Home battery backup: what keeps running in a Melbourne blackout

Does a solar battery work in a blackout?

Only if it is set up for backup. Solar Victoria says standard solar systems, with or without a battery, are “designed to switch off during a power outage to protect homes from power surges and those potentially working on nearby electricity lines”. A hybrid (‘multimode’) inverter wired for backup changes that: it disconnects the house from the grid and keeps supplying the circuits on its backup output, sending nothing to the grid. Solar Victoria calls this ‘islanding’.

1. Grid on: the backed-up circuits run as normal, from the panels, the battery and the grid.
2. Grid fails: the inverter disconnects from the grid and switches the backed-up circuits to the battery, in under 10 ms on Sungrow’s hybrids.
3. During the outage: the battery, and the panels in daylight, run those circuits; nothing is exported.
4. Grid back: the inverter reconnects and the home returns to normal operation.

Because backup depends on the inverter and the system design, Solar Victoria advises telling your installer you want it. It also notes that outages are rare in Victoria but can reach any grid-connected home (Solar batteries: what you need to know about backup power; Power outages and all-electric homes).

How a solar battery works

Whole-home vs essential-circuits backup vs a backup outlet

Which set-up a home gets depends on the inverter and the switchboard wiring. Solar Victoria notes that some inverters run the home continuously in an outage while others back up only one or two priority circuits such as your fridge and lights.

Whole-home backup still runs on the inverter’s backup rating, 10 kW on a Sungrow SH10RS, so the loads on at once must stay under it, and the battery empties faster with the whole house on.

Backup on each battery, compared

Switchover time and what will run

Switchover time

Sungrow states a backup switch time under 10 ms, a hundredth of a second, for its SH-RS and SH-T hybrids, and calls it a “seamless switch”. Alpha ESS lists the SMILE-G3’s backup as “UPS” (uninterruptible power supply) and states no switch time on its Australian datasheet.

What will run

The inverter’s backup rating caps how much can run at once (kW); the energy left in the battery caps how long (kWh). After dark, with no solar, the battery’s own power rating applies too.

An example, by our arithmetic: Sungrow rates the 12.8 kWh SBR128 battery at 7.68 kW. On a 10 kW SH10RS at night, the backed-up circuits get the smaller of the two, about 7.68 kW; in daylight the panels add to it. Sungrow notes that its three phase peak figures “can be reached only if PV and battery power is sufficient”.

If someone at home relies on powered medical equipment, Solar Victoria advises getting specialist advice on backup, because typically home battery systems are not appropriate to service the required power load as a primary source.

Work out your backup kWh

How Sungrow SBR and Alpha ESS SMILE handle backup (makers’ specs)

Sungrow’s SBR and Alpha ESS’s SMILE-G3 set out backup differently in their makers’ datasheets. Your written quote names the exact battery, inverter and backup set-up.

Sungrow SBR high-voltage battery

The SBR battery has no backup output of its own: backup comes from the Sungrow hybrid inverter it connects to. The single phase SH8.0RS and SH10RS and the three phase SH15T to SH25T have a built-in 63 A bypass for whole-home backup; the SH5.0RS and SH6.0RS back up the circuits on their backup port, at up to 5 or 6 kW. All switch over in under 10 ms.

Its power rating rises with the stack: 3.84 kW for 6.4 kWh, up to 15.36 kW for 25.6 kWh (SBR datasheet).

Sungrow batteries in Melbourne

Alpha ESS SMILE-G3 home battery system

The SMILE-G3 is battery and inverter in one system. Its backup is a 5 kVA “UPS” backup port: the circuits wired to it keep running, up to 5 kVA in all; Alpha ESS doesn’t state whole-home backup. Both the S5 hybrid and the B5 for existing solar have the port.

It uses 9.3 kWh modules: 1 to 4 modules (9.3 to 37.2 kWh) on the single phase S5 or B5; up to 6 (55.8 kWh) on a three phase SMILE-G3-T5 to T20.

Alpha ESS batteries in Melbourne

Of the other batteries in our comparison, Tesla Powerwall 3 and Sigenergy SigenStor state whole or partial-home backup through their own gateways.

Sizing for blackouts and the backup reserve

Backup storage is the load you keep on (kW) times the hours you want it: 1.5 kW for 10 hours is 15 kWh. Solar Victoria says how much backup power you get and how long it lasts depend on the size of your solar system, how much electricity you use during the outage and what the appliances you keep running draw.

• It may not be full. A blackout can start late at night, after the evening has used the battery.
• The backup reserve. Some systems keep part of the battery for outages. Sungrow says its iSolarCloud app sets “how much battery percentage should be kept as reserve” (Sungrow backup factsheet for its SH5.0–10RT three phase hybrid, Europe). A 20% reserve on a 12.8 kWh battery holds back about 2.56 kWh that your evenings don’t use.
• Winter refills less. Spare solar can recharge the battery during an outage on systems built for it, but winter leaves less to spare: PVGIS models a north-facing 6.6 kW system in Mount Waverley, Melbourne at about 14.4 kWh a day in June against 34.9 in January.
• Size it the first time. There is one battery claim per property, and modules added later don’t get the rebate if the first battery already claimed it.

Battery size calculator: backup option

Backup on three phase homes

On a three phase home, backup depends on whether the inverter is single or three phase. A single phase hybrid connects to one phase, so only circuits on that phase, or moved onto it, can be backed up, and anything that needs three phase power won’t run in a blackout. A three phase hybrid can back up all three phases.

Sungrow’s SH15T to SH25T have a 63 A bypass on each phase for whole-home backup, supply 15, 20 or 25 kW in backup, and support “100 % unbalance output in backup and on grid mode”, so the three phases don’t need equal loads. Among the batteries in our comparison, Sigenergy SigenStor, Sungrow SBR, Alpha ESS SMILE-G3, GoodWe Lynx Home F G2, Fox ESS EQ5500, BYD Battery-Box Premium HVS / HVM and Enphase IQ Battery 5P with FlexPhase have three phase set-ups; Tesla Powerwall 3 is a single phase unit.

Three phase solar and batteries

Check whether your home is three phase

Hybrid vs off-grid: why Melbourne homes stay connected

A hybrid solar system is grid-connected solar with a battery and, when wired for it, backup; the grid stays as the fallback. An off-grid (stand-alone) system has no grid connection at all: Solar Victoria describes it as solar panels, inverters, a battery bank and a backup petrol or diesel generator, the generator primarily used to support the system during cloudy winter periods (Solar Battery Buyers Guide, section 3).

That winter gap is why a Melbourne home with a grid connection usually keeps it: the June solar output above is about 41% of January’s, and the grid covers the shortfall without a generator. Stand-alone systems follow their own standard, AS/NZS 4509.1:2009 (Stand-alone power systems, Part 1: Safety and installation), which Energy Safe Victoria lists beside the battery standard AS/NZS 5139:2019, and Solar Victoria notes that some retailers and installers specialise in them. This guide covers backup on grid-connected homes.

Export or store your solar

What backup adds to an install

Backup is mostly switchboard work: the circuits you want kept on move to the inverter’s backup output, or the whole supply runs through the inverter’s bypass or the maker’s gateway. Energy Safe Victoria says every circuit supplied from a backup output needs RCD (safety switch) protection of the type the inverter maker specifies; with partial backup, only the backed-up circuits count. Type AC RCDs may not be installed in Australia since 1 May 2023 (Energy Safe Victoria).

• A backup output: on the hybrid inverter, or a separate gateway for batteries that use one.
• Switchboard work: moving the chosen circuits and fitting the right safety switches.
• A compliant spot for the battery: under AS/NZS 5139:2019, a battery can’t go in a ceiling space or wall cavity, or under a stairway or access walkway, or in an escape route, among other places.
• Look, don’t touch: all of it is work for a licensed electrician, inspected before switch-on.

How a battery install is inspected

Battery backup: your questions