid mains. The GTX4200 Pro is pure sine wave throughout, EURO 5 emissions-compliant, RCM-approved for the Australian market, and EPA-certified. For caravanners with medical equipment on board, that combination of waveform quality and certified compliance is the baseline, not a premium upgrade.
Noise, Parks, and the 65 Decibel Threshold
Most Australian national parks set a generator noise limit of around 65 decibels at a reasonable distance, though the exact figure and measurement method vary between NPWS in New South Wales and equivalent agencies in other states. Some bush camps require generators to be off entirely after nine in the evening. Some popular sites have stricter local limits during the school holiday peak.
The GTX4200 Pro runs at 58 decibels measured at seven metres. That is roughly the volume of a television playing low in the next room, comfortably inside the typical 65-decibel park threshold. The eco throttle setting pulls the figure lower again when the load is light, which is true for most of a typical caravan trip.
Tourers planning extended park stays should still check site-specific rules before arrival. A generator that meets the threshold on paper can still draw complaints if it is sited a few metres from a neighbouring tent. Common sense placement, behind the van and away from neighbours, matters as much as the decibel rating itself.
When Two Smaller Units Beat a Big One
Parallel capability is often listed as a feature. It is more useful to think of it as a sizing strategy.
Two 3.5-kilowatt units in parallel deliver around 7 kilowatts of combined rated output. The buyer gets the option to leave one at home for shorter trips, redundancy if one fails on a remote outback leg, and lighter individual lifting than a single high-capacity machine. A single 4.2 kilowatt unit, by contrast, is simpler to operate, has fewer points of failure, and uses one fuel tank rather than two.
The choice depends on the use pattern. Tourists doing long off-grid trips with full electric setups tend to prefer a single larger unit, because a simpler equipment list matters more on a six-week lap than the redundancy does. Food vendors and event operators who cannot afford downtime lean parallel. The GTX4200 Pro is parallel capable in its own right, so a buyer can start with one unit and add a second later if power demands grow over the ownership window.
The Bottom Line
Sizing a generator with the four-step method takes ten minutes with a calculator and the compliance plates inside the caravan. It saves thousands of dollars in wrong purchases and hours of frustration at the first camp. For most full-size Australian caravans running reverse-cycle air conditioning, the answer lands cleanly in the 3.5 to 4.2 kilowatt class, and the Gentrax GTX4200 Pro sits at the top of that band with the certifications, 36-month warranty, and parts availability that a five-year ownership decision deserves.
