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title: "Scenario guide: large home with ducted air conditioning — 10 kW PV + 20 kWh battery" description: "How a larger Perth household with high A/C demand might approach battery sizing — balancing summer cooling loads, export credits and capacity taper deadlines." lastUpdated: "27 Feb 2026"

Scenario guide: large home with ducted air conditioning

This is an illustrative scenario based on typical Perth household data, not a specific customer story.

Home profile

• Occupants: 2 adults, 3 children
• Home size: 280 m², double-storey
• Cooling/heating: Ducted reverse-cycle (12 kW rated), runs 4-8 hours daily in summer
• Hot water: Heat pump (runs at 10 am via timer)
• Daily usage: ~30-40 kWh (summer), ~18-22 kWh (winter)
• Grid connection: Single-phase, Synergy A1 flat tariff

System

• Solar: 10 kW PV (split north/west roof, installed 2 years ago)
• Battery: 20 kWh usable capacity
• Backup: Essential circuits plus A/C circuit on one zone
• Goal: Cover evening A/C load through summer without exporting cheaply during the day

Why larger homes need different sizing

The standard advice of "match your battery to your evening load" breaks down for larger homes with significant air conditioning demand. A typical 3-bedroom home might use 8-10 kWh overnight, but a larger home running ducted A/C can use 15-25 kWh between 3 pm and midnight on a 38-degree day.

Summer peak demand: On hot days (35 degrees+, which Perth sees 30-50 days per year), ducted A/C draws 2-4 kW continuously. Over a 6-hour evening window (4 pm to 10 pm), that alone accounts for 12-24 kWh, before adding lights, cooking, entertainment, and other appliances. A 10 kWh battery would be flat by 7 pm.

The sizing trade-off: A 20 kWh battery costs roughly $18,000-$24,000 before rebates. But it captures more of the 30-40 kWh that a 10 kW solar system generates on a summer day, avoiding export at 2-3c/kWh and instead displacing import at 32c/kWh. The payback improves when the battery is genuinely utilised rather than sitting at 80% state of charge most evenings.

STC capacity taper for larger batteries

The capacity taper in force since 1 May 2026 has the biggest impact on systems above 14 kWh. A 20 kWh battery falls into the 14-28 kWh band, which receives 60% of STCs, currently ~$3,101 in federal rebates versus the ~$6,384 the same system would have received under the legacy pre-May rates.

For large-capacity battery buyers, this makes right-sizing the most important ROI lever. A 13.5 kWh battery sized to actual evening A/C demand often delivers better ROI per dollar than a 20 kWh battery that sits partly unused.

How this typically plays out

Summer (Oct-Mar): Solar generates 35-45 kWh/day. The battery charges fully by 1-2 pm. A/C starts drawing from the battery around 3-4 pm. On moderate days (30-33 degrees), the battery covers the entire evening. On extreme days (38 degrees+), the battery may deplete by 9-10 pm, requiring 2-4 hours of grid import.

Shoulder months (Apr-May, Sep): Solar drops to 25-35 kWh/day. A/C usage drops significantly. The 20 kWh battery charges fully and provides more than enough evening coverage, with energy left at sunrise.

Winter (Jun-Aug): Solar drops to 18-25 kWh/day. Heating load is moderate (reverse-cycle is efficient in heating mode). The battery may not fully charge on overcast days. A forced-charge window on off-peak rates can restore full coverage.

Budget consideration

At 20 kWh under the current framework, the combined rebates are still substantial. WA rebate ($$1,300 for 10 kWh cap) plus federal STCs (~$3,101 with the 60% capacity taper) gives approximately $4,401 in total rebates. On a system costing $20,000-$24,000 installed, the net cost is $15,600-$19,600. Downsizing to 13.5 kWh keeps the system in the full-rate STC band.

Typical electricity savings for this household profile: $1,800-$2,800/year (high A/C usage means high self-consumption value). Payback period: 5-7 years, well within the 10-15 year battery warranty.

Key takeaways

• Don't under-size for A/C loads. A 10 kWh battery in a high-A/C home will run flat by early evening on hot days. For genuine coverage, 15-20 kWh is the realistic range.
• The May 2026 capacity taper is steepest above 14 kWh. A 20 kWh system receives about $3,300 less in federal rebates than under the legacy pre-May rates. Right-sizing matters more than ever for large households.
• Timer management matters. Running the heat pump hot water, dishwasher, and pool pump (if applicable) during 10 am-2 pm solar peak ensures the battery starts the afternoon fully charged.
• One-zone A/C backup is a practical compromise. Backing up the entire ducted system requires very high inverter power; backing up one zone provides comfort during outages without oversizing the inverter.