Most people who walk into the wrong-sized inverter generator do one of two things. They oversize for fear of tripping the unit, then haul a heavy machine they never fully use. Or they underbuy on price, then discover the caravan air conditioner refuses to start on the first 35-degree day.
Both mistakes are avoidable with about ten minutes of arithmetic. At Outbax, we have spent years helping Australian campers, caravanners and homeowners size their first generator, and the questions we hear most often boil down to the same calculation. This guide walks through it.
How to Calculate the Inverter Generator Size You Actually Need
The single biggest mistake buyers make is sizing based on rated wattage alone. Anything with a motor or compressor draws far more power for a fraction of a second when it kicks on, and that surge is what trips small generators.
Running Watts Versus Starting Watts
Running watts is the steady draw an appliance pulls once it is up and going. Starting watts, sometimes called peak or surge wattage, is the brief spike at switch-on. For a fridge, a caravan air conditioner, or a power tool, the starting figure can be two to three times the running figure. Your generator has to cover the running load of everything switched on, plus the largest single surge happening at the same time.
A Simple Three-Step Load Calculation
The method is straightforward:
- List every appliance you want to run at the same time.
- Add up the running watts of all of them.
- Add the largest single starting surge to that total.
That final number is the minimum continuous wattage your inverter generator needs to deliver. Then add roughly 20 to 30 per cent on top as headroom. A unit running flat out at 100 per cent load wears faster, runs louder and burns more fuel. Aim for the 70 to 80 per cent zone at peak draw.
Gentrax GT800 Pro Inverter Generator
Australian Appliance Wattage at a Glance
Use this as a starting point. Always check the data plate on your own appliances for accurate figures.
| Appliance | Running watts | Starting watts |
|---|---|---|
| 200L caravan fridge (240V) | 150 | 450 |
| Reverse cycle caravan air con (2.5kW) | 600 | 1,800 |
| Domestic kettle | 2,200 | 2,200 |
| Microwave (800W output) | 1,200 | 1,500 |
| LED lighting (full van) | 30 | 30 |
| CPAP machine | 60 | 90 |
| Laptop charger | 65 | 65 |
| 1,500W induction cooktop | 1,500 | 1,500 |
What Size Generator Do You Need for a House?
For most Australians, the home backup question is really a storm season question. You do not need to power the whole house. You need to keep the essentials going until the grid comes back.
Sizing for Blackout Backup (Essentials Only)
The fridge, freezer, lights, modem, phone chargers, and a single fan or small heater will sit comfortably under 2,500 running watts. A 3kVA inverter generator handles this with room to spare and is the most popular choice for short outages.
Sizing for Whole of Home or Extended Outage
If you want to add a microwave, kettle, washing machine or a small split system air conditioner into the mix, you are looking at 4kVA to 5.5kVA territory. Larger Australian homes with multiple people staying through a multi-day outage typically size up to a 5.5kVA to 6kW unit.
When 3kVA Is Enough and When It Is Not
3kVA covers the basics for one or two people. It struggles the moment you ask it to run a kettle and a heater together, or start a 2.5kW air conditioner. If your blackout plan includes any of those, jump up a tier.
Gentrax GTX3500 Inverter Generator
What Size Generator Do You Need for a Caravan?
Caravan sizing comes down to one question: Do you have an air conditioner, and do you actually run it?
The Caravan Air Conditioner Test
A typical 2.5kW reverse cycle caravan air conditioner draws around 600 running watts but surges to 1,800 watts at startup. Add a fridge, lights, and a couple of phone chargers, and you are sitting near 2,500 watts at peak. That is why 3.5kW rated is the practical floor for any caravan with AC.
Weekend Camping Versus Full-Time Off-Grid
Weekend campers running lights, a small fridge, and device chargers can manage on a 1kW to 2kW unit. Families touring full-time, with a caravan air conditioner, induction cooktop, washing machine and multiple devices on charge, are realistically in 4kW to 5.5kVA territory.
Caravan Park Noise Rules and How They Affect Sizing
Most Australian caravan parks set generator noise limits around 65 decibels measured at seven metres. That rules out larger open frame generators and pushes most buyers toward quieter inverter models in the 2kVA to 3.5kW range. Going bigger is only worthwhile if you are free camping where the rules do not bite.
Gentrax GTX4200 Pro Inverter Generator
Inverter Generator Features That Change Your Sizing Decision
Pure Sine Wave for Sensitive Electronics
Modern electronics, including CPAPs, laptops, induction cooktops, and smart TVs, need clean, stable power. A pure sine wave inverter generator delivers that. Outbax stocks pure sine wave across the entire Gentrax range for exactly this reason. Without it, you risk damaging the gear you bought the generator to power.
Eco Mode and Fuel Burn at Part Load
Most quality inverter generators have an eco or economy mode that throttles the engine down when the load is low. This is where correct sizing pays back: a properly sized unit spends most of its time at part load, burning less fuel, running quieter and lasting longer per tank.
Pairing with Solar to Size Down Safely
If your caravan has solar panels and a decent battery bank, the battery handles brief surges, and the generator only tops up the system. That often lets buyers size down by one tier and still cover the same loads.
Gentrax GT2000 Inverter Generator
Matching Your Sizing Result to the Right Outbax Inverter Generator
Once you have your number, here is where each Gentrax model fits in the Outbax range.
Solo and Ultralight Camping (Under 1,000 Watts of Draw)
- Gentrax GT800 Inverter Generator — the lightest option, suited to a single camper running lights, charging devices and a small fan.
Here’s what one of our customers said:
“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 it’s not. At this price, it is an absolute bargain.”
Weekend Camping with a Small Fridge (1,500 to 2,000 Watts)
- Gentrax GT2200 Pro Inverter Generator — the everyday workhorse for most weekend campers, easy to lift and quiet enough for most sites.
Caravan Park Stays Where Noise Rules Apply (2,000 to 2,500 Watts)
- Gentrax GT2500 Inverter Generator — the noise-rated option for buyers prioritising caravan park compliance.
Single AC Caravans and Most Home Backup (2,500 to 3,500 Watts)
- Gentrax GTX4200 Pro Inverter Generator — the most common caravan choice, comfortably starts a 2.5kW air conditioner.
Here’s what one of our customers said:
“Amazing unit, was surprised how economical it was and covered all our camp out needs. Best 2026 purchase thus far.”
Larger Caravans and Extended Home Outages (4,000 to 5,500 Watts)
- Gentrax GTX6600 Inverter Generator — for full-time tourers, large Australian homes and any setup running multiple appliances simultaneously.
All five sit within the Outbax inverter generator collection, so you can compare specs side by side once you know which capacity tier suits you.
Get the Size Right and the Rest Is Easy
Three numbers do the work: total running watts, the biggest single startup surge, and a 20 to 30 per cent headroom buffer. Add them together, match the result to a capacity tier, and the model choice falls out naturally.
The Outbax inverter generator collection is built around exactly these tiers, and our team is happy to talk through the specifics if you want a second opinion before you buy. Browse our range today and choose the best unit for your next outdoor adventure.
