Solar battery storage, and what happens in a brownout
Most people buy solar to cut a bill and assume the lights will stay on when the grid drops. Those are two different systems, and the difference is a battery and the inverter that goes with it. A grid-tied system without one goes dark in a brownout along with the whole street, and the usual time to discover that is during a brownout.
This page explains why, because the reason is not a fault or a cheap component. It is a safety requirement, and it is the right one.
Why it switches itself off
When the utility line goes dead, a linesman may be about to work on it. An inverter still pushing power into that line would be energising a wire somebody believes is dead. So a grid-tied inverter is required to detect the loss and disconnect, which is called anti-islanding: it must not create a live island in a dead network.
It is written into the rules here. Department Circular DC2026-08-0017 sets out what counts as micro-solar, and one of its three conditions is that the micro-inverter has an automatic anti-islanding disconnection. The same protection is what an interconnection standard exists to require of anything larger.
The three permit routes, and which is yours
Grid tied, hybrid and off grid, and what the three solar system types mean
Three names cover every rooftop system sold here, and they describe one thing only: what the system is connected to. Everything else people argue about, panel brand, inverter brand, how many, sits inside whichever of the three you have picked.
- Grid tied, or on grid
- Connected to the utility and to nothing else. It cuts the bill while the sun is up and it has no store, so it has nothing to offer at night or during a brownout. This is what most people are quoted when they ask for solar.
- Hybrid
- Connected to both the utility and a battery, through one hybrid inverter that can talk to each. That second connection is the whole difference: it is what allows the system to keep chosen circuits alive while the street is dark, which the section below explains.
- Off grid
- Connected to a battery and to no utility at all. There is no bill to cut and no application to make, and the sizing question changes completely, because the system has to carry the worst week of weather rather than an average one. Built from separate parts, it also needs a charge controller between the panels and the battery.
Net metering belongs to the first two. A system with nothing to export has a lighter route through the paperwork, and the single line diagram on the net metering page shows where the meter sits in it.
Three arrangements, one bad night
Drawn in the page rather than generated. The grid has gone down at two in the morning. Each row says what that arrangement is doing.
- Panels
- Grid-tied inverter
- Your board
- At night, normally
- Nothing. The sun is down, so the panels make nothing and the house runs on the grid.
- When the grid drops
- Dark, like everyone else.
- Why
- The inverter is required to disconnect when the line is dead. With no battery there is nothing else to draw on.
- Panels
- Hybrid inverter
- Battery
- Backed-up circuits
- At night, normally
- The battery runs whatever is on the backed-up circuits, until it is empty.
- When the grid drops
- The backed-up circuits stay live.
- Why
- The hybrid inverter separates the backed-up circuits from the dead line, so it can keep them alive without feeding the street.
- Panels
- Off-grid inverter
- Battery bank
- Everything in the house
- At night, normally
- The battery bank runs the house. There is nothing else.
- When the grid drops
- Unaffected. There is no grid to lose.
- Why
- Nothing is connected to the utility, so a utility failure is not an event here.
The middle arrangement is the one people mean when they say they want solar so the power stays on. It is a different system from the one that only cuts a bill, and it costs more because there is more in it.
What to ask before you buy one
Two questions settle most of it, and neither needs a specification sheet.
- Which circuits stay live?
- A battery does not keep the whole house running. It keeps whatever was wired to the backed-up side, and that choice is made at installation. Lights, a fan, a router and the fridge is a normal answer. Air conditioning usually is not, because a night of cooling can take most of a battery on its own; the aircon and solar page works it out, and the battery sizing page does the same for a whole evening of named appliances.
- For how long?
- That is the battery size against what you put on the backed-up circuits, and it is arithmetic rather than opinion. Tell us what you want to stay on and we will do it with you before anything is quoted.
Tell us what you want to stay on
Send the property and the circuits that matter to you, and you will get a written answer with a size and a next step.
Tell us the property and what you are trying to cut, and you will get a written answer with a size and a next step. No visit is booked and nothing is charged for asking.