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.

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.

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.

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.

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.

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 — 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.

Sizing recommendation for AC-heavy Perth homes

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

| Scenario | Recommended system | |---|---| | Daytime occupancy (family at home during summer) | 10–13.3kW solar — high self-consumption of midday generation | | Work-away household (AC evening peak) | 10kW solar + 10kWh battery — battery covers evening AC | | Large 4+ bedroom with ducted AC | 13.3kW solar — allows margin for peak AC draw | | Pre-existing ducted + planning to add EV | 13.3kW + battery — covers both simultaneous loads |

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.

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.