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Backup power depends on discharge ratings, inverter capacity, circuit selection, and the way the system is wired. This guide covers what to check before signing a quote.
Whether a battery delivers backup power depends on the complete system design, not the battery model. The same battery can be backup-capable or not, depending on the inverter, wiring, transfer switch, and installation choices. Confirm backup scope with your installer before signing a quote.
Many battery systems are grid-tied only and stay off during a blackout, even when fully charged. This is a safety requirement: the system must not backfeed electricity into the grid while Western Power crews are working on the lines.
To provide backup power, a system needs:
Backup capability typically adds cost: extra hardware (transfer switches, backup boxes) and additional install labour to rewire selected circuits.
Three specifications determine what backup actually delivers:
Determines how long backup lasts. A 10 kWh battery at a 500 W load runs about 20 hours; at 3,000 W it runs about 3 hours.
Ask: "What's the usable capacity for backup?"
Determines how much you can run at once. A 5 kW output cannot run a 7 kW air conditioner.
Ask: "What's the continuous backup output rating?"
Handles startup surges from motors such as pumps and compressors. Fridges may draw three times their running wattage when the compressor kicks in.
Ask: "What's the peak output for motor startup?"
Most backup systems are designed for essential circuits only, typically two or three circuits covering the loads that matter during an outage. Whole-home backup is possible, but it needs a much larger inverter and battery.
The most common and cost-effective approach. The installer wires specific circuits to the battery's backup output.
Typical loads:
Total: ~300–600 W continuous. Most backup-capable systems handle this load.
Needs larger inverter capacity (often 8–15 kW or more) and larger batteries, and costs noticeably more.
Additional loads:
Total: can exceed 10 kW. Requires careful system sizing.
Some battery systems have low continuous discharge ratings: they can store energy but only release it slowly. These systems may not be suitable for backup use, even when they technically support islanding.
For example, a battery with a 2.5 kW maximum discharge rate cannot run a 3 kW load, regardless of stored energy. If backup matters, ask your installer about the continuous discharge rating and whether it matches your expected backup loads.
"What is the continuous and peak discharge rating of this system in backup mode, and will it handle the startup surge of my fridge / freezer?"
Whether your panels can recharge the battery during a long blackout depends on system design:
Ask your installer: "If there's a blackout during the day, will my solar panels recharge the battery, or do I have to wait for grid power to return?"
Before signing a quote, confirm what backup capability the system actually provides:
"All batteries provide backup." No. Many systems are grid-tied only and provide zero power during a blackout, even when fully charged.
"A bigger battery means longer backup." Partly true. The inverter's output rating sets what you can actually run. A 20 kWh battery paired with a 3 kW inverter still cannot run a 5 kW air conditioner.
"I can run my whole house on battery backup." Possible, but it needs careful system design with large inverter capacity (8–15 kW or more). Most installations are sized for essential circuits only.
"Solar will keep me going indefinitely during a blackout." Only if the system supports solar charging in backup mode, and only during daylight hours with adequate generation.
Backup capability varies widely between systems. Ask the installer to spell out what backup you're getting, which circuits are covered, and the limitations. Get the backup scope documented on the quote before signing.