Protecting Your Caravan GVM: How to Add 3.5 kW of Off-Grid Backup Power Without Blowing Your Tow Ball Weight

A 3.5 kW generator can run your caravan aircon, but every kilogram it adds comes out of a payload budget that is often already thin. Here is how to add power and stay legal.

By Outbax Editorial Team

Adding a generator to a touring setup sounds simple. Buy a 3.5 kW unit, strap it to the drawbar and head bush. The catch is that the generator, its fuel, and the box it rides in all land somewhere on your weight budget, and most Australian rigs have very little of that budget to spare.

Roadside checks show how thin the margin has become. At a 2026 NSW Highway Patrol education day near Appin, officers weighed 44 caravan and tow vehicle combinations. They found 17 vans over their aggregate trailer mass and 16 tow vehicles over their gross vehicle mass, according to What’s Up Downunder. In Victoria, police operations found 57 per cent of weighed caravans were overweight, Caravan World reports.

Vans also keep getting heavier. Federal BITRE data shows about three-quarters of caravans newly registered in 2025 had a tare weight above 2,500kg.

This guide draws on Gentrax manufacturer data and specifications from Outbax, which has sold more than 60,000 Gentrax generators in Australia since 2010. Here is how a caravan generator affects your weights, where to carry it, and how to compare 3.5 kW models by kilograms per kilowatt.

» MORE: How to calculate your generator power needs

 What Do GVM, ATM, GTM, and Tow Ball Weight Mean for a Generator?

Each weight on your compliance plates is a separate legal limit. A generator can push you over one of them while the others still look fine.

Gross vehicle mass (GVM): The maximum your tow vehicle can legally weigh when loaded. It includes passengers, fuel, accessories and the downward load from the caravan coupling.

Aggregate trailer mass (ATM): The maximum your caravan can weigh fully loaded, measured while unhitched.

Gross trailer mass (GTM): The maximum weight carried on the caravan’s own axles when hitched. RACQ notes it always sits below ATM because part of the van’s weight transfers to the tow vehicle.

Tow ball weight: Also called ball download or tow ball mass, this is the vertical load the coupling places on your tow ball. A loaded van typically puts 8 to 12 per cent of its weight there, and that load counts against the tow vehicle’s GVM, not the van’s.

Gross combination mass (GCM): The maximum combined weight of the tow vehicle and caravan together.

Payload is simply the gap between what something weighs and its limit. For the tow vehicle, it is GVM minus kerb weight. For the caravan, it is ATM minus tare.

The key point for generator buyers is that tow ball weight is not fixed. Move 40kg forward or backward inside the van, and the ball figure moves with it.

Where Should You Carry a Caravan Generator?

Where the generator rides decides which limit it eats into. The same 28kg unit can be harmless in one spot and the thing that tips you over GVM in another.

How generator placement shifts load between your caravan and tow vehicle limits.

1. Drawbar and Front Boot

Drawbar mounts keep fumes and noise outside, and the generator is easy to lift down. The catch is leverage. Weight ahead of the van’s axles pushes the ball down, so on a rig near its GVM or tow ball limit, a drawbar box holding a generator and fuel can be a step too far.

2. Rear Bumper

Hanging weight off the back of a van does the reverse. It levers the nose up and lightens the tow ball. Too little ball weight is a recognised cause of trailer sway, which makes a heavy rear-mounted generator the riskiest option even when every plate figure looks legal.

3. Over the Axles or in the Ute

Over the van’s axles is the most neutral position for handling, though the generator must never be run inside. Carrying it in the ute protects the van’s numbers but takes its full weight off the vehicle payload.

Don’t Forget the Fuel

A generator is rarely weighed empty. Petrol weighs roughly 0.74kg per litre, and a full 20-litre jerry can adds about 15kg before you count the can itself.

Estimates based on petrol at approximately 0.74kg per litre.

The Gentrax user manual is clear: drain the fuel before transporting the generator in a vehicle. That cuts weight and removes a spill and fire risk. Carry spare fuel in an approved, secured container.

How Much Does a 3.5 kW Inverter Generator Weigh?

A 3.5 kW inverter generator can weigh from about 28kg to more than 40kg, depending on whether 3.5 kW is its maximum or its rated output. That is the most misunderstood figure on any spec sheet.

Rated Versus Maximum Output

Maximum output is what a generator can supply briefly, such as when an air conditioner compressor starts. Rated output is what it can deliver continuously. A caravan air conditioner runs for hours, so rated output is the number to match.

Gentrax 3.5 kW Specifications from the User Manual

Source: Gentrax Inverter Generator User Manual. Specifications subject to change without notice.

Comparing the Range By Weight Per Kilowatt

Dividing weight by rated output gives a simple measure of how much payload each kilowatt of continuous power costs you.

Weights and outputs from the Gentrax user manual and Outbax product listings, and technical calculations October 2026.

Here’s what one of our customers said about the Gentrax GT3500 Inverter Generator:

“Connected to the caravan and ran the aircon with ease, also started the microwave in conjunction with the aircon running. Great product so far.”

 Which Is the Best Generator for a Caravan Aircon on a Tight Payload?

Gentrax GT3500: The payload saver. At 28kg, it is the lightest 3.5 kW peak option in the range, and its 3.0 kW rated output covers many caravan air conditioners once running. Check your unit’s start load, and consider a soft starter.

Gentrax GTX3500: Adds key and remote start plus a 7.5-litre tank for an extra 9.5kg.

Gentrax G3500: This is not a good option. While its 3.0 kW continuous rated output is mathematically perfect for overcoming the high startup surge of most rooftop caravan air conditioners, its structural build works heavily against a strict weight budget.

Gentrax GTX4200 Pro: The choice when you genuinely need 3.5 kW continuous, such as aircon plus a kettle. It is parallel-capable, has a key, remote, and pull start, and a 36-month warranty. At 41.5kg, it suits rigs with real payload headroom.

» MORE: What will a 3.5kVA generator run?

How to Check Your GVM Payload Before Adding a Generator

You can work out whether a generator fits with a pen, your compliance plates and one trip to a weighbridge.

1. Read Both Compliance Plates

Note your tow vehicle’s GVM, GCM, braked towing capacity, and maximum tow ball mass. On the van, note tare, ATM, GTM, and any tow ball figure. Your working tow ball limit is whichever is lower: the vehicle’s or the van’s.

2. Do the Payload Maths

Here is an illustrative example using a typical dual cab ute. 4X4 Australia uses the same 3,200kg GVM and 2,300kg kerb weight to show a 900kg payload. Your own numbers will differ.

Illustrative example only. Weigh your own rig to confirm.

Both the Gentrax GT3500 and GTX4200 Pro fit within this example's remaining payload, but only just. A full jerry can would eat into that margin further. This is where the decision comes down to the last 50kg of available payload.

3. Weigh the Rig as You Travel

Load up exactly as you would for a trip, generator included, then visit a public weighbridge or mobile weighing service. Portable tow ball weight scales are a cheaper way to check ball download at home. In Queensland, the VanSafe program runs free weighing sessions.

Need help matching output to your appliance list? The Outbax team can talk through your numbers on 02 888 10 333.

Can You Get Off-Grid Caravan Power Without the Weight Penalty?

Yes, by sharing the load between a lighter generator and a power station. A generator does not need to run all night if a lithium power station handles the quiet hours.

The VoltX Topband V1200 Portable Power Station weighs 13kg, stores 1,152Wh in a LiFePO4 battery, and supplies 1,200W continuous with a 2,400W peak. VoltX estimates it can run a 60-litre fridge for roughly three to four days. It recharges in about 1.5 to 2 hours from 240V and accepts up to 550W of solar.

It cannot run aircon for long, so treat it as a partner, not a replacement: generator for afternoon cooling, battery, and solar for everything else. Because Outbax sells both Gentrax generators and VoltX power stations, you can build that mix around your actual weights.

» MORE: Portable power stations

Running a Generator Safely at Camp

The Gentrax manual lists several rules that matter in a caravan setting:

•     Never run the generator in an enclosed or poorly ventilated space. Exhaust fumes can cause carbon monoxide poisoning.

•     Keep it away from canvas and plastic structures, including annexes and tents.

•     Place it on firm, level ground with the exhaust unobstructed.

•     Stay under 75 per cent of rated capacity for the first 25 hours while the engine breaks in.

•     Stop the engine before refuelling and wipe up any spills before restarting.

The 12V DC outlet is for battery charging only, and the manual’s procedure assumes a lead-acid battery. For lithium systems, use a charger designed for that chemistry.

Choosing a Caravan Generator That Fits Your Power and Your Weights

A 3.5 kW generator and a legal rig can coexist. Start with your plates, decide where the unit will ride, compare kilograms per rated kilowatt, then confirm on a weighbridge. On tight payloads, a 28kg unit carried without fuel is the safest path. If you need true 3.5 kW continuous, budget for the extra weight first.

Browse the Outbax inverter generator range, or call the team with your compliance plate figures before you choose.

» MORE: Shop 3.5 kW inverter generators