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Size an inverter properly — continuous rating, surge rating for motor startup, and the DC current it will pull from your battery. The surge figure is what most people get wrong.
Your system
| Measure | Value |
|---|
| Continuous | Typical surge | Suits |
|---|---|---|
| 300 W | 600 W | Laptop, phone charging, small TV |
| 600 W | 1,200 W | Adds a small fridge or blender |
| 1,000 W | 2,000 W | Caravan basics, small power tools |
| 2,000 W | 4,000 W | Microwave, kettle, most household items |
| 3,000 W | 6,000 W | Off-grid cabin, larger tools |
| 5,000 W+ | 10,000 W+ | Whole-home off-grid — 48 V only |
Surge is usually rated for a few seconds only. Check the manufacturer's stated duration.
Every inverter carries two numbers. The continuous rating is what it can supply indefinitely. The surge rating is what it can supply for a few seconds. Buy on the continuous figure alone and the inverter will trip the moment a motor tries to start.
Anything with a motor or compressor draws a large inrush current at the instant of starting — the rotor is stationary and the motor briefly behaves close to a short circuit. It lasts a fraction of a second, but the inverter has to supply it.
| Load type | Starting surge | Why |
|---|---|---|
| Resistive — heater, kettle, toaster | ×1 | No surge at all |
| Incandescent / halogen lighting | ×1.5 | Cold filament briefly draws more |
| LED lighting, electronics, laptop | ×1.2 | Small inrush from power supplies |
| Microwave | ×2 | Magnetron draws above its cooking rating |
| TV, audio equipment | ×1.5 | Capacitor charging on switch-on |
| Fridge / freezer compressor | ×5 | The classic inverter killer |
| Water pump | ×4 | Motor starting under load |
| Air conditioner (non-inverter) | ×4 | Compressor starting current |
| Air conditioner (inverter type) | ×1.5 | Soft-starts by design |
| Power tools — drill, grinder | ×3 | Motor inrush |
| Air compressor | ×5 | Starts against pressure |
Running watts are usually on the appliance label. Starting watts almost never are — which is why this table exists. The last two columns show what each appliance pulls from a 12 V battery through an inverter.
| Appliance | Running | Starting | 12 V running | 12 V starting |
|---|---|---|---|---|
| Refrigerator | 150 W | 750 W | 14 A | 71 A |
| Chest freezer | 120 W | 600 W | 11 A | 57 A |
| Microwave | 1,200 W | 2,400 W | 114 A | 227 A |
| Kettle | 2,200 W | 2,200 W | 208 A | 208 A |
| Coffee machine | 1,200 W | 1,200 W | 114 A | 114 A |
| Toaster | 1,000 W | 1,000 W | 95 A | 95 A |
| TV, 55-inch | 120 W | 150 W | 11 A | 14 A |
| Laptop | 90 W | 99 W | 9 A | 9 A |
| Phone charger | 15 W | 16 W | 1 A | 2 A |
| CPAP machine | 40 W | 48 W | 4 A | 5 A |
| Water pump | 600 W | 2,400 W | 57 A | 227 A |
| Washing machine | 500 W | 1,500 W | 47 A | 142 A |
| Power drill | 600 W | 1,800 W | 57 A | 170 A |
| Angle grinder | 900 W | 2,700 W | 85 A | 256 A |
| Air conditioner (non-inverter) | 1,500 W | 6,000 W | 142 A | 568 A |
| Air conditioner (inverter) | 1,500 W | 2,250 W | 142 A | 213 A |
| Hair dryer | 1,800 W | 2,160 W | 170 A | 205 A |
Starting watts use the surge multipliers above; at 88% inverter efficiency. Your appliance's own specification always beats a table.
Only the largest starting load matters for surge, because two motors almost never start at the same instant. Everything else is treated as steady continuous draw.
| Step | What to do |
|---|---|
| 1. Add up continuous load | Everything that may run simultaneously, in watts |
| 2. Identify the largest starting load | Usually a fridge, pump or air conditioner |
| 3. Apply its surge multiplier | From the table above, or the appliance's own specification |
| 4. Check both against the inverter | Most inverters surge to about twice continuous, for a few seconds only |
| 5. Check the DC current | This determines cable, fuse and whether 12 V is viable at all |
This is the number that decides your whole system architecture, and it is invisible if you only think in watts. An inverter draws its power from the battery at battery voltage — and low voltage means enormous current.
| AC load | 12 V cont. | 12 V surge | 24 V cont. | 24 V surge | 48 V cont. | 48 V surge |
|---|---|---|---|---|---|---|
| 300 W | 28 A | 85 A | 14 A | 43 A | 7 A | 21 A |
| 600 W | 57 A | 170 A | 28 A | 85 A | 14 A | 43 A |
| 1,000 W | 95 A | 284 A | 47 A | 142 A | 24 A | 71 A |
| 1,500 W | 142 A | 426 A | 71 A | 213 A | 36 A | 107 A |
| 2,000 W | 189 A | 568 A | 95 A | 284 A | 47 A | 142 A |
| 3,000 W | 284 A | 852 A | 142 A | 426 A | 71 A | 213 A |
| 5,000 W | 473 A | 1420 A | 237 A | 710 A | 118 A | 355 A |
At 88% efficiency, surge shown at 3× continuous.
This is the practical ceiling on 12 V systems. Above roughly 1,500 W the cable, fusing and connection quality required at 12 V stop being sensible, which is why larger off-grid systems run at 24 V or 48 V. Check what your cable run actually costs you with the voltage drop calculator — inverter cables are short precisely because the current is so high.
This follows directly from the DC current table. Pick the voltage that keeps continuous current at a workable level for the inverter size you need.
| Inverter size | Recommended | Current at 12 V | at 24 V | at 48 V |
|---|---|---|---|---|
| Up to 1,000 W | 12 V | 95 A | 47 A | 24 A |
| 1,000–2,000 W | 24 V | 189 A | 95 A | 47 A |
| 2,000–5,000 W | 48 V | 473 A | 237 A | 118 A |
| Above 5,000 W | 48 V | 758 A | 379 A | 189 A |
Continuous DC current at the top of each band, at 88% efficiency.
Twelve volts remains the right answer for caravans and 4WDs, because the whole vehicle is already 12 V and most appliances have 12 V versions that avoid the inverter entirely. The moment you want to run mains appliances at any scale, higher voltage stops being optional.
A 300 W continuous load — laptop, lights, charging — of which 150 W is a fridge compressor with a ×5 surge, on 12 V.
| Step | Working | Result |
|---|---|---|
| Continuous requirement | Given | 300 W |
| Surge requirement | (300 − 150) + (150 × 5) | 900 W |
| Suggested inverter | Next size up | 300 W continuous |
| Does 2× surge cover it? | 600 W vs 900 W needed | No |
| DC current continuous | 300 ÷ 0.88 ÷ 12 | 28 A |
| DC current at surge | 900 ÷ 0.88 ÷ 12 | 85 A |
A household drawing 4,500 W continuous in total, of which 2,500 W is a non-inverter air conditioner that starts at ×4.
| System voltage | Continuous current | Surge current | Practical? |
|---|---|---|---|
| 12 V | 426 A | 1,136 A | No |
| 24 V | 213 A | 568 A | Marginal |
| 48 V | 107 A | 284 A | Yes |
The surge requirement here is 12,000 W, which needs a 6,000 W inverter surging to 12,000 W. At 12 V that is over a thousand amps for an instant — not a system anyone should build. This is the arithmetic behind the rule that whole-home off-grid runs at 48 V.
| Pure sine wave | Modified sine wave | |
|---|---|---|
| Output | Smooth, matches mains | Stepped approximation |
| Cost | Higher | Lower |
| Motors and compressors | Run normally | Run hotter, less efficiently, shorter life |
| Sensitive electronics | Fine | May buzz, misbehave or refuse to run |
| Medical devices — CPAP | Required | Do not use |
| Variable-speed tools | Fine | Speed control may not work |
| Simple resistive loads | Fine | Fine — heaters, kettles, incandescent lamps |
An inverter consumes power simply being switched on, whether or not anything is plugged in. Typical no-load draw is 8–30 W depending on size, and it runs the whole time the unit is on.
| Standby draw | Left on 24 h | At 12 V | Per year |
|---|---|---|---|
| 10 W | 0.24 kWh/day | 20 Ah/day | 88 kWh |
| 20 W | 0.48 kWh/day | 40 Ah/day | 175 kWh |
| 30 W | 0.72 kWh/day | 60 Ah/day | 263 kWh |
Grid-connected solar inverters are sized differently again, and the governing number is the ratio of panel capacity to inverter capacity.
Clean Energy Council rules permit oversizing the array relative to the inverter by up to 133%. Deliberate oversizing is good practice: panels rarely reach their rated output, so a slightly larger array keeps the inverter working nearer its efficient range through mornings, afternoons, winter and cloud. That limit is precisely why 6.6 kW on a 5 kW inverter became the default Australian residential system.
| Inverter | Maximum array at 133% | Common pairing |
|---|---|---|
| 3 kW | 3.99 kW | 3.96 kW |
| 5 kW | 6.65 kW | 6.6 kW |
| 8 kW | 10.64 kW | 10.5 kW |
| 10 kW | 13.3 kW | 13.2 kW |
Size the array first with our solar panel calculator, then choose the inverter that keeps you inside the ratio. Note that your distributor may separately cap export capacity, which can constrain inverter size regardless of the array.
| Check | What to look for |
|---|---|
| Waveform | Pure sine for anything with a motor, compressor, microprocessor or medical function. Modified sine only for simple resistive loads |
| Continuous rating | Covers everything running at once, with normal operation sitting around 60–80% of rating |
| Surge rating and duration | Peak watts is meaningless without the seconds. "2× for 5 seconds" and "2× for 0.1 seconds" are very different products |
| Efficiency curve | Peak efficiency is quoted at optimal load. Ask what it does at 10–20% load, where many systems actually sit |
| Standby draw | 8–30 W typical. Check whether it has a search or eco mode, and whether your loads will wake it reliably |
| Battery voltage | Must match your bank. Check the low-voltage cutoff suits your chemistry — lead acid and lithium want different thresholds |
| Warranty and support | Two years is common, five is better. Australian support matters when something fails in a remote location |
| Protection features | Over-temperature, over-load, low-voltage, reverse-polarity and short-circuit protection |
| IP rating if exposed | Most inverters are indoor-rated. Under a caravan, in an engine bay or in a boat locker needs a sealed unit |
| Cooling | Fan-cooled units are noisier but handle sustained load better. Fanless units are silent and better suited to intermittent use |
| Type | What it does | Typical use |
|---|---|---|
| Off-grid / standalone | Battery DC to AC, no grid connection | Caravans, boats, remote cabins |
| Grid-tie (string) | Solar DC to AC, synchronised with the grid | Standard rooftop solar |
| Microinverter | One small inverter per panel | Shaded or complex roofs |
| Hybrid | Handles solar, battery and grid together | Solar with battery storage |
| Inverter-charger | Inverts from battery and charges from shore power or a generator | Caravans, boats, backup systems |
An inverter sits between very high DC current on one side and mains-voltage AC on the other. Both sides deserve respect.
A fuse between battery and inverter is not optional. Its job is to protect the cable, not the inverter — an unfused short at these currents will glow a cable red before anything else reacts.
| Inverter at 12 V | Continuous current | Typical fuse |
|---|---|---|
| 300 W | 28 A | 40 A |
| 600 W | 57 A | 80 A |
| 1,000 W | 95 A | 125 A |
| 2,000 W | 189 A | 250 A |
| 3,000 W | 284 A | 400 A |
| 5,000 W | 473 A | 591 A |
Around 1.25× continuous current, rounded to the next standard size. The cable must be rated above the fuse, not the other way round.
What size inverter do I need?
Add up everything that may run at once for the continuous rating, then take your largest motor load and multiply it by its surge factor for the surge rating. A 300 W load including a 150 W fridge compressor at ×5 needs 300 W continuous but 900 W surge — which means a 600 W inverter, not a 300 W one.
What is inverter surge rating?
Surge is the power an inverter can deliver briefly — usually a few seconds — to start motors and compressors. Most inverters surge to roughly twice their continuous rating. Motors draw far more current at the instant of starting than while running, so the surge figure decides whether an appliance will start at all.
Why won't my inverter start my fridge?
Almost certainly surge. A fridge compressor can demand around five times its running wattage for a fraction of a second at startup, so a 150 W fridge briefly needs about 750 W. If your inverter's surge rating is below that it will register an overload and shut down, even though the running load is well within its continuous rating.
How much current does an inverter draw from the battery?
Divide the AC watts by the inverter efficiency and then by battery voltage. A 2,000 W load at 88% efficiency on 12 V draws about 189 A continuously, and around 568 A on a 3× surge. The same load on 48 V draws 47 A and 142 A — which is why larger systems use higher voltages.
What is the difference between pure sine and modified sine?
Pure sine produces a smooth waveform matching mains supply; modified sine produces a stepped approximation. Motors, compressors and sensitive electronics run hotter and less reliably on modified sine, and some refuse to work at all. Modified sine is only really suitable for simple resistive loads such as heaters and work lights. Medical devices including CPAP machines require pure sine.
Can I run a 2,000 W inverter on a 12 V system?
Technically yes, but it is rarely sensible. At 12 V a 2,000 W inverter draws about 189 A continuously and can surge past 500 A, which demands very heavy cable, a high-rated fuse and excellent terminations. Above roughly 1,500 W, 24 V or 48 V is the practical answer.
How much power does an inverter use on standby?
Typically 8–30 W depending on size, drawn continuously whenever it is switched on. At 20 W that is 0.48 kWh or 40 Ah a day at 12 V — comparable to a caravan fridge. Many inverters offer a search or eco mode that sleeps until a load appears; otherwise switch the unit off when not in use.
What is the 133% rule for solar inverters?
Clean Energy Council guidelines allow a solar array to be oversized relative to the inverter by up to 133%. Oversizing is deliberate — panels rarely produce their rated output, so a larger array keeps the inverter closer to its efficient operating range in weak light and winter. It is why 6.6 kW of panels on a 5 kW inverter became the standard Australian residential system.
Will my solar keep working during a blackout?
Not with a standard grid-tie inverter. It is required to shut down when the grid fails, to protect line workers from an energised network. Continuing to run during an outage needs a hybrid inverter, battery storage and a properly configured protected-loads circuit.
How efficient are inverters?
Good units achieve 85–95% at moderate to high load, but efficiency drops at very low loads where fixed overheads dominate. Running a 5 W phone charger from a 2,000 W inverter is very inefficient once the standby draw is counted. Where a 12 V version of an appliance exists, running it directly off the battery avoids the loss entirely.
Do I need a soft starter?
A soft starter ramps a motor up rather than applying full voltage instantly, typically cutting inrush by 50–70%. On large air conditioners and pumps it can allow a considerably smaller inverter, and often costs less than the inverter upgrade it avoids. Inverter-type air conditioners already soft-start by design, which is why their surge multiplier is around ×1.5 rather than ×4.
Can I install an inverter myself in Australia?
A standalone inverter running from a battery with appliances plugged directly into it is extra-low voltage on the DC side and generally not restricted. But anything wiring the inverter's AC output into a building's fixed wiring, a switchboard or a changeover switch is licensed electrical work and must be done by a licensed electrician. Grid-connected solar inverters additionally require a Clean Energy Council accredited installer.
Two numbers decide whether an inverter works: the surge it can deliver for a few seconds, and the DC current it will pull from your battery. Get the first wrong and appliances will not start. Get the second wrong and you have a cable and fusing problem that is genuinely dangerous.
Complete the system with the battery calculator for storage, the solar panel calculator for generation, and the voltage drop calculator for the heavy DC cable an inverter demands.