The most common mistake we see with solar quotes isn't a bad inverter or a poor-quality battery — it's the wrong size for the actual household. A system that's undersized leaves you back in the dark by hour three of an outage; one that's oversized means paying for capacity you'll rarely use. Sizing properly starts with your load, not the marketing on a box.

Step 1: List What You Actually Want Backed Up

Be specific rather than vague. "Lights and WiFi" draws very little power. Add a fridge, a few plug points, a TV and a pool pump, and your load grows substantially. Write out every device you want running during an outage — this list is the real starting point, not the size of your house.

Step 2: Understand Watts vs Watt-Hours

Your inverter size (measured in kW) needs to cover the combined wattage of everything running at once — this determines whether the system can handle the load, not how long it lasts. Your battery size (measured in kWh) determines how many hours that load can run before the battery is empty. Both numbers matter, and they solve different problems.

Step 3: Match Inverter Size To Your Peak Load

  • 3kW — Suitable for WiFi, lighting, a TV and general plug points. A common entry point for smaller households or a "keep the essentials on" setup.
  • 5kW — Comfortably covers the above plus a fridge, a pool pump and more simultaneous plug-point use. The most common size we install for family homes.
  • 8kW and above — Needed where multiple high-draw appliances run together, larger homes, or properties combining backup power with daytime solar generation for a geyser or aircon.

Step 4: Match Battery Size To Desired Runtime

Once the inverter can handle your peak load, the battery decides how long you can run it. A 2.5kWh battery might comfortably cover a few hours of essentials. A 5–10kWh bank extends that toward a full overnight outage on a heavier load. There's no universally "right" answer here — it comes down to how long your outages typically run and your budget.

Why Battery Chemistry Matters

LiFePO4 (lithium iron phosphate) batteries have become the standard for good reason: they handle deep daily discharge without the rapid degradation older lead-acid batteries suffer, generally last several times longer in cycle count, and need essentially no maintenance. Lead-acid is cheaper upfront but usually works out more expensive over the system's lifetime once replacement costs are factored in.

Sizing solar isn't about buying the biggest system you can afford — it's about matching the system to a load you've actually measured, not guessed at.

A Simple Way To Start

Rather than guessing, note your typical load-shedding stage and how many hours you're without power on a normal day. Combine that with your device list from Step 1, and a proper installer can size an inverter and battery combination that actually matches your household — not a generic package.

Don't Forget The DB Board

A correctly sized inverter and battery still need to be integrated into your distribution board safely, with the correct isolators and circuit separation. This is electrical work that should be signed off with a Certificate of Compliance — not just bolted on by whoever sold you the panels.