What size home battery you actually need (and where bigger stops paying)

Size around your evening, not your daily total

Start with what the battery is actually for. On a normal grid-connected home with solar, its job is simple: soak up the surplus your panels make during the day and give it back after the sun goes down, so you buy less power at the expensive end of the day.

That means the number that matters is what your house uses between late afternoon and the following morning, not what it uses over 24 hours. Daytime consumption is already being covered by the panels. Sizing a battery to your total daily use is the single most common way people end up with more battery than their house can ever cycle.

You can find this properly. If you have a smart meter, your retailer can give you interval data, and your solar monitoring will show what you are currently exporting. Two useful figures fall out of that: your overnight consumption, and how much surplus you actually have to store. A battery cannot store energy you never generated, so a big stack on a small array will spend most of its life part full.

As a rough orientation before you go and check, a typical Queensland family home lands somewhere in the region of 8 to 15 kWh of overnight use. Yours could be well outside that in either direction, which is exactly why the interval data is worth pulling.

The discount is tiered, and that changes the answer

Here is the part that most sizing conversations leave out, and it is the reason this article exists.

The federal battery discount is not a flat rate per kilowatt-hour. It is tiered on usable capacity, and the tiers step down hard:

Why value per kilowatt-hour collapses on a very large stack

Put those tiers together and something clear falls out. The kilowatt-hours at the bottom of your stack are heavily discounted. The ones at the top are barely discounted at all.

In practice, a kilowatt-hour in that top band attracts roughly a seventh of the support that a kilowatt-hour in the first band does. You are still paying close to full freight for the hardware, but the discount has largely stopped helping.

So there are two sensible answers rather than one. Around 14 kWh usable is the point where every kilowatt-hour you have bought is fully supported, and for a lot of households it also happens to be close to their real overnight use, which is a nice coincidence. Around 28 kWh usable is the practical ceiling for most homes: it comfortably covers a heavy evening with capacity to spare, and it stays inside the second tier.

Past 28 kWh, keep going only if you want the storage for a specific reason, not because you expect the discount to carry it. That is the honest position, and it is the opposite of what a bigger sale would have me tell you.

When a big stack genuinely is the right call

None of the above means large systems are wrong. It means capacity past a point has to earn its place on its own merits rather than on the rebate. Here is when it genuinely does:

You are off-grid or heading that way. With no grid behind you, the battery is not a savings device, it is the supply. Sizing then runs off worst-case days and days of autonomy, and it lands much higher. Different problem, different maths, and we cover it on our off-grid page.

You want to ride out real outages. If the goal is running a house through a multi-day event rather than trimming a power bill, capacity is the point.

You have genuinely large evening loads. Ducted air conditioning through a Queensland summer, a pool, a workshop, or two electric vehicles charging at home will move your overnight figure a long way up.

You are electrifying. If gas is coming out over the next few years and an EV is coming in, size for the house you are about to have rather than the one you have now. That is a legitimate reason to buy ahead.

Backup changes the question completely

If you want the house to keep running in a blackout, sizing stops being about bill savings and starts being about how long you want to last and what you want alive.

The two questions to answer are which circuits and for how long. Essential-circuit backup covering a fridge, lights, power points and the internet asks far less of a battery than running the whole house including air conditioning. We compare the two approaches in whole-home backup vs essential-circuit backup.

Worth knowing before you plan around it: backup is separate equipment from the battery itself, it is scoped against your actual switchboard, and the changeover time when the grid fails varies widely between systems, from under ten milliseconds to a couple of seconds, so it is worth asking for the number.

The date that sets your discount is the install date

This one costs people real money, so it gets its own section.

Your discount is determined by when the system is installed, not when you accepted the quote. The certificate factor steps down at the start of each calendar year.

So if you are quoting late in a year for an install that might slip into the next one, ask the question directly: which year is this discount calculated on, and what happens to my price if the install date moves. A quote that is silent on that is a quote with a hole in it.

It also means the run-up to the end of a year gets busy, and the constraint is rarely the battery. It is switchboard work, network paperwork and installer availability. If your timing matters, start earlier than feels necessary.

How I would actually size it

The short version, in the order I would do it:

One. Pull your interval data and find your real overnight consumption, from late afternoon to morning.

Two. Check how much surplus your solar actually exports on a normal day. That is your ceiling on what a battery can usefully store.

Three. Size to cover a typical evening comfortably, not the worst evening of the year. Chasing the worst case is how a battery ends up half used for the other 360 days.

Four. Decide whether backup is in scope, because that is a separate build and it changes both the size and the price.

Five. Add capacity for what is coming, an EV or gas coming out, if it is genuinely coming.

Six. Then look at where that lands against the tiers, and see whether nudging the size slightly puts more of your capacity in a better-supported band.

Do it in that order and the number falls out of your own house rather than a price list. If you would like a hand with the sizing, that is exactly what our solar and battery design service is for.

Common questions