Ducted air conditioning and solar in Perth: sizing, timing, and costs

Air conditioning is the largest electricity load in most Perth homes — by a wide margin. In summer, a ducted system running for 8 hours on a 40°C day can consume more electricity in that one day than your fridge does in a month.

Solar panels change the economics significantly, but the timing relationship between when Perth heats up (afternoons) and when solar peaks (midday) means the benefit requires some thought.

How much electricity does ducted AC use?

A typical Perth ducted reverse-cycle air conditioner:

| System type | Cooling capacity | Typical draw at full load | |---|---|---| | Small ducted (4-zone) | 8–10kW cooling | 2.5–3.5kW electrical | | Medium ducted (5–6 zone) | 12–14kW cooling | 3.5–5kW electrical | | Large ducted (whole home) | 16–20kW cooling | 5–7kW electrical |

Running a medium ducted system at 4kW for 8 hours costs:

• At A1 rate (33.26c/kWh): 4kW × 8hr × $0.3326 = $10.64/day
• At Midday Saver super off-peak (8.85c/kWh): 4kW × 8hr × $0.0885 = $2.83/day
• At Midday Saver peak (55.33c/kWh): 4kW × 8hr × $0.5533 = $17.71/day

On a typical Perth summer week with 5 full-load AC days: the cost ranges from $14.15/week (Midday Saver off-peak) to $88.55/week (Midday Saver peak) — a 6× difference based purely on timing.

The timing problem: Perth heat vs solar generation

Perth's characteristic summer pattern creates a timing mismatch:

• Solar generation peaks from approximately 10am to 2pm
• The temperature peaks from approximately 2pm to 6pm
• Electricity demand peaks from approximately 3pm to 8pm (when people are home and AC is running)

Under Midday Saver, that 3pm–9pm window is the peak rate (55.33c/kWh). This is precisely when Perth households most want to run their AC — and precisely when it's most expensive to do so.

Running AC on solar (10am–2pm): A 6.6kW solar system generating 5–6kW at midday can offset all or most of a medium ducted system's load. During this window, you're effectively running AC on solar at near-zero marginal cost.

Running AC in the afternoon (2pm–6pm): Solar output is declining (from 5kW at noon to 0kW at sunset). On a clear 4pm Perth summer afternoon, a 6.6kW system might be generating 2–3kW — partially offsetting a 4kW AC load. You'd still be importing grid electricity, and on Midday Saver that's at the 55.33c/kWh peak rate.

The pre-cooling strategy

The most effective way to use solar generation for afternoon cooling is pre-cooling — running AC hard during the solar peak (10am–2pm) at Midday Saver's 8.85c/kWh rate, then reducing or stopping AC as temperatures hit peak in the afternoon.

The principle: thermal mass in your home (concrete slab, brick walls, furniture, water) stores the cool temperature. A well-insulated Perth home with a concrete slab can maintain temperatures 3–5°C below the outdoor peak for 2–3 hours if pre-cooled to 20–22°C before the outdoor temperature peaks.

In practice:

1. Set AC to 22–23°C and run from 9am–2pm (Midday Saver super off-peak / solar peak)
2. Reduce setpoint to 25–26°C from 2pm–3pm as outdoor temperatures rise
3. After 3pm, use ceiling fans to extend comfort and minimise AC run time in the expensive peak window
4. Resume normal cooling after 9pm (Midday Saver off-peak at 24.34c/kWh)

This strategy requires a house with reasonable insulation and few heat sources. Poorly insulated houses with large single-glazed west-facing windows gain heat quickly and are harder to pre-cool effectively.

Adding solar: how big a system for AC users?

If air conditioning is a primary motivation for solar, the sizing question changes. A 6.6kW system is the standard Perth choice, but a 10kW or larger system may be justified for households with large ducted AC loads.

Why larger for AC: A 6.6kW system generating 5kW at peak covers a medium ducted system running at full capacity. A 10kW system generating 8kW covers the same system AND other midday loads — leaving more to self-consume across the whole house, less to export at 2c/kWh DEBS.

Network limits: Standard Perth single-phase connections limit inverter capacity to 5kW, allowing up to 6.6kW panels. Three-phase connections allow larger inverters. If you have three-phase power (ask your electrician to check your switchboard), a 10kW system may be feasible without a network upgrade.

A battery for AC: A battery changes the AC equation — you can charge the battery from midday solar and use it to power afternoon AC in the peak window (saving 55.33c/kWh on Midday Saver vs exporting at 2c/kWh). A 10kWh battery might provide 2–3 hours of medium-sized ducted AC operation. Given the peak rate savings, the maths for a battery are more compelling for heavy AC users.

Reverse-cycle vs evaporative cooling

Perth is one of the few places in Australia where evaporative cooling still has a strong case:

Evaporative cooler advantages:

• Running cost: 200–500W vs 2,500–5,000W for reverse-cycle ducted — roughly 5–10× cheaper per hour
• Initial cost: significantly cheaper ($1,500–$4,000 installed vs $5,000–$15,000 for ducted)
• Cooling effect: on a dry Perth summer day (common in November–February), evaporative cooling is highly effective
• Works with windows open: houses can breathe, reducing stuffiness

Evaporative cooler disadvantages:

• Doesn't work in humid conditions: on humid summer days (particularly during February–March with weather coming off the ocean), efficiency drops significantly
• Can't heat: evaporative coolers don't heat. Perth winters are mild but evenings reach 8–12°C — a separate heating solution is needed
• Water usage: evaporative coolers use significant water (5–25 litres/hour depending on size)
• Climate change: Perth summers are getting hotter and more humid over time; evaporative cooling's advantage over reverse-cycle is gradually reducing

The practical split: Many Perth households run evaporative cooling for the bulk of summer (November–January, typically dry) and reverse-cycle for the humid shoulder periods (February–March). Others have moved entirely to reverse-cycle for the year-round flexibility (heat in winter, cool in summer).

If you're upgrading from evaporative to reverse-cycle for solar self-consumption, the higher capital cost of reverse-cycle is partially offset by the dramatically better solar pairing — a 500W evaporative cooler can't absorb much excess solar midday capacity, while a 4kW reverse-cycle can.

Ceiling fans: the underrated tool

Ceiling fans use 50–150W — less than 5% of the electricity of a ducted AC system. For Perth's spring and autumn (when outdoor temperatures are 22–32°C), ceiling fans often provide enough comfort without any AC at all.

In summer, ceiling fans extend the AC comfort window — running AC to pre-cool the house to 23°C, then switching to fans to maintain comfort at an apparent 25–26°C through the peak temperature window, uses a fraction of the energy of continuous AC.

If your home doesn't have ceiling fans, adding them is a high-ROI improvement: typical installation $150–$350 per fan, energy savings of 1–3 kWh/day on heavy summer days compared to AC-only approach.

AC power consumption varies significantly with system efficiency rating, house size, insulation, and outdoor temperature. The figures above are typical ranges — check your system's specifications for more precise estimates. Tariff rates effective 1 July 2026.