Comparing solar system sizes for Perth: 6.6kW vs 10kW vs 13kW

The 6.6kW system became Perth's entry-level size because it maximises the STC rebate relative to the common 5kW single-phase inverter limit (inverters are sized to 133% of inverter capacity to capture morning and evening generation). But panel costs have fallen to the point where 10kW and 13kW systems have become cost-competitive options worth comparing.

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What each system size generates in Perth

Using Perth's 5.0 peak sun hours and approximately 80% performance ratio:

Daily ranges vary with exact weather; annual figures are indicative.

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How system size affects self-consumption and export

Larger systems generate more surplus. But whether that surplus benefits you depends on what happens to it:

Self-consumption: Each kWh generated and consumed in your home displaces grid electricity (33.26c/kWh on A1, or up to 55.33c/kWh peak on Midday Saver). This is the most valuable use of solar generation.

Battery storage: A battery captures surplus that would otherwise export at 2c/kWh, storing it for 55.33c/kWh evening use. But a battery has fixed capacity (typically 10kWh). Once the battery is full, additional generation still exports.

DEBS export: Generation that can't be self-consumed or battery-stored is exported at 2c/kWh (off-peak) or 10c/kWh (peak). At 2c/kWh, the financial value of exported electricity is minimal.

Key insight: If your household daytime consumption + battery capacity = 15kWh, a 6.6kW system that generates 28kWh/day sends 13kWh to export at 2c. A 10kW system that generates 44kWh/day sends 29kWh to export at 2c. The larger system exports more surplus at low value, unless your consumption is high enough to use it.

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When a 10kW or 13kW system makes sense

Add an EV: An EV charging 9am–3pm at 7.4kW uses 44kWh in a 6-hour session. This single load can use all the surplus that a 6.6kW system generates (and more). A 10kW system with an EV and household consumption can potentially achieve 80%+ self-consumption.

Add a battery: A 10kWh battery with a 10kW solar system means the battery fills comfortably in most weather conditions and you still export during peak midday periods. The combination increases self-consumption by capturing surplus that would otherwise export.

High household consumption: A household using 35–50kWh/day (large family, ducted air conditioning, WFH, pool) can self-consume most of what a 10kW system generates.

Commercial premises on residential connection: Home-based businesses with significant electricity use during business hours can benefit significantly from larger systems.

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Cost comparison for Perth 2026

Approximate installed prices (after STC rebate):

The cost per kW typically reduces as system size increases. Going from 6.6kW to 10kW adds approximately $2,000–$3,500 and 51% more generation capacity.

Marginal return on extra capacity: If the additional generation is mostly exported at 2c/kWh, the financial return on the extra investment is poor. If it's self-consumed or battery-stored, it's valuable.

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Three-phase consideration

Standard Perth homes have single-phase power. Export is capped at 5kW in total across all phases, on single-phase and three-phase connections alike. A bigger system needs:

• Inverter export limiting (limiting the system's export to 5kW even if it generates more): means the extra panels only help when the household can self-consume the generation
• Western Power approval and a generation-limit setting if the inverter goes above the single-phase generation limit (10kVA or 15kVA, depending on the connection). This doesn't require a three-phase upgrade, and a three-phase upgrade wouldn't raise the export limit

Most Perth households installing 10–13kW systems stay single-phase with an export-limited inverter setup. Your installer confirms what your connection allows.

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Decision framework

Choose 6.6kW if:

• Average Perth household consumption (15–25kWh/day)
• No EV and no battery planned immediately
• Limited roof space
• Budget-conscious choice

Choose 10kW if:

• Higher consumption (25–40kWh/day)
• Adding an EV now or within 2 years
• Adding a battery alongside solar
• Roof space available (55–65m²)
• Export-limited setup is acceptable if self-consumption justifies it

Choose 13kW if:

• Very high consumption household (40kWh+/day)
• Multiple EVs
• Commercial home-based electricity use
• Planning future expansion (battery + EV)
• Three-phase connection available or planned

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System sizing depends on your specific consumption profile, roof orientation, and budget. Use the BillWise calculator to model system sizes against your actual Synergy usage data.