How long will your battery last in a Perth blackout? A guide to backup duration

During a grid outage, your battery powers whatever loads you run — until it's depleted. How long that takes is simple arithmetic: battery capacity ÷ load power = hours. But Perth households often overestimate backup duration because they underestimate their continuous load.

The basic calculation

Backup duration = (battery usable capacity in kWh) ÷ (continuous load in kW)

Example:

• 10kWh battery (80% usable depth of discharge = 8kWh usable)
• Household running refrigerator (0.15kW) + LED lights (0.1kW) + router + phone chargers (0.05kW) = 0.3kW continuous load
• Duration: 8kWh ÷ 0.3kW = 26.7 hours

Example with air conditioning:

• Same 10kWh battery (8kWh usable)
• Add a ducted reverse-cycle AC at 3kW continuous
• New total load: 3.3kW
• Duration: 8kWh ÷ 3.3kW = 2.4 hours

Air conditioning changes backup duration by an order of magnitude.

Common Perth household appliances: power draw

| Appliance | Typical power draw | |---|---| | Ducted reverse-cycle AC (large, running) | 2.5–6kW | | Split-system AC (2.5kW cooling, running) | 0.7–1.2kW | | Electric oven | 2–3kW | | Electric cooktop (one element) | 1–2kW | | Induction cooktop (one element) | 0.5–2kW | | Electric hot water system (element on) | 2–4kW | | Refrigerator | 0.1–0.25kW (cycling, not continuous) | | Chest freezer | 0.05–0.15kW | | LED lighting (whole house) | 0.1–0.3kW | | TV (55") | 0.08–0.15kW | | Wi-Fi router | 0.01–0.02kW | | Laptop | 0.04–0.1kW | | Phone charging | 0.01–0.02kW each | | Pool pump (variable speed, low) | 0.2–0.5kW | | Pool pump (single speed, running) | 0.75–1.5kW |

Battery usable capacity vs rated capacity

The full rated capacity of a battery (e.g. 10kWh) is not all usable:

Depth of discharge (DoD): Most batteries are warranted with a maximum DoD of 80–90%. A 10kWh LFP battery with 90% DoD has 9kWh usable. Going to 100% DoD accelerates degradation and typically voids the warranty.

State at outage time: If the grid goes down at 8pm and your battery is at 50% charge (5kWh remaining on a 10kWh battery), you have 5kWh of charge available — not 10kWh.

Inverter efficiency losses: The inverter converting battery DC to household AC has approximately 95–97% efficiency. A 10kWh battery delivers approximately 9.5–9.7kWh of usable AC energy.

Realistic usable capacity (10kWh battery, full charge at outage start):

• 10kWh × 90% DoD × 96% inverter efficiency ≈ 8.6kWh

Perth summer vs winter blackout considerations

Perth summer blackouts: Summer storms and transformer failures are the most common Perth outage causes. If the outage occurs during daylight hours, your solar panels (if installed with backup capability) continue to generate — this extends backup duration significantly.

Important: Standard grid-connected solar systems without a battery with backup functionality STOP generating during a grid outage (safety requirement — anti-islanding protection). A system with backup capability (specific hybrid inverters with EPS — Emergency Power Supply mode) can continue to self-power from solar + battery during an outage.

Inverter backup capability: Not all hybrid inverters support backup/EPS mode. Confirm with your installer whether your inverter supports off-grid backup operation. Sungrow SBH, BYD HVS/HVM, Goodwe Lynx, and Enphase systems all support various forms of backup — but the specific configuration matters.

Summer backup load recommendation: In a Perth summer, running AC for an extended blackout is battery-intensive. Prioritise:

• Refrigeration (critical for food safety and medication)
• Essential lighting
• Charging phones and devices
• Fan cooling (much lower power than AC)

Planning backup duration by scenario

Basic comfort backup (no AC, no cooking):

• Refrigerator + freezer + lighting + devices: ~0.4–0.6kW
• 10kWh battery (8.6kWh usable): 14–21 hours

Medical equipment backup:

• CPAP: ~0.05–0.1kW
• Oxygen concentrator (if applicable): 0.3–0.6kW
• Refrigerator for medication: 0.15kW
• Lighting and devices: 0.15kW
• Total: ~0.7–1.0kW
• 10kWh battery: 8–12 hours

If medical equipment is essential and grid reliability is a concern, a separate UPS (Uninterruptible Power Supply) for the specific equipment is a more targeted solution than whole-home battery backup.

Summer comfort backup (with split-system AC):

• Split-system AC (2.5kW, moderate operation): 0.8–1.2kW average
• Refrigerator + lighting + devices: 0.4kW
• Total: ~1.2–1.6kW
• 10kWh battery: 5–7 hours

Summer comfort backup (ducted AC):

• Ducted AC (large home): 3–5kW
• Other loads: 0.4kW
• Total: ~3.4–5.4kW
• 10kWh battery: 1.5–2.5 hours

Extending backup duration

Prioritise loads: During a blackout, turn off non-essential loads. Disconnecting the ducted AC and using a single fan extends backup from 2 hours to 14+ hours.

Solar recharging (if backup-capable inverter): During a daytime outage, a backup-capable system recharges the battery from solar. A 6.6kW system in Perth summer generates approximately 2–4kW during midday — partially offsetting any running loads and recharging the battery.

Thermal mass: A fridge maintains safe temperature for approximately 4 hours unopened. A full freezer maintains temperature for 48 hours. Minimising fridge opening during a blackout reduces the time you need refrigeration power.

For most Perth households, a 10kWh battery provides meaningful backup for essential loads (fridge, lights, devices) for 12–24 hours without AC — and approximately 2–5 hours if ducted AC runs continuously. Backup planning means deciding in advance which loads to prioritise.