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

The 6.6kW system became the Perth standard 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.

What each system size generates in Perth

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

| System size | Summer daily generation | Winter daily generation | Annual generation | |---|---|---|---| | 3.3kW | 13–16kWh | 5–8kWh | ~4,800kWh | | 6.6kW | 26–32kWh | 10–16kWh | ~9,600kWh | | 10kW | 40–48kWh | 16–24kWh | ~14,600kWh | | 13kW | 52–62kWh | 20–30kWh | ~18,900kWh |

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

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.

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.

Cost comparison for Perth 2026

Approximate installed prices (after STC rebate):

| System size | Approximate installed cost | Cost per kW installed | |---|---|---| | 6.6kW | $6,500–$9,000 | ~$985–$1,360 | | 10kW | $8,500–$12,000 | ~$850–$1,200 | | 13kW | $11,000–$15,000 | ~$846–$1,154 |

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.

Three-phase consideration

Standard Perth homes have single-phase power. A single-phase connection has a maximum export limit of 5kW per phase. Systems above 10kW (or above 5kW export) require either:

• 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
• Three-phase power upgrade (Western Power application, upgrade cost $1,000–$3,000) — gives 5kW export per phase (15kW total export)

Most Perth households installing 10–13kW systems choose an export-limited inverter setup rather than upgrading to three-phase.

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

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.