What Size Solar System Do I Need?

The correct solar system size balances your annual electricity usage, the amount of sunlight your roof receives and the physical space available. Start with your annual usage (in kilowatt-hours, or kWh) from your last 12 months of power bills, then divide by your local peak sun hours per year—this tells you the minimum kW capacity you need to generate that energy.

Most of Australia receives 4–5.5 peak sun hours per day (higher in the inland west, lower in Tasmania and southern Victoria). For example, if you use 6,000 kWh per year and live in a 5-sun-hour zone, you need 6,000 ÷ 5 ÷ 365 = 3.3 kW just to break even over the year. However, system efficiency losses (inverter, wiring, temperature) reduce this by about 23%, so you'd need a 4.3 kW system to truly offset your usage. In practice, most households add another 20–40% to account for cloudy days, future usage growth, and shading—landing at 5–6 kW.

The second check is roof space. A modern 400 W panel is roughly 2 square metres, so a 6 kW system (15 panels) needs about 30 square metres of unshaded, north-facing roof. If your roof is south-facing, smaller or heavily shaded, you'll need a smaller system or a battery to maximise value. Our accredited electricians carry out a full roof assessment during the free site visit, including drone imagery to map shading from trees and buildings throughout the year.

Let's say you live in suburban Brisbane (postcode 4000–4999), use 6,500 kWh per year, have a north-facing tile roof with no tree shade and a modern switchboard. Brisbane sits in STC deeming zone 3 (roughly 5.2 peak sun hours per day).

Step 1: Annual generation. A 6 kW system in Brisbane generates roughly 6 × 5.2 × 365 × 0.77 (system efficiency) = 8,785 kWh per year. This covers your 6,500 kWh usage with 2,285 kWh left over to export to the grid or store in a battery.

Step 2: Panel and roof layout. Fifteen 400 W panels take up about 30 square metres in a 3×5 grid. Your 10° tile roof can handle the weight (tile roofs hold about 75 kg/m²; a solar system is 15 kg/m²). A single-phase 6 kW inverter (not 10 kW, because oversized inverters waste money) sits inside your garage.

Step 3: Rebates. Your 6 kW system qualifies for Small-scale Technology Certificates. The deeming calculation is 6 kW × 1.654 (zone 3 multiplier) × 5 years × $38/STC ÷ 120 = about $1,558 upfront discount. Many Queensland council areas also offer small rebates—check with your local energy authority.

Step 4: Price and payback. The full 6 kW system costs $5,500–$8,500 depending on panel brand (tier-one panels cost more but last longer and have higher efficiency). Subtract $1,558 rebate, leaving $3,942–$6,942 net cost. At an 8¢/kWh export price and 35¢/kWh grid import price, your payback is 5–8 years. Add a 10 kWh battery ($9,000–$15,000 after rebates) and the payback stretches to 7–11 years but your self-consumption rises to 60–70% and your grid bills drop much faster.

Solar Panel Installation costs $4,000–$9,500 for a typical 5–7 kW residential system; pricing rises with panel quality, inverter brand, roof complexity and your state. A budget 5 kW system on a simple tile roof in NSW with standard panels costs around $4,000–$5,500. A premium 6 kW system with tier-one panels on a slate roof in Tasmania or with complex multi-storey wiring costs $8,500–$9,500.

The main cost drivers are system size (kW), panel efficiency grade, inverter brand (SMA and Fronius cost more than local-brand options but have longer warranties and better monitoring), roof type (metal is easiest, slate or heritage is hardest), roof pitch (flat roofs need special racking), and your postcode's labour rates (inner Melbourne and Sydney cost more than rural areas).

Small-scale Technology Certificates give an automatic upfront discount of about $200–$300 per kW (exact amount depends on your postcode's deeming zone and the current STC price, which changes quarterly). The Clean Energy Regulator publishes STC prices on its website; confirm the current rebate with your installer because amounts change on set dates.

Adding a battery costs $7,000–$15,000 for 8–13 kWh; the federal Cheaper Home Batteries Program (from 1 May 2026) offers rebates of up to $252/kWh for the first 14 kWh, $151/kWh to 28 kWh and $38/kWh to 50 kWh (confirm current amounts with DCCEEW). Your state may also offer additional rebates or loan schemes—check with your state energy agency.

We always include site assessment, roof check, system design, installation by an accredited electrician, grid connection paperwork and monitoring app setup in our quotes. No hidden fees.

Add a larger system (8–10 kW) now if you use most power during the day (working from home, pool pump, industrial fridge) or expect rapid usage growth. Add a battery if your peak usage is in the evening or night (reverse-cycle heating, electric vehicle charging at 6pm, all-night heater). These two choices have different paybacks and both are valid.

A larger-only system (no battery) costs less upfront ($7,500–$9,500 for 8 kW) but exports surplus power to the grid at the low export tariff (5–8 cents/kWh in most states). Over 10 years, you recover money slowly unless your network raises export prices significantly.

A smaller system plus battery (5 kW + 10 kWh, $12,000–$24,000 after rebates) costs more upfront but keeps 60–70% of solar energy for evening use at your full tariff rate (25–40 cents/kWh depending on your state and plan). The payback is faster if electricity prices rise, which they have done consistently over the past decade. The battery also gives you 8–12 hours of blackout protection, a useful bonus if your area suffers supply cuts.

The federal Cheaper Home Batteries Program makes battery-first thinking more attractive from 1 May 2026 (rebate rates are $252/kWh for the first 14 kWh, $151/kWh to 28 kWh and $38/kWh to 50 kWh; confirm current amounts). If you don't have the full budget now, solar-only is the right choice; battery additions cost only 10–15% more when added later because the AC wiring and switchboard upgrades are already done.

Your state's peak sun hours, climate, network rules and rebate programs all affect which size system makes financial sense. Northern Australia (northern Queensland, Northern Territory, Western Australia) receives 5.5–6 peak sun hours per day and benefits from large systems because the payback is faster. Southern Australia (Victoria, Tasmania, South Australia) receives 4–4.5 hours and typically suits smaller systems, or systems paired with a battery to improve self-consumption.

Your distribution network's export limits are also crucial. Most networks in Australia allow up to 10 kW single-phase export without formal approval; anything larger needs a network application (usually free, takes 2–4 weeks). If your network has a 'postcode cap' (some regional areas have limits of 30–50 kW total export across all homes in a zone), you may need to size down or apply for special permission.

State rebates change regularly and vary widely. New South Wales has no state rebate, but South Australia, Victoria and Queensland offer various grants or loan schemes for batteries or heat pumps. Check your state energy agency's website (search 'solar rebate [your state]') and confirm current amounts before committing to a size.

Your postcode's deeming zone for Small-scale Technology Certificates affects your STC rebate amount. NSW zones 1–3 have lower deeming multipliers (1.282–1.494); WA and NT zones have higher (1.822+). We confirm your exact rebate during the quote.

Choosing system size based on a dealer's sales target, not your actual usage. Always base sizing on your last 12 months of bills, not the installer's recommended 'standard' size. A 6 kW system is not right for everyone; for a single person in a small flat it's wasteful; for a family with a pool and heat pump it's undersized.

Ignoring shading. Trees, buildings and chimneys can cut output by 30–50% in some months. A system sized for full sun on a heavily shaded roof will disappoint. We always assess shading with drone imagery and software modelling.

Oversizing the inverter. A 10 kW inverter on a 6 kW panel system wastes money and sits idle most of the time. The inverter should match the panel array; oversizing by 10–15% is acceptable for future panel additions, but avoid 50%+ oversizing.

Forgetting switchboard space or three-phase limits. Some older switchboards can't fit a solar inverter breaker without replacement. Three-phase homes have different export limits and may need network approval for systems over 10 kW. These issues are caught during the site assessment, but knowing them upfront helps you budget.

Assuming you can add panels later without more electrical work. You can, but only up to your inverter's rated power and your network's export cap. A second addition usually means a new inverter. We discuss expansion limits in the first visit.

An accredited electrician and a solar designer will spend 60–90 minutes at your property. They'll review your electricity bills (last 12 months), measure your roof (or use a drone), check for shading, inspect your switchboard, confirm your phase type (single or three) and discuss your usage patterns and future plans.

They'll take photos and measurements, run shading software (like Sunwiz or PVSyst) to forecast generation in your exact location, and estimate whether a switchboard upgrade or three-phase conversion is needed. All of this feeds into a detailed quote with itemised costs, rebate estimates and a production forecast for your first year.

We'll also discuss roof condition (if your tiles or metal looks worn, we may recommend re-roofing before solar to avoid double-handling), inverter placement (garage, laundry or outside wall), and cable routing to the switchboard. If you have plans to add a battery, EV charger or heat pump later, we design the system to make those easy and cost-effective.

The assessment is free and takes place at a time that suits you. You'll walk away with a written quote, a detailed system design and honest advice on payback, rebates and next steps. No pressure, no sales pitch.

Once you've decided on a system size and accepted the quote, we lodge the Small-scale Technology Certificate application with the Clean Energy Regulator (this is automatic and adds no cost to you). We then submit your system design to your local distribution network for export approval; most single-phase systems under 10 kW are approved within 2 weeks.

You'll receive a final invoice and a contract outlining the installation date, warranty terms (all our systems include a 10-year panel warranty and 5-year inverter warranty as standard; check your specific installer's terms), and the handover process. We'll also arrange any council permits if needed (most suburbs don't require a permit for residential solar, but some heritage zones do).

Installation is usually a single day for a residential system. Our accredited electricians will install the racking, panels, inverter, isolators and wiring, then test the entire system. Once testing is complete and the network gives the final approval (usually within 3–5 days), your system goes live and you can see generation and export in real time via the monitoring app.

After installation, we include a 12-month free health check and cleaning service. Beyond that, an annual clean (removing dust, pollen and bird droppings) keeps output at peak efficiency; this costs $150–$450 depending on your system size and roof access.