Sizing solar to run your air conditioner in Perth

Perth's climate makes air conditioning essential for most households. AC is also the single biggest driver of electricity bills — especially the 3pm–9pm Midday Saver peak period when cooling demand is highest and solar generation is declining. Getting the solar system size right for your AC usage is one of the most important sizing decisions you'll make.

How much electricity does AC use in Perth?

Typical AC electricity consumption by type:

| AC type | Draw range | Typical Perth use | |---|---|---| | 2.5kW split system (small room) | 0.6–1.2kW | 3–6h/day in summer | | 5–7kW split system (medium room or large space) | 1.2–2.5kW | 4–8h/day in summer | | Ducted reverse cycle (10kW+) | 2.5–5kW | 5–10h/day in summer | | Evaporative cooling (ducted) | 0.7–1.5kW | 4–8h/day in summer |

Annual AC electricity consumption for a typical Perth household:

• Single split system (medium): 1,500–2,500kWh/year
• Ducted reverse cycle: 4,000–10,000kWh/year (highly variable with house size, insulation, thermostat settings)

At A1 (33.26c/kWh), a typical ducted reverse cycle might cost $1,300–$3,300/year in electricity.

The timing mismatch: when AC runs vs when solar generates

Perth solar generation profile (6.6kW north-facing):

• 8am: 0.5–1kW
• 10am: 3–4kW
• 12pm (peak): 5–6kW
• 2pm: 4–5kW
• 3pm: 3–4kW (DEBS peak window starts)
• 4pm: 2–3kW
• 5pm: 1–2kW
• 6pm: 0.3–0.8kW
• 7pm+: near zero

Perth AC demand profile (summer, house occupied):

• Morning: low (house cool from overnight)
• 11am–2pm: medium (heating up, AC often starts)
• 2pm–6pm: high peak (hottest part of the day)
• 6pm–10pm: medium to high (evening cooling down)
• 10pm+: low (house and outdoor temp cooling)

The timing problem: AC demand peaks (2pm–8pm) while solar generation is declining (3pm+) and hits zero by 7–8pm. This means a significant portion of peak summer AC consumption cannot be offset by solar generation — it requires either battery storage or grid import at peak Midday Saver rates (55.33c/kWh).

How solar offsets AC electricity

What solar covers well:

• Morning AC use (10am–2pm): solar peak coincides with AC ramping up
• Lunchtime AC: well-covered by solar generation
• Pre-cooling strategy: running AC hard at 10am–2pm (when solar is at peak) to cool the house, then reducing AC from 3pm onward

What solar covers poorly (without battery):

• Evening AC use (5pm–10pm): solar has mostly stopped
• Overnight AC: solar is zero

Sizing solar to target AC offset

Rough sizing guide:

For a single split system (medium, 5kW unit): Drawing approximately 1.5kW on average during operation. A 6.6kW solar system generates enough during daytime to fully cover this during the 9am–4pm window on most summer days, with surplus for other loads.

For ducted reverse cycle (10kW system): Drawing approximately 3–4kW during operation. A standard 6.6kW system partially offsets ducted AC — covering roughly half the draw during peak generation hours. A 10–13kW system covers more of the daytime AC load.

For maximum AC coverage (large ducted + battery pre-cooling strategy): A 10kW+ solar system paired with a 10kWh battery enables:

• Battery charges from solar surplus during 9am–3pm
• AC runs hard until 3pm on solar
• Battery powers evening AC from 3pm–8pm (discharged using stored solar)

The pre-cooling strategy for Perth homes

One of the most effective ways to align AC with solar generation:

1. Set AC to pre-cool the house during 9am–2pm — run it to 20–22°C while solar covers the cost
2. Turn AC up to 25–26°C at 2pm — the house retains cool temperature from the pre-cooling
3. Run AC minimally or off from 6pm — house is still cool from the afternoon pre-cool

This strategy can reduce AC consumption during the 55.33c/kWh Midday Saver peak by 30–50% without reducing comfort.

Home features that enable pre-cooling:

• Good insulation (ceiling, walls, double-glaze)
• External window coverings (external roller blinds, awnings) — prevent heat gain before the house heats up
• Ceiling fans to distribute cooled air
• Curtains drawn on west-facing glass from 1pm–4pm

Ducted vs split system for solar homes

Split systems for solar homes: Individual room control means you can cool only occupied rooms during solar hours and avoid cooling empty rooms. More efficient for partial-occupancy households.

Ducted systems for solar homes: More efficient for whole-home cooling. Higher upfront power draw, but covers multiple zones simultaneously. The pre-cooling strategy is most effective with ducted systems (cooling the whole house thermal mass, not just room air).

Zone control for ducted: If your ducted system has zone control, switch to 2–3 zones during solar hours (occupied rooms) and expand to full-home cooling by 2pm–3pm when solar is still strong.

Practical system sizing recommendation

For a Perth household with a single medium split system: Standard 6.6kW solar is adequate for AC offset during daytime. Evening AC is covered by grid import (or battery if installed).

For a Perth household with large ducted reverse cycle:

• 6.6kW: covers approximately 30–50% of daytime AC load
• 10kW: covers approximately 60–80% of daytime AC load (may require three-phase or second inverter)
• 6.6kW + 10kWh battery + pre-cooling: covers most AC load including early evening

For highest AC offset without battery: A larger panel array (10kW+) combined with aggressive pre-cooling during 10am–2pm will minimise 55.33c/kWh peak imports. The battery is not strictly required but significantly improves evening AC economics.

Your actual AC load and solar offset depends on your AC type, usage patterns, house insulation, and system size. Upload your Synergy bill to BillWise to calculate your AC's share of your electricity cost and model the savings from different solar configurations.