How to work out what size solar system you need
The starting point is your annual electricity usage in kilowatt-hours. You can read this off your electricity bill — either as quarterly kWh totals or, if your bill only shows a dollar amount, by dividing that amount by your import rate (typically 28–35¢/kWh in 2026).
Once you have your usage, the sizing formula is straightforward: divide your target annual generation by your zone's yield per kW. A household in Zone 3 (Sydney, Melbourne, Adelaide) can expect around 1,380 kWh of generation per kW of installed capacity per year. So if you use 6,000 kWh per year and want to cover 80% of that with solar, you need a system that generates 4,800 kWh — which is 4,800 ÷ 1,380 ≈ 3.5 kW.
The estimator handles this calculation and also shows you the panel count, approximate roof area required, and how that generation would typically split between power used in your home (self-consumed) and power exported to the grid.
Target offset: 80% vs 100%
An 80% target means solar covers four-fifths of your annual electricity on paper — a sensible default that avoids over-sizing the system for a handful of winter days when generation is lower. Many households choose 80–90% because the marginal cost of the last 20% of generation (oversizing for winter) rarely pays off.
A 100% target means the system generates as much electricity annually as you consume. This does not mean you're off-grid — you still draw from the grid at night and export surplus during the day. Without a battery, even a 100%-offset system typically leaves you buying 60–70% of your daily electricity from the grid, because solar only generates during daylight hours. For true round-the-clock self-sufficiency, combine a well-sized solar system with a home battery.
Self-consumption vs export: why the split matters
A larger solar system does not automatically mean larger savings — it means more export if your household load profile doesn't change. Every kWh exported earns only the feed-in tariff (typically 5–10¢ in 2026), while every kWh self-consumed is worth your full import rate (28–35¢). The gap between these rates can be 25¢/kWh or more.
Without a battery, most households self-consume 25–35% of their solar generation. Shifting appliance use to daylight hours is the cheapest way to increase this: run the dishwasher, washing machine, and dryer between 10 am and 3 pm; set your hot water system to heat during the middle of the day; charge an EV from solar if possible. These changes alone can push self-consumption to 40–50%, improving the economics of the same system.
If you want to push self-consumption to 70–90%, a home battery is the most reliable approach. Use the Solar Battery ROI Calculator to model whether the extra cost pays off for your usage pattern.
From size to savings: the next step
Once you know your target system size, the Solar Payback & Savings Calculator takes you from kW to dollars — including the STC rebate, 25-year savings projection, and a plain-English verdict on whether the numbers stack up. The STC rebate alone on a 6.6 kW system in Zone 3 is currently around $1,600, bringing the net installed cost down to roughly $6,300 for a quality mid-market system.
The payback period depends on your self-consumption rate and electricity import rate far more than it depends on system size. For most households on a 30¢+ tariff with 30% self-consumption, a quality system pays back in 5–8 years — well inside the 25-year panel warranty period.
Frequently asked questions
How big a solar system do I need for my home?
The right size depends on how much electricity you use and how much of that you want solar to cover. The basic formula is: required kW = (annual usage in kWh × target offset %) ÷ your zone's annual yield per kW. For a typical Australian household using around 5,000–7,000 kWh per year targeting 80% solar coverage in Zone 3 (Sydney/Melbourne), that works out to roughly 3–5 kW. If you're in a sunnier zone (Zone 2 — Brisbane/Perth), the same usage requires a slightly smaller system because each kW generates more electricity per year.
What is a typical residential solar system size in Australia in 2026?
The most common residential system sizes in Australia are 6.6 kW and 10 kW. The 6.6 kW figure became standard because it matches the typical single-phase inverter limit of 5 kW output while allowing for the gap between DC panel capacity and AC inverter output. For households targeting 100% offset of a 20+ kWh/day usage, 10–13 kW systems are increasingly popular, especially where export limits no longer apply. Homes with batteries often size the solar system larger to ensure the battery fills daily.
How many solar panels do I need for a 6.6 kW system?
A 6.6 kW system using standard 400 W panels requires approximately 16–17 panels. The exact count depends on the panel wattage your installer uses — 400 W is the dominant size in 2026, but some installers use 410–430 W panels, which would reduce the count slightly. Each panel occupies roughly 1.8 m² of roof space, so a 6.6 kW system needs around 28–30 m² of suitable roof area. You need unshaded, north-facing (or close to it) roof space; east and west-facing panels can work but produce less per panel.
What does STC zone mean for solar sizing?
The STC (Small-scale Technology Certificate) zone is set by your postcode and is used by the Clean Energy Regulator to determine your solar rebate amount and as a proxy for your location's solar resource. Zone 1 (Darwin, far north QLD) has the highest solar yield — around 1,650 kWh per installed kW per year. Zone 2 (Brisbane, Perth, Townsville) yields roughly 1,550 kWh/kW/yr. Zone 3 (Sydney, Melbourne, Adelaide, Canberra) is the most common at about 1,380 kWh/kW/yr. Zone 4 (southern Tasmania, alpine areas) is lowest at around 1,100 kWh/kW/yr. A smaller system achieves the same annual generation in a higher-yield zone.
Can I size my solar system to fully offset my electricity usage (100%)?
Yes — a 100% annual offset target means the system generates as much electricity over the year as you consume. However, 'annual offset' does not mean you never draw from the grid. At night and on cloudy days you still import electricity; on sunny days you export excess to the grid. True self-sufficiency (minimal grid use day and night) requires a battery in addition to a solar system sized for 100% or more. Without a battery, most households with 100% annual offset still draw 60–75% of their daily electricity from the grid, because the timing of generation and consumption don't align.
What if my roof can't fit the ideal system size?
If your roof is smaller than what the ideal system requires, the calculator caps the recommended size at your roof limit and shows you the actual coverage percentage you'll achieve. In that case, you can offset less of your usage (lower the target %) or look at ways to increase self-consumption to maximise the value of the smaller system — for example, by shifting loads to daytime hours or adding a battery. Some households with limited roof space install higher-efficiency (and higher-cost) panels to squeeze more capacity into the same area; your installer can advise on the premium-efficiency options available.
What is self-consumption and why does it matter for sizing?
Self-consumption is the percentage of solar generation your household actually uses directly, rather than exporting to the grid. It matters for savings (not sizing) — a larger system doesn't change your self-consumption rate, it just produces more export. At 30% self-consumption with a 32¢/kWh import rate and 6¢ FiT, each kWh generated is worth a blended average of about 14¢. At 50% self-consumption the blended value jumps to 19¢/kWh. Strategies to increase self-consumption include daytime appliance scheduling, hot water timer adjustment, EV daytime charging, and home battery storage.
Solar yield figures (kWh/kW/yr) are derived from Clean Energy Regulator zone ratings and SolarQuotes installer averages as at July 2026. Actual system output depends on panel orientation, roof pitch, shading, inverter efficiency, and soiling. Panel count assumes 400 W panels; roof area assumes 1.8 m² per panel. Self-consumption percentages are indicative estimates — actual figures depend on your household load profile and whether a battery is installed. This estimator does not account for time-of-use tariff optimisation, grid export limits, or system performance guarantees. Figures are estimates only. Not financial, energy, or investment advice.
This article is for general information only and does not constitute financial, tax or legal advice. Individual circumstances vary. Consult a licensed financial adviser or energy consultant before making decisions based on this information.