A rooftop air conditioner turns a camper trailer from a hot box into somewhere you can actually sleep in January. Running one away from a powered site is harder. Your generator has to survive the compressor's start surge, keep up through a 40-degree afternoon and still be light enough to lift into the ute.
So, is the 28kg Gentrax GT3500 enough to run your camper trailer air conditioner off-grid? In many cases, yes. If your air conditioner has a soft starter and runs on its own, the Outbax Gentrax GT3500 (3.0 kW rated, 28kg) will usually do the job. If you cool overnight, run a kettle at the same time, or camp in serious outback heat, the Gentrax GTX4200 Pro (3.5 kW rated, 4.2 kW peak) offers more headroom.
We compared both using the Outbax product listings and the Gentrax user manuals, so you can choose a generator for caravan air conditioner duty that suits your setup rather than the brochure.
» MORE: Best portable generator for caravan air conditioning
How Much Power Does a Camper Trailer Air Conditioner Actually Draw?
Most 240V rooftop units fitted to camper trailers and hybrids draw somewhere between 1,000W and 2,500W once running. Outbax uses about 2,000W as its reference figure for a 2.5hp caravan air conditioner. The bigger challenge is the first second after the compressor switches on.
Starting Watts vs Running Watts
An air conditioner's compressor is driven by a motor. When a motor starts, it briefly pulls several times its normal running current. That's why every generator lists two output figures.
• Rated output: the power the generator can supply continuously, hour after hour.
• Max output: the short peak it can deliver while a motor gets up to speed.
If the start surge exceeds max output, the generator's overload protection cuts power. Both Gentrax manuals note that the red overload light may glow for around four seconds as an inductive load starts. That's normal. If it stays on, the load is too big.
Check Your Unit's Compliance Plate First
Dometic, Truma and other brands publish running current and power figures for each model, and the numbers vary widely. Check the compliance plate on your unit or the manufacturer's spec sheet before you buy anything. If only amps are listed, multiply running amps by 240 to estimate running watts.
What Soft Start Changes
A soft start module ramps the compressor up gradually instead of all at once. That trims the start surge sharply, and it's often the difference between a 3 kW class generator coping and tripping. Many newer rooftop units have soft start built in. Older units can usually be retrofitted by an air conditioning technician.
» MORE: How to calculate your generator power needs
GT3500 vs GTX4200 Pro: How the Specs Compare
Both are pure sine wave inverter generators, so both deliver clean power for the electronic control board inside a modern rooftop unit. The differences come down to capacity, weight, fuel, and convenience. The table below draws on the Gentrax user manuals and the current Outbax listings.
Table 1. GT3500 vs GTX4200 Pro specifications
| Specification | Gentrax GT3500 | Gentrax GTX4200 Pro |
|---|---|---|
| Max output | 3.5 kW | 4.2 kW |
| Rated output | 3.0 kW | 3.5 kW |
| Waveform and voltage | Pure sine wave, 240V | Pure sine wave, 240V |
| Engine | 149cc, air cooled, 4 stroke | 223cc, air cooled, 4 stroke OHV |
| Fuel and tank | 91 RON unleaded, 5.7L | 91 RON unleaded, 8.8L |
| Run time (manual) | 6.2 hours at 50% load | 4.5 hours at 100% load |
| Run time (Outbax listing) | 4.1 hours at 50% load | 6.8 hours at 50% load |
| Engine oil | SAE 10W30, 0.9L | SAE 10W30, 0.7L |
| Starting | Recoil (pull) start | Recoil, key and remote (10m) |
| Noise at 7m | 62dB | 58dB |
| Net weight | 28kg | 41.5kg |
| Parallel capable | Not listed in manual | Yes, same model only |
| Warranty | 12 months | 36 months |
| Price | $899 | $1,449 |
Sources: Gentrax GT3500 and GTX4200 Pro user manuals; Outbax product listings, October 2026. Specifications are subject to change.
Here’s what one of our customers said about the Gentrax GT3500 Inverter Generator:
“Absolutely thrilled - keeps our caravan's aircon running all night off-grid, I slept like a baby super quiet! and even powers the creature comforts like a barista-style coffee in the morning!”
Output and Headroom
The GTX4200 Pro gives you 500W more continuous power and 700W more peak power. With a 2,000W running load, the GT3500 is working at about two thirds of its rated output. The GTX4200 Pro sits at just over half, which leaves room for a second appliance and for hot days, when compressors work harder.
Table 2. How much rated output a running load uses (illustrative)
| Running load | GT3500 (3.0 kW rated) | GTX4200 Pro (3.5 kW rated) |
|---|---|---|
| Aircon alone, 2,000W | 67% | 57% |
| Aircon plus 500W of other loads | 83% | 71% |
| Aircon plus 1,000W of other loads | 100% (at limit) | 86% |
| Break-in limit (75% of rated) | 2.25 kW | 2.63 kW |
Based on the 2,000W caravan air conditioner reference on the Outbax GT3500 listing. Check your own unit's figures.
Here’s what one of our customers said about the Gentrax GTX4200 Pro Inverter Generator:
“This inverter generator well exceeded my expectations with the recent bad weather. It depended on providing power and it certainly did.”
Weight, Fuel, and Run Time
The GT3500's 28kg weight is an important selling point for campers who move around. Most adults can lift it into a ute tray or drawbar box without help. The GTX4200 Pro weighs 41.5kg. It rolls on wheels, which helps on flat ground, but you'll want two people to lift it.
Run time figures need care. The manuals and listings measure them differently, as the table shows. Plan around the lower figure: about four hours per tank for the GT3500 and close to seven hours for the GTX4200 Pro at half load.
Starting, Noise, and Warranty
The GT3500 uses a pull start. The GTX4200 Pro adds key and remote start with a 10m range, so you can restart the aircon from bed. It's also quieter on paper, at 58dB at 7m compared with 62dB, and carries a 36-month warranty against 12 months.
Which Generator Suits Your Camper Trailer Setup?
Choose the GT3500 If:
1. Your rooftop unit has a soft start.
2. You mainly cool in the afternoon and evening, then switch off.
3. You run little else while the aircon is on.
4. Weight and storage space matter most.
At $899, it also leaves budget for a battery. Owner experience backs this use. One verified reviewer reported running a caravan aircon and a microwave together on a GT3500. Another said it ran a 240V fridge and aircon for three days in 37-degree heat.
Choose the GTX4200 Pro If:
1. You want to cool through the night.
2. Your unit has no soft start, or you aren't sure.
3. You boil a kettle, use a microwave, or charge batteries while the aircon runs.
4. You regularly camp in 35 to 45-degree heat.
The extra $550 buys headroom, a bigger tank, and remote start. For many hybrid owners, that's the difference between a comfortable night and a 2 am restart.
Middle Options and Bigger Setups
The Outbax Gentrax GTX3500 sits between the two. It offers 3.2 kW rated output, a 7.5L tank, and remote start; weighs 37.5kg and costs $1,199. If you run two air conditioners, the GTX4200 Pro is parallel-capable. Its manual says a linked pair of the same model delivers 90% of their combined rated output, or about 6.3 kW. The Gentrax GTX6600 offers 6 kW rated output, but at 89kg, it suits a worksite better than a camper trailer.
» MORE: 3.5kVA generator: what will it run?
Running Rooftop Aircon Off a Generator in Australian Free Camping Conditions
The Gentrax manuals contain several rules that matter for air conditioners. Many owners never read them. Here are the ones to know before your first trip.
Table 3. Manual guidance that matters for aircon
| Topic | What the Gentrax manuals say |
|---|---|
| Eco mode | Switch the economy control off for appliances with a large starting current, such as compressors (GTX4200 Pro). |
| Break-in | For the first 25 hours, don't exceed 75% of rated output, then change the oil (GT3500). |
| Start surge | Red and green lights together for about four seconds as a motor starts is normal. If red stays on, reduce the load. |
| Overload reset | Reduce the load, then hold the reset button for two seconds to restore output (GTX4200 Pro). |
| Refuelling | Don't refill while the engine is running or hot. Close the fuel cock first. |
| Placement | Keep away from canvas and plastic structures (GT3500) and at least 1m from other equipment (GTX4200 Pro). |
| Oil | Check before every use. Low oil triggers automatic shutdown. Use SAE 10W30. |
| Remote storage | Turn the engine switch to STOP when stored, or the remote receiver drains the starter battery (GTX4200 Pro). |
Sources: Gentrax GT3500 and GTX4200 Pro user manuals.
Eco Mode and the Break-in Period
Eco mode lets the engine slow down to match the load. That saves fuel and noise, but it can struggle when a compressor kicks in. The GTX4200 Pro manual says the economy switch must be off for appliances with a large starting current, such as a compressor. Some owners report running aircon in eco mode once it settles. Start with eco off and only experiment once you know your setup.
The first 25 hours also matter. During break-in, the manual says not to exceed 75% of rated output. On the GT3500, that's about 2.25 kW, which leaves little room beyond the aircon on your first trip. Change the oil once the break-in is done.
Placement and Carbon Monoxide Safety
Never run a generator inside a camper, annexe, or tent. Exhaust contains carbon monoxide, which can kill within minutes. The GT3500 manual warns against placing the generator near canvas or plastic structures, and the GTX4200 Pro manual asks for at least 1m of clearance. Set it on firm, level ground with the exhaust pointing away from your canvas and your neighbours. Don't run either unit in rain.
Fuel Planning for 91 RON and Outback Heat
Both generators run on 91 RON unleaded petrol. The manuals say not to refuel while the engine is running or hot. In practice, an overnight run on the GT3500 will likely mean a shutdown, a cool down and a refill. Carry fuel in approved containers and allow extra for hot weather, when the aircon cycles more often.
Before you set up in a national park, check generator rules with the managing agency, such as NSW National Parks and Wildlife Service. Rules vary between parks and campgrounds. If you're unsure which unit suits your aircon, the Outbax support team in Sydney can talk it through on 02 888 10 333.
Pairing Your Generator with a Power Station for Quiet Hours
What a Portable Power Station Can and Can't Run
A portable power station is silent and produces no exhaust, but most can't sustain a 240V rooftop compressor. The VoltX Topband V1200 offers 1,200W continuous output and 1,152Wh of LiFePO4 capacity. That suits a fridge, lights, and devices, not a rooftop unit.
Building a Hybrid Camper Power Setup
Run the generator through the hottest hours and use that time to recharge the V1200, which accepts up to 700W from AC power. Switch the generator off during quiet hours and let the power station carry your essentials. Adding solar panels tops it up during the day and cuts generator hours further.
» MORE: Browse portable power stations
The Bottom Line: Match the Generator to Your Aircon, Not the Brochure
For a single soft start unit and light travel, the GT3500 is the value pick. For overnight cooling, stacked loads, or summer in the outback, the GTX4200 Pro earns its extra weight. Check your unit's running watts first, then compare both across the Outbax inverter generator range.
