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This is an illustrative scenario based on typical Perth household data, not a specific customer story.
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.
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, where the STC value must be calculated from the current factor, certificate price, eligible capacity, and taper rules rather than copied as a fixed rebate number.
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.
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.
At 20 kWh under the current framework, the combined rebates can still be substantial, but the dollar value must be calculated from the WA rebate cap, current STC factor, certificate price, and capacity taper. Apply that calculator result to the installed quote before comparing it with a 13.5 kWh option.
For this household profile, high A/C usage can create high self-consumption value, but the electricity savings and payback period are calculator outputs. Model them from evening A/C load, usable battery capacity, tariff, export profile, install price, and warranty assumptions.
A step-by-step method covering evening usage and seasonal AC demand.
AS/NZS 5139 clearance rules, garage vs outdoor walls, and what installers check on-site.
Synergy customers get $130/kWh back, capped at $1,300.
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