Most inverter shutdowns are not faults but sizing errors that surface months later. Here is the arithmetic that prevents them.
A 12V inverter lets you run 240V appliances from a battery bank when you’re off-grid. But the inverter and battery need to be matched properly, especially when you’re powering high-draw appliances like kettles, microwaves, or coffee machines.
Sizing is the step most people skip. They buy based on the wattage printed on the inverter, connect the unit to whatever battery is already in the caravan, and find the numbers don’t line up. The kettle trips the inverter. The bank reads flat by morning. A season on, the battery holds less charge.
None of that is bad luck. It is arithmetic, and it takes five minutes to do it properly.
» MORE: Complete power guide for camping caravans and RVs
What Sizing a 12V Inverter Actually Means
Sizing means matching three things: the load you run, the current your battery bank can deliver, and the cable and fuse between them. Get one wrong, and the system either shuts down early or quietly damages itself.
Four figures do most of the work.
Continuous watt rating: what the inverter sustains indefinitely.
Surge rating: the short burst it holds while a motor starts.
Usable battery capacity: what you can take out of the bank, not what is on the label.
DC current draw: the amps pulled from the battery to produce those 240V watts.
VoltX 12V 1kW Pure Sine Wave Inverter
Continuous Watt Rating and Surge Rating Are Different Numbers
The continuous watt rating is what an inverter holds for hours without derating. The surge rating is a brief allowance, usually specified in seconds, covering inrush current when a motor starts.
A caravan air conditioner might settle at 1,800W once running but demand two to three times that at start-up unless it has a soft starter. That is why a 2000W inverter 12V unit can run a 1,500W load comfortably and still trip when a 900W compressor kicks in.
Waveform matters too. A pure sine wave inverter produces output that matches grid-quality power, which compressors, CPAP machines, and anything with a microprocessor expect. Modified sine wave units run hotter and make motor-driven appliances work harder for little savings.
Build a Load List You Will Actually Use
Add up only what runs at the same time. Take running watts from the compliance plate, then identify the largest start-up demand. The working rule: pick a continuous rating at least 20% above your simultaneous load, and check that the surge covers your worst motor start.
Work Out What Your Battery Bank Can Actually Give You
Battery amp-hours are a nameplate figure. Usable capacity is what remains once you apply depth of discharge, and the gap is wider than most buyers expect.
Depth of discharge is how far you can draw the bank down before the chemistry or the BMS stops you. AGM gives roughly 50% for sensible cycle life. LiFePO4 gives 80% to 90%.
Usable watt-hours equals amp-hours multiplied by nominal voltage multiplied by depth of discharge. A 100Ah AGM at 12V and 50% yields about 600Wh. An Outbax VoltX 12V 100Ah LiFePO4 Battery at 12.8V and 80% yields about 1,020Wh: same headline number, nearly double the usable energy. A VoltX 12V 200Ah Pro LiFePO4 Battery delivers around 2,050Wh.
Capacity is only half the question. The bank also has to deliver current. Every lithium battery has a maximum continuous discharge rating set by its BMS, and a 2000W inverter, such as the VoltX 12V 2 KW Pure Sine Wave Inverter under load can ask for 180A or more. If the BMS is rated below that, the system cuts out with charge still in the cells.
» MORE: Deep cycle lithium batteries and discharge ratings
How to Size a 12V Inverter in Five Steps
1. Total Your Simultaneous 240V Load
List what runs together, in watts. A 1,800W kettle, a 200W laptop setup and a 60W charger total 2,060W when they’re running at the same time. If the kettle isn’t running, the load drops to 260W.
2. Add Surge Headroom
Take your largest motor load, multiply its running watts by two to three, and confirm the surge rating covers the result. A 900W compressor may need 2,700W for a second or so.
VoltX 12V 2 kW Pure Sine Wave Inverter
3. Convert Watts Into DC Current Draw
This is where undersized systems are made. DC amps equals AC watts divided by battery voltage multiplied by inverter efficiency. A 2,000W load at 12.5V through an inverter running at 88% draws 182A. For a field check, divide AC watts by 10. It overstates slightly, which is the direction to err in.
4. Subtract Efficiency Losses and Idle Draw
Conversion is never free. Good pure sine wave units run at 85% to 92% under load, so every 100Wh at the power point costs about 110Wh at the battery. Idle draw catches more people out: a unit switched on with nothing plugged in still pulls 0.5A to 2.5A. At 1.5A over twelve hours, that is 18Ah, nearly a fifth of a 100Ah bank. Fit a remote switch or a VoltX Inverter LCD Display.
5. Check Runtime Against Usable Capacity
On a 200Ah lithium bank with 2,050Wh usable: three kettle boils a day at 1,800W for three minutes costs about 307Wh; 200W of laptops and lighting for five hours costs about 1,136Wh; idle draw at 1.2A over fourteen hours adds 200Wh. Total 1,643Wh, leaving close to 20% in reserve.
The 12V fridge is deliberately absent because it should run directly off 12V. Converting DC to 240V and back through a plug pack throws away 15% to 20% for no benefit.
Cable and Fuse Rating: The Step That Undoes Good Maths
A correctly sized inverter on undersized cable will still cut out, and the fault looks identical to a flat battery. Voltage drop pulls the input below the low voltage cut-out while the bank is half full.
Size the cable to carry the DC current from step three, holding voltage drop under 3% across the run. For runs under two metres:
1000W inverter: roughly 16mm sq to 25mm sq.
2000W inverter: roughly 35mm sq to 50mm sq.
3000W inverter: roughly 50mm sq to 70mm sq.
Longer runs need the next size up, and the manual overrides any general table. Fuse at roughly 1.25 times continuous draw, putting a 2,000W inverter on a 250A ANL or MEGA fuse within 150mm of the battery positive terminal. The fuse protects the cable, not the inverter, which is why it belongs at the battery end.
One compliance point worth stating plainly. A portable inverter you plug appliances into is yours to install. Any fixed 240V installation in a caravan or camper falls under AS/NZS 3001.2 and must be certified by a licensed electrician.
VoltX 12V 3 kW Pure Sine Wave Inverter
Three Setups and the Inverter Size That Suits Each
The examples below use the VoltX range stocked at Outbax, though the arithmetic applies to any brand.
Weekend Camping and Light Touring
Lights, charging, a laptop, and the occasional small appliance. A VoltX 12V 1 kW Pure Sine Wave Inverter with a 100Ah lithium battery covers this, keeping draw near 90A.
Full Time Touring Caravan
A kettle, a toaster, and a work setup, used one at a time. A VoltX 12V 2 kW Pure Sine Wave Inverter on a 200Ah bank handles it, as long as the BMS clears 180A continuously.
Heavy Draw Touring and Home Backup
Induction cooking, air conditioning with a soft starter, or on-site power tools. An Outbax VoltX 12V 3 kW Pure Sine Wave Inverter needs 300Ah or more behind it. At 3,000W the draw approaches 270A, where many owners move to 24V or 48V.
Putting Energy Back In: Solar, DC-DC, and Generators
Sizing the discharge side only matters if you can replace what you take out. Solar is the default for touring but unreliable in winter or on shaded sites. A DC-DC charger pulls 25A to 50A from the alternator while you drive, which suits people who move camp often. Neither helps when you are parked for a week in poor weather.
That is where an inverter generator earns its place. It recharges the bank in a few hours and runs heavy 240V loads in the meantime. The Outbax GT3500 Inverter Generator delivers 3 kW rated and 3.5 kW maximum, runs 4.1 hours at 50% load, and sits at 62dB at seven metres. The Gentrax GTX4200 Pro Inverter Generator gives 3.5 kW rated and 4.2 kW maximum, enough for air conditioning start-up on most vans. Another 200Ah of lithium usually costs more and still leaves recharging unsolved, so price both ways at Outbax.
Generator safety is not negotiable. Never run one indoors, in a garage or in an annexe. Keep it at least six metres from windows, doors and vents, and refuel only once cooled.
Deye 48V 8KW Hybrid Solar Inverter MPPT Charger Regulator Low Voltage
» MORE: How to calculate your generator power needs
The Short Version
Start with the appliances you want to run, work out their combined load and check the inverter’s surge rating for anything with a motor. Then factor in efficiency losses, idle draw and your battery’s usable capacity to make sure the system can handle the load for as long as you need it.
Finally, make sure the battery, cables and fuse are all rated for the current the inverter will draw. The goal isn’t simply to choose the biggest inverter you can afford. It’s to choose one that your battery bank can safely and reliably support.
Once you know the size you need, explore Outbax’s range of 12V inverters and find an option that suits your setup.
