Solar and air conditioning for heat-sensitive households in Perth

For most Perth households, air conditioning is a comfort appliance. For households with heat-sensitive members — people living with multiple sclerosis, heart conditions, chronic lung disease, or elderly residents vulnerable to heat stress — air conditioning is a medical necessity. This changes how to size solar and battery storage.

Why heat sensitivity matters for solar design

The standard solar design question is: what size system maximises my return on investment?

For a household with a heat-sensitive member, the question is different: what system ensures I can run air conditioning reliably through Perth summer, including during grid outages?

Perth's summer grid is stressed. The SWIS (South West Interconnected System) runs at peak load during extreme heat events — precisely when heat-sensitive households most need cooling. Grid-connected solar plus battery storage provides a layer of resilience that has direct health implications for these households.

Sizing solar for high cooling loads

A standard Perth household solar design assumes self-consumption of 30–40% and sizes around average daily usage. For heat-sensitive households, the calculation centres on cooling load:

Split system air conditioner: 0.7–1.2 kW running load (2.5–3.5 kW rated). A split system running 8 hours per day uses 5.6–9.6 kWh.

Ducted air conditioning: 2.5–6 kW running load. Ducted running 10 hours per day uses 25–60 kWh.

For a household running split systems heavily during summer (say 12–15 kWh/day in cooling loads), a standard 6.6kW system generating ~10,000 kWh/year (27 kWh/day average) may still leave significant grid import during peak cooling periods if the household is otherwise a high consumer.

Rule of thumb for high-cooling households:

• Calculate your summer cooling load (hours × rated kW × 0.5 for average running)
• Add your base load (fridge, lighting, appliances: typically 5–8 kWh/day)
• This is your total summer daily usage
• Size the system to cover 80–100% of this on a clear Perth summer day

For a household with 15 kWh/day summer total, a 6.6kW system generating ~28 kWh on a good Perth summer day provides adequate coverage. But on overcast days in summer (which do occur), the shortfall returns — this is where battery storage becomes medically important rather than merely financially useful.

Battery storage: minimum backup for cooling

The standard advice is to size battery storage around 1–1.5× your average evening usage. For heat-sensitive households, the more relevant calculation is:

How long can I run cooling if the grid goes down?

A 10kWh battery (9.0kWh usable at 90% DoD) running a split system at 1.0 kW average: 9 hours. Running a ducted system at 3.5kW average: 2.6 hours.

For overnight backup during a summer heat event (a critical scenario for heat-sensitive households), battery storage needs to cover the hours when solar isn't generating (roughly 6pm to 7am = 13 hours). At 1.0 kW cooling load, that requires ~13 kWh of usable storage — two standard 10kWh batteries.

Practical recommendation for heat-sensitive households:

• Minimum one battery (10kWh) if primary concern is daytime outage coverage
• Two batteries or a 20kWh system if overnight heat event resilience is required
• EPS (Emergency Power Supply) or backup-capable inverter is essential — standard grid-tied inverters shut down during outages per anti-islanding requirements

The EPS/backup inverter requirement

Standard grid-tied solar systems cannot power your home during a grid outage — the inverter shuts down to protect line workers. This is a critical distinction for heat-sensitive households: having solar panels on the roof does not mean you have backup power.

To run cooling (or any load) during a grid outage, you need:

• A battery storage system and
• An inverter/battery system with EPS (Emergency Power Supply) or "backup" mode capability

EPS-capable systems include: Sungrow SH series, SolarEdge Energy Hub, Goodwe ES/EH series, Tesla Powerwall (whole-home backup), Alpha ESS (backup-capable models), Sigenergy. Most quality hybrid inverters sold in Perth include this capability — but verify explicitly with the installer, as not all configurations are set up for backup mode out of the box.

Whole-home vs partial backup: Some systems can power the entire home during an outage; others are limited to a "backup circuit" (essential loads panel) that includes specific circuits (typically 1–2 circuits). For a household that needs air conditioning during outages, confirm the AC is wired to the backup circuit.

Financial assistance and concessions

Pensioner Electricity Credit: Eligible pensioners and low-income households receive a concession credit applied to Synergy bills. This does not directly reduce the cost of solar but reduces the overall electricity burden.

Energy Assistance Payment: One-off payment for eligible households facing energy hardship. Contact Synergy or the Department of Communities.

Medical Equipment Energy Subsidy: Households with approved power-dependent medical equipment (home dialysis, oxygen concentrators, powered wheelchairs, etc.) may qualify for energy subsidy through the WA Department of Communities — Energy Assistance Team. Phone: 1800 065 596. This is separate from the general concession.

Solar and battery rebates: The WA Battery Incentive ($130/kWh, max $1,300, 10kWh cap) applies to Synergy-connected battery installations regardless of medical need. STCs apply to the solar panels. There is no specific medical-need uplift for solar or battery rebates in WA.

Tariff choice for high cooling households

A1 (standard flat rate, 33.2621c/kWh as of 1 Jul 2026): Simple. Cooling any time costs the same.

Midday Saver: 8.8511c/kWh from 9am–3pm, 44.1326c/kWh 3pm–9pm (peak), standard rate other times. For households that must run cooling primarily during daytime, Midday Saver can reduce costs significantly. But for households that need cooling through the evening peak (3pm–9pm), the Midday Saver peak rate is punishing — especially since peak heat events often extend through late afternoon.

For heat-sensitive households where cooling runs through the evening: A1 is likely safer than Midday Saver unless you have battery storage to cover the evening peak period.

Checking your insurance and building provisions

Some household insurance policies cover loss of cooling equipment (freezers, refrigerated medications). Speak to your insurer about any endorsements for medically-required cooling if your household has specific requirements.

For homes with heat-sensitive members, it's also worth verifying that your home's ceiling insulation is adequate — Perth homes lose significant heat through the roof, and poor insulation makes the cooling load worse regardless of your solar and battery configuration.

For heat-sensitive households in Perth, the solar and battery design question is less about financial optimisation and more about reliability. Ensure your system includes EPS-capable backup, size batteries for overnight heat event scenarios if needed, and confirm the AC circuit is wired to the backup load panel. The Medical Equipment Energy Subsidy through the Department of Communities is worth investigating if you have power-dependent medical equipment.