One of the biggest mistakes I see people make is either undersizing their solar system (trying to save money) or oversizing it (because bigger must be better, right?). The right size depends on your specific circumstances, and calculating it isn’t rocket science.

Here’s my step-by-step guide.

Step 1: Understand your consumption

Pull your last 12 months of electricity bills and find your daily average consumption in kWh. Most bills show this somewhere. If not, your distributor’s website usually has detailed usage data.

Typical ranges for Australian households:

• 1-2 person household: 8-15 kWh/day
• Family of 3-4: 15-25 kWh/day
• Large family with pool: 25-40 kWh/day
• All-electric home (no gas): Add 5-10 kWh/day for hot water and cooking

Write down your daily average. Mine is about 18 kWh/day (family of three, all-electric home, no pool).

Step 2: Determine your self-consumption goal

This is a philosophical question as much as a technical one:

Option A: Offset most of your daytime consumption. A smaller system (5-8kW) that produces roughly what you use during daylight hours. Good financial return because most generation is self-consumed. Low export reliance.

Option B: Offset your total daily consumption. A larger system (8-13kW) that generates enough over the day to cover 24-hour consumption. Without a battery, you’ll export heavily during the day and import at night. With a battery, you can achieve near-zero grid reliance.

Option C: Maximum generation. Fill your roof (13kW+ on single-phase). Makes sense if you have or plan to get an EV, battery, or both. High export during the day, but every kWh you self-consume or store saves money.

For most households, Option B with a plan for future battery storage is the sweet spot.

Step 3: Size the solar

A rough rule of thumb for Australian conditions: each kW of solar panels generates about 3.5-4.5 kWh per day depending on your location, orientation, and tilt.

• Brisbane: ~4.3 kWh/kW/day
• Sydney: ~4.0 kWh/kW/day
• Melbourne: ~3.6 kWh/kW/day
• Perth: ~4.5 kWh/kW/day
• Adelaide: ~4.1 kWh/kW/day

To offset 18 kWh/day in Brisbane, you’d need roughly 18 / 4.3 = 4.2kW minimum. But this assumes 100% self-consumption, which isn’t realistic without a battery. In practice, a 6.6kW system gives you enough headroom to cover your consumption comfortably and have surplus for a future battery.

Most installers default to 6.6kW because it’s the maximum you can connect to a 5kW inverter (common on single-phase) while staying within CEC guidelines. It’s a good default for most households consuming 15-25 kWh/day.

If you’ve got an EV or plan to get one, go bigger. My 13.2kW system covers my house (18 kWh/day) plus EV charging (7-10 kWh/day on solar) plus battery charging (13.5 kWh Powerwall). On a good day, I’m genuinely self-sufficient.

Step 4: Size the battery (if applicable)

Battery sizing is about bridging the gap between your daytime solar surplus and your evening/night consumption.

Measure your evening and overnight consumption. For most households, this is about 8-15 kWh between 4pm and 8am. That’s roughly the amount of battery storage you need to cover that period.

Common battery sizes and who they suit:

• 5 kWh: Covers 3-4 hours of evening consumption. Good for small households on a budget.
• 10 kWh: Covers most of the evening for an average household. The sweet spot for families.
• 13.5 kWh (Powerwall size): Covers evening and most of the overnight period. Good for larger households or those wanting near-complete grid independence.
• 20+ kWh: For all-electric homes with high consumption or those wanting extended blackout protection.

Don’t oversize your battery. A 20kWh battery in a house that only consumes 10kWh overnight will never fully cycle, which means you’re paying for capacity you don’t use. Better to match the battery to your actual consumption patterns.

Step 5: Check your roof

Before finalising sizes, verify your roof can accommodate your planned system:

• Available roof area: Each panel needs about 1.7-1.9 square metres. A 6.6kW system (roughly 15 panels) needs about 28 square metres. A 13kW system needs about 55 square metres.
• Orientation: North-facing is ideal. East-west is good. South-facing is poor (but can still work on steep pitches).
• Shading: Even partial shading significantly reduces output. Be honest about trees, chimneys, and neighbouring buildings.
• Roof condition: If your roof needs replacing within 10 years, do it before solar installation. Removing and reinstalling panels for a roof replacement costs $1,500-3,000.

The quote comparison trick

When you get quotes, normalise them to a cost-per-watt figure:

• Total installed price / system size in watts = cost per watt
• For example: $7,200 / 6,600W = $1.09/W

Good values for 2026 (after STC discount):

• 6.6kW system: $0.75-1.10/W
• 10kW+ system: $0.70-1.00/W
• Battery (per kWh): $700-1,100/kWh installed

If a quote is dramatically cheaper than these ranges, ask why. If it’s dramatically more expensive, get more quotes.

Right-sizing your system isn’t about spending the least or the most. It’s about matching the technology to your actual energy life. Get that right and the numbers work out beautifully.