Sizing, Worked Examples and Quote Checklists
Home battery backup in a blackout: essential circuits vs whole-home backup and what it costs
Does a home battery work in a power outage? How backup works in Australia, essential-circuit vs whole-home options, power limits, sizing for outages and quote questions.
A home battery only keeps your lights on in a blackout if it is installed with backup. Grid-connected solar inverters must shut down when the grid fails so they don't send power into lines that workers may be repairing; a battery with backup isolates your home from the grid and keeps supplying power. There are two main approaches: essential-circuit backup, where selected circuits such as lights, the fridge and internet are kept running, and whole-home backup, where the whole switchboard stays live within the battery's power limit. Backup usually adds hardware such as a gateway or changeover switch, plus wiring changes, so it is a common line item that differs between quotes. How long backup lasts depends on the battery's stored energy at the time and your load, not just its capacity.
For general sizing, see the battery sizing guide. Prices below are illustrative.
How backup works
| Situation | Without backup | With backup |
|---|---|---|
| Grid fails during the day | Solar inverter shuts down; no power at home | Battery isolates the home; solar may keep charging the battery (system-dependent) |
| Grid fails at night | No power | Battery supplies backed-up circuits until its reserve runs out |
| Changeover | – | Automatic, in a fraction of a second to a few seconds depending on equipment; some devices may reset |
| Grid returns | Normal | System reconnects automatically |
Not every battery or inverter offers backup, and some offer it only with an extra device. Some systems can restart solar charging during a long outage; others cannot. Ask the installer exactly how your proposed system behaves.
Essential circuits or whole home?
| Feature | Essential-circuit backup | Whole-home backup |
|---|---|---|
| What stays on | Chosen circuits: lights, fridge, internet, some power points | Everything on the switchboard, within the battery's power limit |
| Hardware | Separate backup circuit or sub-board | Gateway or changeover device for the whole supply |
| Cost tendency | Lower | Higher |
| Risk of overload | Low, because large loads are excluded | Higher; air-con, oven and hot water at once can exceed the inverter |
| Battery energy use | Slow, lasts longer | Faster, unless you manage loads |
| Good for | Occasional short outages | Frequent or long outages, rural and storm-prone areas, home medical needs |
Large loads such as ducted air-conditioning, electric hot water, induction cooktops and pool pumps can exceed a battery's continuous output. Whole-home backup systems may shed those loads or trip. Plan what you actually need running.
Sizing for an outage
Work out the energy and power you need during an outage.
Illustrative essential load list:
| Appliance | Power (W) | Hours over 12 hours | Energy (kWh) |
|---|---|---|---|
| Fridge-freezer | 150 average | 12 | 1.8 |
| LED lights | 100 | 5 | 0.5 |
| Modem and router | 20 | 12 | 0.24 |
| Phone and laptop charging | 60 | 4 | 0.24 |
| TV | 100 | 3 | 0.3 |
| Ceiling fans | 120 | 6 | 0.72 |
| Total | 3.8 kWh |
These essentials use about 3.8 kWh over 12 hours. A 13.5 kWh battery that is half-full when an evening outage starts has about 6.75 kWh available, enough for these loads overnight. Add a reverse-cycle air conditioner at, say, 1.5 kW for six hours (9 kWh) and the same battery would run out.
Backup reserve
Most batteries let you set a backup reserve: a percentage kept aside for outages and not used for daily savings. Setting a 30% reserve on a 13.5 kWh battery keeps about 4 kWh for outages, but reduces the energy available for daily bill savings by the same amount. This lowers the payback benefit; see the payback worked example.
If you join a VPP, ask how the operator treats your reserve and whether events are paused when storms are forecast. See virtual power plants explained.
Medical equipment and life support
If someone in your home relies on life support or medical equipment, register with your electricity retailer and distributor as a life support customer, whatever battery you have. A battery is not a replacement for that registration or for a medical backup plan. Ask your installer whether the backup changeover is fast enough for the specific equipment.
Illustrative cost differences
| Backup option | Illustrative extra cost |
|---|---|
| No backup | $0 |
| Essential-circuit backup with sub-board | $600–$1,500 |
| Whole-home backup gateway | $1,200–$2,500 |
These ranges are assumptions for comparing quotes, not market prices. Enter the extra in the "extra site work" box of the installed cost estimator.
Questions to ask installers
- Does this battery provide backup, and is the backup hardware included in the quote?
- Which circuits will be backed up?
- What is the continuous power during backup, and which large loads are excluded?
- Can the solar keep charging the battery during a long outage?
- How fast is the changeover, and will any devices reset?
- Can I set a backup reserve, and how does a VPP affect it?
- Will a switchboard upgrade be needed?
FAQ
Will my solar panels work in a blackout without a battery?
Normally no. Grid-connected inverters shut down for safety when the grid fails. Some inverters have a limited daytime-only emergency outlet; check your model's manual.
Does backup affect the federal discount?
No. The STC discount depends on usable capacity and eligibility, not on whether backup is installed.
Can I add backup later?
Often, depending on the system. It may need extra hardware and a visit from the installer. Ask before you buy if backup might matter later.
Sources
- Clean Energy Regulator — battery eligibility and installation requirements
- Solar Accreditation Australia — accredited battery installers
- NSW Climate and Energy Action — guide to owning a battery
- Energy Safe Victoria — electrical safety guidance for batteries
