A generator rated well above your caravan can still stall the moment the air conditioner starts. Here is why the label lies, and how to size for the load you really pull.
You buy a portable generator rated comfortably above what your caravan should need. You plug in on the first hot afternoon, switch on the air conditioner, and the whole thing stumbles. The engine surges, the overload light flicks on, and you are left sweating over a machine that looked more than big enough on paper. It is one of the most common complaints we hear from Australian caravanners, and it almost always comes back to the same thing. The number printed on the box is not the number your setup draws.
This guide explains why that gap exists, what a caravan really pulls once the air conditioner and the kettle fight over the same supply, and how to size an inverter generator for caravan use so it copes on the days that matter. Every Gentrax unit sold through Outbax lists both its maximum and its rated output, so you can judge the honest figure before you spend a cent.
Rated Watts, Maximum Watts, and the Number You Actually Get
Most sizing mistakes start with one misread figure. A portable generator carries at least two power ratings, and buyers almost always anchor on the larger one.
Starting Watts Versus Running Watts
Running watts, sometimes called rated output, is the power a generator supplies continuously, hour after hour. Starting watts, or maximum output, is a short burst it can hold for a few seconds while a motor spins up. The two figures can differ by 15 to 20 per cent. The Gentrax GT2000 Inverter Generator is a clear example. It peaks at 2.0 kW, but its rated output is 1.7 kW. The 1.7 kW figure is the one you live with every day.
Here’s what one of our customers said about the GT2000 Generator:
“Great product. I bought this to power our caravan aircon while we are off grid. 2kva is just enough to power our Dometic harrier unit which has a soft start up and is roughly about 1800kwh, unsure how many hours it’ll run for on a full tank of fuel yet. Really happy with the purchase so far.”
Kuller 4200 Inverter Generator
What Surge Load Means at the Moment of Startup
Anything with a motor, a fridge compressor, a water pump, an air conditioner, demands a surge load when it kicks in. That inrush can be two to three times the appliance's running draw for a heartbeat. If the generator cannot cover the combined surge, it trips before it ever settles into steady running.
Why the Headline Is the Maximum, Not the Everyday Figure
Marketing leads with maximum output because it is the bigger number and it sells. For daily caravan life, you should size around rated output and treat the peak as emergency headroom only. Read the two figures on any unit and quietly ignore the larger one when you do your sums.
What a Caravan Really Draws, Appliance By Appliance
A caravan rarely runs one appliance at a time, and the totals climb faster than most people expect.
Caravan Air Conditioner Power Draw and Startup Surge
The rooftop air conditioner, similar to the Cold King 12V Rooftop Air Conditioner, is the heaviest load on board and the usual culprit behind a tripped generator. A common 13,500 BTU unit draws roughly 1,300 to 1,600 watts once running, but its compressor can spike past 2,500 watts at startup unless it has a soft starter fitted. A larger 15,000 BTU unit pushes those numbers even higher.
Stacking Appliance Load
The air conditioner seldom runs alone. Add a 1,200-watt kettle, a near-1,000-watt microwave, a compressor fridge, a battery charger, and a couple of phone chargers, and appliance load stacks up in seconds. Even two or three items overlapping for a minute can push a modest generator past its limit.
The BTU Reality for RV Air Conditioners
As a rough guide, budget around 2,000 watts of running capacity for the air conditioner alone (including surge), then add whatever else you expect to run at the same time. The Outbax G3500 Inverter Generator, rated at 3.0 kW with a 3.5 kW peak, leaves plenty of room for a single air conditioner and a few small loads.
Here’s what one of our customers said about this generator,
“I bought this as a power backup for my home.
Big enough to run the essential appliances, fridge, TV, PC, fans, lamps, etc. Not too heavy at 32kg net; Would be nice if it had wheels, using a Dolley trolley to move it around.
I'm happy, thanks.”
» MORE: How to choose a portable generator for caravan air conditioning
TrekGen 3500 Inverter Generator
The Hidden Losses: Power Factor, Derating, and Your 15-Amp Lead
Even a correctly sized generator can underdeliver, because three quiet factors eat into the number on the label.
Power Factor and Why kVA Does Not Equal Usable kW
Generators are often rated in kVA, but appliances consume kW, and power factor links the two. A reactive load such as an air conditioner compressor may run at a power factor of 0.7 to 0.8, so a 2kVA generator, like the Gentrax GT2200 Pro Inverter Generator, might deliver only about 1.6 kW of usable power. Always convert kVA to kW before comparing a generator to your load, or you will overestimate what you bought.
Derating in Heat, Altitude, and a Full Load
Engines lose output as conditions get tougher. Available power falls in high ambient temperatures and at altitude, both common on an Australian summer trip inland. This derating means a generator that looked strong on a cool bench test has less to give on a 38-degree afternoon, which is exactly when the air conditioner is working hardest.
The 15-Amp Lead Ceiling
Your lead also sets a hard limit. A standard 10-amp lead caps supply near 2,300 watts, while a 15-amp lead lifts that to roughly 3,450 watts at 230 volts. If your van is wired for a 15-amp inlet, matching the lead to the generator matters. For clean, stable power to sensitive onboard electronics, the Gentrax GT2200 Pro Inverter Generator delivers a pure sine wave output that will not upset a laptop, a television, or a modern fridge.
How to Size a Generator to Your Real Caravan Load
Once you understand the losses, sizing becomes a short, repeatable exercise rather than a guess. Work through three steps.
- Add the running watts of everything you expect to use at the same time.
- Add the single largest startup surge on top, which is usually the air conditioner.
- Add a buffer of about 30 per cent for derating, power factor and general headroom.
Matching Capacity to Your Setup
A weekender running lights, a fridge, and chargers is well served by a compact unit like the Gentrax GT2000 Inverter Generator. A caravan with an air conditioner needs more. The Gentrax GT3500 Inverter Generator and the electric-start Outbax GTX3500 Inverter Generator both offer 3.5 kW peak output with strong rated figures, enough for an air conditioner plus everyday loads without living on the edge.
Gentrax GS2-5000IE 4.5kW Portable Inverter Generator
Choosing a Quiet, Fuel-Efficient Inverter Generator for Camping
Noise and running cost matter at a caravan park. An inverter generator adjusts its engine speed to match demand, which saves fuel and keeps noise down to around 58 to 62 decibels on these Gentrax models, close to a normal conversation. The full Outbax portable generator range lists rated and maximum output side by side, so you can compare on the honest figure rather than the marketing one.
» MORE: Generator buying guide- what to consider before you buy
When One Generator Is Not Enough: Running in Parallel
Some setups draw more than a single portable unit can supply, especially caravans with a large air conditioner or two.
How a Parallel Kit Combines Two Units
A parallel kit links two compatible inverter generators and combines their output through one heavy-duty outlet. Two light units are far easier to lift and pack than one large machine; they stay quieter, and you can run just one when demand is low to save fuel. If a unit ever needs servicing, you still have power on the other.
A Parallel Pair Versus One Large Unit
The alternative is a single high-output generator. The Gentrax GTX6600 Inverter Generator supplies 6.0 kW rated and 6.6 kW peak, ample for a large air conditioner and the rest of the van at once. Outbax stocks compatible parallel kits and leads for both approaches, so the decision comes down to weight, budget and how you like to travel.
Gentrax GT800 Inverter Generator
The Bottom Line
The lesson is simple enough. Match a generator to the load your caravan actually pulls, running watts plus the biggest surge plus a sensible buffer, rather than to the confident number on the box. Do that once and you avoid both the overload light and the cost of buying twice. Compare the rated and maximum output on each unit in the Outbax portable generator range, and size for the hot afternoon when you will need it most.
