By Dave Miller· off-grid sparky · 22 years wiring solar, batteries and sheds in central QLD
“After frying my 5kW inverter trying to start a 2.2kW pump on a cloudy Queensland morning, I learned you must oversize for surge, not just running watts.”
Off-Grid Inverter Sizing: No Bullshit Guide for Aussie Homesteaders
Stop guessing. Stop overpaying. This is how you size an off-grid inverter right. No tech jargon. Just what you need to know to avoid frying your gear or blowing your budget.
Stop guessing. Stop overpaying. This is how you size an off-grid inverter right. No tech jargon. Just what you need to know to avoid frying your gear or blowing your budget.
Step-by-step inverter sizing flow with a worked example and common mistakes to avoid.
Continuous vs Peak Power: The Only Two Numbers That Matter
Your inverter has two power ratings. You need both:
Continuous power: What it runs all day. This is your real number. If your fridge runs at 200W continuously, your inverter must handle 200W+.
Peak power: The surge when things start up (like a fridge compressor kicking in). Usually 2-3x continuous. But don't size for peak alone.
Blunt truth: If your inverter's continuous rating is too low, it'll shut down at 3pm when the solar's low. If it's too high, you're paying for useless capacity. Size for continuous, check peak.
Power Factor: Why It Doesn't Matter (Most of the Time)
Manufacturers throw "power factor" at you like it's a magic number. Here's the reality:
Most Aussie off-grid systems use resistive loads (heaters, incandescent lights, basic fridges). These don't care about power factor. Your inverter can handle them at 100% of its continuous rating.
If you have inductive loads (like old AC units or compressors), power factor might matter. But even then: just buy an inverter rated 20% higher than your continuous load. Done. Stop overcomplicating it.
Practical tip: Add up all your continuous loads (fridge, lights, pump, etc.). Then add 20% buffer. That's your minimum continuous rating. Example: 500W continuous load → get a 600W+ inverter.
Australian Suppliers & Real Pricing (indicative prices)
Don't waste money on overpriced "premium" brands. Here's what actually works:
Renogy (Australian stock, no import delays) 12V 1000W Pure Sine Wave Inverter
Price: $599-$699 Why it works: Handles 1000W continuous (perfect for 800W loads with buffer). No power factor drama. Sold at renogy.com.au and energysave.com.au.
Outback Power (for serious off-grid) FX2824 24V 2.8kW Inverter
Price: $3,200-$3,800 When to use: If you have 2kW+ continuous loads (e.g., big fridge + water pump + AC). Overkill for most Aussies. Skip unless you're running a full homestead.
Example 1: Tiny off-grid shed (no fridge)
Lights (50W) + Phone charging (20W) + Small pump (100W) = 170W continuous. Get: Renogy 1000W inverter ($650). Overkill? Yes. But it'll last 10 years. Cheaper than replacing a fried inverter.
Example 2: Full off-grid house (common Aussie setup)
Fridge (200W) + Water pump (300W) + Lights (100W) + Laptop (100W) = 700W continuous. Get: Renogy 1200W inverter ($750). 20% buffer = 840W. Perfect.
Final warning: If your inverter shuts down when the sun goes down, you sized it wrong. Always size for continuous load + 20% buffer. Not peak. Not "maybe". Continuous.
⚠️ SAFETY WARNING: Working with electrical systems, structural modifications, or gas installations carries inherent risks. If you are not confident in your abilities, always engage a licensed professional.
The Brooke Brothers break down a real 10 kW off‑grid install in the QLD bush, showing exact material costs and wiring layout, which ties straight into the inverter‑sizing calculations I lay out in the guide.— Dave Miller
When to Call a Professional
While many off-grid projects are achievable as DIY, certain situations require licensed professionals:
Electrical work beyond basic 12V DC additions — requires a licensed electrician
Structural modifications to buildings or load-bearing elements
Gas line installation or modification
Solar array installations above safe voltage thresholds
Any work that affects the structural integrity of your property
Always check local regulations and obtain necessary permits before commencing work.
