Solar System Size Calculator: What Size Solar Do I Need?

Written by Jacob, Last reviewed 2026. This guide is for informational purposes only — see methodology.

Choosing the right system size is the most important decision you will make when going solar. Too small and you will barely dent your bill. Too big and you are exporting most of the power at a low feed-in tariff, stretching the payback.

The right size depends on three things: your electricity usage, your roof space, and your plans for the future. Here is how to work it out.

The Basic Sizing Rule

In most of Australia, 1kW of solar panels generates roughly 3.5 to 4.5 kWh per day on average across the year. More in summer, less in winter. So a 6.6kW system produces about 23 to 30 kWh per day.

The general rule: match your system size to roughly your daily usage divided by your local generation factor.

System size (kW) ≈ Daily usage (kWh) / Generation factor (kWh/kW/day)

Common System Sizes and Who They Suit

• 3kW — Small homes, low usage (under 15 kWh/day), limited roof space, or tight budget
• 5kW — Medium homes, 15–20 kWh/day usage, moderate daytime occupancy
• 6.6kW — Common residential size, 20–25 kWh/day, often paired with a 5kW inverter
• 8–10kW — Large families, higher usage (25+ kWh/day), EV charging plans
• 10kW+ — Very high usage, multiple EVs, home office, or commercial needs

Worked Example: Home Using 18 kWh/Day

Take a home in Sydney that uses about 18 kWh per day on average, with summer usage hitting 25 kWh (air conditioning). The local generation factor is about 4 kWh per kW per day.

18 divided by 4 gives roughly 4.5kW. But since systems are sold in standard sizes, a 5kW system is the closest match. It would generate about 17.5–20 kWh per day on average — covering most of their typical usage.

In summer, the 5kW system pushes out 25+ kWh per day, matching their higher cooling load. In winter, when generation drops to maybe 12–14 kWh/day, they will still import some from the grid — normal for almost every home.

Could they go to 6.6kW? Yes, and that would cover more of their summer usage and leave room for future needs. It would also export more in spring and autumn. The extra upfront cost versus extra savings is the trade-off.

Roof Space: Can You Fit It?

Solar panels are roughly 1.7m by 1m and produce 400–500 watts each. That means each panel needs about 2m² of space. A 6.6kW system with 16 panels needs about 32m² of usable roof area.

North-facing is ideal in Australia — it gets the most sun through the middle of the day. But east and west facingwork well too. An east-west split actually produces a flatter generation curve, with power in the morning from east-facing panels and afternoon from west-facing panels. That can match household usage better than a single north array.

Shading is the enemy. Even partial shade on one panel can drag down the whole string. If your roof has chimneys, trees, or neighbouring buildings casting shade, you may need microinverters or optimisers — which adds cost.

Generation Factor by Location

The generation factor varies by location. It is a rough average across the year and accounts for your local climate. Typical factors:

• 3.5 — Tasmania, southern Victoria, cooler inland areas
• 4.0 — Sydney, Adelaide, Perth, most of coastal NSW
• 4.5 — Brisbane, Queensland, Northern Territory

These are averages. An online tool^[1] or your installer will have more precise local data for your specific postcode.

Should You Size for the Future?

If you are planning an electric vehicle, heat pump, or battery in the next 3–5 years, sizing up now is usually cheaper than adding panels later. An EV alone adds 10–15 kWh of daily charging load. A household currently using 18 kWh/day with plans for an EV might want 8kW rather than 5kW.

That said, oversizing too far means more exports at the FiT rate. There is a balance. A good installer will walk through your current bill and future plans before recommending a size.

STC Discounts by System Size

Under the Small-scale Renewable Energy Scheme (SRES), every kW of eligible solar capacity earns Small-scale Technology Certificates (STCs).^[2]More certificates mean a bigger upfront discount, which most installers pass through. The rate is the same per kW regardless of size (up to 100kW), so the discount scales linearly with your system size.

Because STCs are calculated per kW of capacity, a 6.6kW system earns proportionally more STCs than a 5kW system, which increases the upfront discount.^[2] That larger discount can partially offset the higher upfront cost, but the final price per watt depends on installer pricing, equipment, roof complexity, and network requirements.

This guide is for informational purposes only. System sizing recommendations are estimates and should be confirmed with a qualified solar installer.