Will my battery keep the lights on in a blackout?

Plenty of people buy a battery assuming it will carry the house through the next storm. Some do. Many do not, because backup is a design choice, not something every battery does on its own. Solar Victoria says it plainly: standard solar battery systems do not provide backup when the grid fails. Here is what decides whether yours will.

Why solar switches off in a blackout

A grid-connected solar inverter shuts down the moment it detects a power outage. This is called anti-islanding and it is a safety rule: it stops your panels sending power back into the street while line workers may be repairing it. So a home with solar and no backup goes dark like everyone else, even on a sunny day.

What a backup system does differently

For a battery to keep running during an outage, the system has to be able to disconnect your home from the grid first, then power the house from the battery as a little island of its own. The federal energy guidance describes two common setups:

• Battery only backup. The solar inverter shuts down and the battery runs your appliances until it is empty. Once it is flat, there is nothing more until the grid comes back.
• Islandable solar and battery. The system separates from the grid and keeps going, and when the sun is up the panels keep charging the battery. This costs more, but with careful use it can supply power over several days.

The part that does the disconnecting is often called a backup gateway or backup box, depending on the brand. Some hybrid inverters have a backup output built in, others need a separate unit and some extra switchboard work. Solar Victoria notes that the ability to run during an outage depends on the battery, the inverter and the whole system design, and that wiring or electrical upgrades may be needed. That is why you have to say you want backup before the system is designed, not after it is installed.

Whole home or essential circuits

Depending on the battery, inverter and setup, backup can cover the whole property or only selected circuits. Essential circuits are usually things like lights, the fridge, the internet modem and a few power points, plus a water pump if you rely on tank water.

The trade off is simple:

• Essential circuits keep the important things going for longer, because big loads like ducted heating, electric ovens and hot water are left off. It is also usually simpler to wire.
• Whole home backup means everything works, but the battery's power limit still applies. If several heavy appliances run at once and ask for more than the battery inverter can supply, it may overload and shut down.

Either way, you will need to manage what you run during a blackout. The federal guidance makes the same point: use less to stretch the stored energy and avoid overloading the battery inverter.

Three phase homes

Some homes have a three phase supply rather than a single phase one; your switchboard or your electricity distributor can tell you which you have. The federal guidance notes that on a three phase property some battery setups back up only a single phase. That can be fine if your essential circuits are all on that phase, but it means anything on the other two phases will be off during an outage. If you are on three phase, ask whether the backup covers one phase or all three, and which circuits sit on the backed up phase.

How long will it last?

That comes down to two numbers: how much usable energy is in the battery when the power goes out, and how much you are using.

A simple way to estimate it is usable kWh divided by your average load in kW. For example:

• A battery with 10 kWh available, running a fridge, lights, modem and a TV that together average about 0.5 kW, would last around 20 hours.
• The same battery running a 2 kW heater as well would be flat in a few hours.

Remember the battery may not be full when the outage hits. If it has been powering your evening, it might be half empty at midnight. Many systems let you set a backup reserve, a share of the battery that is held back for outages and not used for everyday savings. A bigger reserve gives you more blackout cover and a bit less day to day use.

If you join a virtual power plant, check how the operator treats your reserve. Some programs can leave you with little stored energy when an outage arrives, others keep part of the battery aside.

Questions to ask before you buy

• Does this battery and inverter provide backup, and what extra hardware does it need?
• Will backup cover the whole home or essential circuits, and which ones?
• If I am on three phase, which phase or phases are backed up?
• Will the solar keep charging the battery during a long outage?
• How fast does it switch over, and will my modem and computers notice?
• Can I set a backup reserve, and can I change it before a storm?

The short answer

A battery can keep the lights on in a blackout, but only if the system is designed and wired for it. Tell your installer you want backup at the start, decide whether essential circuits are enough, and size the battery with that in mind. If you would like to see a backup ready system priced for your home, start a design on your own roof and add the battery there.