Solar and air conditioning in Perth: sizing, timing, and savings

Air conditioning accounts for 30–50% of a Perth household's electricity bill in summer. A well-sized solar system can eliminate most of this cost — but only if your air conditioning runs during solar generation hours. Perth's brutal afternoon heat, which continues into the evening, creates a timing challenge that solar alone can't fully solve.

How much electricity does air conditioning use?

Perth air conditioning loads vary by system type and house size:

| System type | Typical consumption | |---|---| | Small split (1 bedroom/small room) | 0.6–1.2kW | | Medium split (living room) | 1.5–3.5kW | | Reverse cycle multi-head (3–5 rooms) | 3–8kW | | Ducted system (whole house) | 4–12kW |

A ducted system running 8 hours/day in summer (8am–4pm) might consume 48–64kWh/day — equal to a typical Perth household's entire daily electricity consumption plus cooling.

When air conditioning aligns with solar

Perth summer heat follows a pattern:

• Morning (6am–10am): mild (25–32°C), low cooling demand
• Midday–early afternoon (10am–3pm): hottest period (35–45°C), peak cooling demand
• Late afternoon (3pm–7pm): still very hot, continued cooling demand
• Evening (7pm–11pm): finally cooling, residual cooling

Solar generation hours:

• Peak generation: 9am–3pm
• Generation ramping down: 3–5pm
• Near zero: after 5pm

The overlap between peak cooling demand (10am–4pm) and peak solar generation (9am–3pm) is very good. A solar system can power air conditioning during the hottest part of the day.

The challenge is the 3pm–7pm period: still hot, but solar is ramping down or off. This is where the Midday Saver tariff and battery decisions intersect.

The Midday Saver tariff and air conditioning

On Synergy's Midday Saver tariff:

• Super off-peak: 9am–3pm at 8.85c/kWh — this window covers peak heat hours
• Peak: 3pm–9pm at 55.33c/kWh — this window covers late afternoon/early evening cooling

This means that on Midday Saver, cooling your house from 10am–3pm costs very little (solar or cheap grid power). But running air conditioning from 3pm–9pm at 55.33c/kWh can be expensive without solar or battery backup.

The pre-cooling strategy: Use the Midday Saver super off-peak window to pre-cool the house to 22–23°C before 3pm. A well-insulated Perth home (double-glazed windows, good ceiling insulation) can hold a lower temperature for 2–3 hours after the air conditioning turns off. This delays the need for air conditioning during the 3pm–9pm peak period.

Sizing solar for air conditioning

If air conditioning is your main motivation for solar, size the system to cover your peak cooling load:

Example: ducted system using 7kW

• To power a 7kW ducted system entirely on solar requires a system large enough to generate 7kW+ at the same time
• A 10kW solar system generates approximately 8–9kW at peak on a clear summer day
• A 6.6kW system generates approximately 5.5–6.2kW at peak — not quite enough to power the ducted system alone, but covers the bulk of it

Important distinction: You don't need to power the air conditioning entirely from solar. You need the solar generation to exceed your air conditioning load less other household consumption. If your household uses 1kW of background load, your 6.6kW solar generates 5.5kW, and your air conditioning uses 5kW — the solar covers everything with 0.5kW surplus.

General rule: For ducted systems, a 10–13kW solar system provides comfortable solar coverage during peak generation hours. For split systems, 6.6–10kW is typically sufficient.

Battery and air conditioning

Batteries can extend solar-powered air conditioning into the evening, but the numbers need checking:

Example:

• Ducted system: 7kW
• 10kWh battery at 3pm, fully charged
• Running air conditioning from 3–6pm: 7kW × 3hr = 21kWh needed
• Battery can provide: 10kWh

Even a 10kWh battery is consumed in approximately 1.4 hours at 7kW. A large ducted system quickly depletes a standard battery. Batteries extend solar coverage meaningfully for split systems (lower kW draw) but provide limited coverage for high-draw ducted systems.

Where batteries help with air conditioning:

• Covering the 3–5pm transition period when solar is ramping down
• Running split systems during early evening (5–8pm) without high draw
• Combining with the pre-cooling strategy (battery tops up split AC as needed in the evening)

Ducted vs split for solar optimisation

Split systems are better matched to solar:

• Lower per-unit consumption (1.5–3.5kW each)
• Individual room control — only cool occupied rooms during solar hours
• Can leave the bedroom unit off during the day and cool it briefly before bed without high draw

Ducted systems cool more rooms but with a high base load:

• 4–12kW continuous draw when running
• Less granular control (usually whole-house or zone-by-zone)
• More solar generation required to power them fully

If you're planning a new air conditioning installation alongside solar, discuss with your solar installer whether split systems zone by zone might provide better solar utilisation than a single large ducted system.

The Perth summer strategy

1. Pre-cool before 3pm: set air conditioning to cool the house to 22–23°C by 2:30pm, then set it back or off during the 3–5pm peak transition
2. Close blinds and curtains by 11am: north and west windows are the primary heat sources in Perth afternoons; blocking direct sun before midday reduces cooling load
3. Use the Fremantle Doctor: Perth's afternoon sea breeze (usually arriving 1–4pm in summer) can provide free cooling on many days — open cross-ventilation windows when the Doctor arrives
4. Ceiling fans reduce perceived temperature by 3–5°C, allowing air conditioning setpoints 2–3° higher — significant electricity saving at 55c/kWh peak rates

Air conditioning power consumption varies with system age, refrigerant type, house insulation, and outdoor temperature. A 5-star rated reverse-cycle system uses approximately 35–40% less electricity than an equivalent 2.5-star system.