Solar and air conditioning in Perth: getting the sizing right

Air conditioning is the defining energy issue in Perth homes. On a 40°C summer day, a ducted reverse-cycle system running at capacity can draw 4–8kW, easily dominating a household's electricity consumption. Integrating solar with AC strategy is one of the most impactful decisions a Perth homeowner can make.

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AC consumption profile in Perth

Perth's AC season peaks in December–March, with average maximum temperatures of 29–35°C and regular extreme heat days over 38°C. During these months:

• Ducted reverse-cycle (3–5 bedroom home): 4–8kW draw when running at capacity; 1.5–3kW in normal cooling mode
• Split system (individual room): 0.7–3kW per unit (size-dependent)
• Evaporative cooler (Breezair/Seeley): 0.3–1.2kW (much lower power draw, but only effective in dry Perth summers, ineffective on humid days)

Annual AC energy use for a Perth home with ducted reverse-cycle: 3,000–8,000 kWh/year, depending on set temperature, home size, insulation quality, and occupancy patterns.

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The solar-AC timing alignment

Perth's solar generation peak and AC demand peak are well-aligned in summer: the hottest part of the day (11am–3pm) coincides with peak solar generation. This makes Perth's solar-AC combination particularly effective:

• Solar generating 7–9kW at noon → AC drawing 3–4kW → net zero or positive export
• The household cools the house from solar for most of the hottest period without grid cost

The timing mismatch: Evening and pre-cooling. A Perth home that reaches 30°C internally by 5pm (after the solar generation drops off) requires grid-powered cooling from 5pm onward. The strategy of pre-cooling the house while solar is generating (setting the AC to 22°C by 2pm, then raising the setpoint to 25°C by 4pm) lets the building's thermal mass do the work during the expensive evening period.

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Sizing solar to offset AC load

If AC is a significant part of your consumption, it should be a primary input in your solar sizing calculation. The question is: how much of the AC load occurs during solar generation hours?

Best case (daytime occupancy): 80%+ of AC use is during the day → a 10kW system covers most of it.

Typical case (work-away household): AC runs overnight to pre-cool before the family leaves (7am–9am), then from when family returns (5pm–10pm) → only a small fraction is during solar generation hours. In this scenario, adding more panels doesn't solve the problem. A battery does.

The right approach: Track when your AC actually runs (most ducted AC controllers have a run-time log). If AC is primarily an evening load, a battery + solar combination addresses it better than larger solar alone.

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Ducted vs split for solar compatibility

Ducted reverse-cycle: Single large load, controllable via a thermostat or smart controller. Can be zoned to reduce load (run 3 of 5 zones). Programmable pre-cooling is straightforward. Power draw varies based on the compressor load: on a mild 28°C day, a ducted system may draw only 2kW; on a 40°C day, it may draw 6–8kW.

Split system (multi-head): Multiple smaller loads. Each unit can be turned on/off independently, giving more granular control over which rooms benefit from solar-powered cooling at any given time. Running 2 of 4 split systems during peak solar is easy.

Evaporative cooler: Very low power draw (0.3–1.2kW), essentially free to run from solar even on a small 6.6kW system. The trade-off: Perth's occasional humid summer days (Fremantle Doctor or tropical air push) make evaporative ineffective. Houses with evaporative have lower AC electricity consumption but can't handle all summer conditions.

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Inverter-driven AC and solar optimisation

Modern split systems and ducted units use inverter-driven compressors (variable speed). The power draw is variable, anywhere from 0.5kW to 5kW depending on how much cooling is needed, rather than a fixed on/off draw.

Why this matters for solar: On a mild day when solar is generating well and the house only needs gentle cooling, an inverter AC unit might draw 1kW, well within solar output. The house cools for essentially zero net cost. On a hot day, the AC ramps up and may exceed solar output, so some grid draw occurs.

Smart AC controllers: Devices like Sensibo or Mysa (3rd-party controller add-ons) can connect your AC to your solar monitoring system and automatically adjust setpoints based on solar generation. This is increasingly popular in Perth: when solar is generating above a threshold, the controller lowers the setpoint to pre-cool the house aggressively; when solar drops, it raises the setpoint to reduce grid draw.

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Sizing recommendation for AC-heavy Perth homes

For a Perth household where AC is the dominant consumption driver:

Avoid undersizing: A 6.6kW system on a house with heavy AC use will offset grid costs in mild weather but barely dent consumption on the peak summer days that drive the highest bills.

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Perth's solar-AC alignment in summer is excellent: peak solar generation coincides with peak cooling demand, making solar the ideal pairing for AC-heavy homes. The key decision point is when your AC actually runs. Daytime users benefit most from larger solar; evening-dominant AC use benefits most from solar + battery. Pre-cooling from solar during the 10am–3pm window is the most cost-effective single behavioural strategy.