Design
How to size a home battery without guessing
Batteries are usually sized by rule of thumb, roof size or whatever the rebate rewards. All three are wrong. The right input is your evening load, and you already have the data.
- Most three-to-four-person Sydney households need 10–14 kWh of usable storage.
- The correct figure is your overnight consumption, roughly 4pm–7am.
- Capped by your daily solar surplus, less round-trip losses and any backup reserve.
- All of it is readable from your interval data before anyone quotes you anything.
Ask three retailers to size a battery for the same house and you will get three different answers. None will come from your consumption.
The usual inputs are your solar size, your roof size and whatever the current incentive rewards. None of those tell you how much energy you need to move from day to night.
1What are the three wrong ways to size a battery?
Matching your solar size, buying the biggest that fits, and buying the size the rebate favours.
“Match it to your solar size”
A 6.6 kW system generates around 30 kWh a day on a good Sydney roof, averaged over a year. That does not mean you need 30 kWh of storage.
You need enough for the part of that generation you would otherwise export, capped by what you actually use after dark. For most households those are very different numbers.
“Get the biggest one that fits”
Storage you never cycle is capital you never recover. A battery that reaches 100% at 1pm and sits there is oversized for the load, whatever the wall space allows.
“Get the size the rebate favours”
This was more defensible before 1 May 2026. Now that the federal factor tapers above 14 kWh, sizing to the incentive produces worse outcomes than it used to.
Size to the load, then check the incentive.
2What number actually matters?
The energy you use between the end of one day’s solar and the start of the next. That is the most a battery can usefully move.
A grid-connected home battery does one thing economically. It moves solar energy you would have exported cheaply into hours when you would have imported expensively.
Useful battery capacity ≈ overnight consumption (sunset to sunrise), capped by daily solar surplus, adjusted for round-trip efficiency and reserve.
3How do I find my evening load?
Get twelve months of interval data and add up 4pm to 7am. Use the median, not the average.
You do not need to estimate this. Every NSW household with a smart meter has half-hourly interval data, and you are entitled to it.
- Request twelve months of interval data from your electricity retailer, or download it from their portal. Some offer it as a CSV directly.
- Sum consumption from roughly 4pm to 7am the following morning.
- Take the median across the year, not the mean. The mean is dragged upward by a handful of extreme days.
- Look at summer and winter separately. In Sydney the two differ considerably: summer has an evening air-conditioning peak, winter has a longer dark period and heating load.
Your interval data shows net import. Add back your self-consumed solar to get true household consumption. Your inverter’s monitoring app will have this.
4Do I have surplus to store?
A battery cannot store energy that was never spare. Your typical daily export is the ceiling.
Check your export figures. If you export 12 kWh on a typical day, that is the most a battery can capture, less round-trip losses.
Buy 20 kWh of storage in that situation and the top 8 kWh cycles only on your best days, or gets charged from the grid. That changes the economics entirely and, on some tariffs, negatively.
| Constraint | Where to find it | What it sets |
|---|---|---|
| Overnight consumption | Interval data, 4pm–7am | Maximum useful discharge |
| Daily solar surplus | Export figures or inverter monitoring | Maximum available charge |
Useful capacity is the lesser of the two. If they are far apart, fix the gap before buying storage: either add panels or shift load.
5How does my tariff change the answer?
A stored kilowatt-hour is worth whatever it displaces. On time-of-use, that often means a smaller battery than you expected.
| Tariff | Value of stored energy | Sizing implication |
|---|---|---|
| Flat rate | Import rate less feed-in tariff, on every kWh | Straightforward; size to overnight load |
| Time of use | Highest when displacing the peak window | Size to peak-window consumption specifically, not all overnight |
| Demand tariff | Additional value from shaving the demand peak | Power rating (kW) matters as much as capacity (kWh) |
| Wholesale-exposed | Highly variable; upside from price spikes | Favours larger capacity and smart control |
On a time-of-use tariff in NSW, the peak window is typically a defined evening block.
- A battery only needs to carry the load through that window at full value.
- The shoulder hours after it are worth considerably less.
The result is routinely a smaller battery than the household expected, cycling harder and returning more per installed kilowatt-hour.
6What did the 14 kWh band change?
Since 1 May 2026, storage above 14 kWh usable costs more per kWh. “Go bigger, the rebate scales” no longer holds.
The federal discount now applies its full rate only to the first 14 kWh of usable capacity, with a reduced rate above. For sizing, that means:
- The marginal cost per kWh of storage increases beyond 14 kWh usable.
- The old argument — “go bigger, the rebate scales” — no longer holds.
- If your genuine need sits near the band edge, there is a real case for landing just under it.
- If you have a genuinely large evening load, going larger is still correct. It just costs more per kWh than it did.
If your overnight load is 20 kWh, buying 13 kWh to sit under the band means importing the balance every single night for the life of the system.
Rebate optimisation should never override load matching.
7How do I size for blackout backup?
On power, not energy. Decide what must stay on, and for how long.
Backup is a separate requirement. Two questions:
- What must stay running? A fridge, lights, internet and a few outlets is a modest continuous load. Air conditioning, an induction cooktop, an EV charger or a pool pump are not. Each can exceed the continuous output of a typical home battery on its own.
- How long? If outages in your area last hours, a standard battery covers it easily. If they last days, you need generation during the outage. That means the battery must support solar recharge in island mode. Not all systems do.
It needs a dedicated backup circuit or changeover device, and correct isolation so the system cannot energise the network during an outage. It is not a software setting and it is not free.
8What does it look like for a real household?
A four-person Blacktown home with 6.6 kW of solar comes out at 11–13 kWh usable.
This household is well served by a battery in the 12–13.5 kWh nameplate range:
- comfortably inside the full federal rebate band
- matched to available surplus
- cycling close to fully every day
A 20 kWh system would cost substantially more, attract a lower marginal rebate and spend most of the year partly empty.
Overnight consumption rises sharply, surplus becomes the binding constraint even harder, and the better first move is usually more panels rather than more storage.
9Quick reference: where do most households land?
Starting points by household type. Use them to sanity-check your own number, not to replace it.
| Household | Typical overnight load | Indicative usable capacity |
|---|---|---|
| 1–2 people, no gas heating | 5–8 kWh | 6–10 kWh |
| 3–4 people, typical suburban | 10–16 kWh | 10–14 kWh |
| All-electric, no gas | 16–24 kWh | 14–20 kWh |
| All-electric plus EV charged at home | 25–40 kWh | Add panels first |
| Pool plus ducted air conditioning | 18–30 kWh | 14–20 kWh, shift pool to daytime |
Treat these as a sanity check on a number you derived from your own interval data — not as a substitute for deriving it.
Common questions
What size battery do I need for a typical Sydney home?
Most three to four person households in Sydney land between 10 and 14 kWh usable.
The correct figure comes from your own overnight consumption between roughly 4pm and 7am, capped by your daily solar surplus — both of which you can read from your interval data.
Should I match my battery size to my solar system size?
No. Solar size tells you what you can generate, not what you consume after dark.
A 6.6 kW system generating 30 kWh a day does not imply 30 kWh of storage — it implies storage sized to whatever portion of that you would otherwise export and later re-import.
Is a bigger battery always better?
No.
Capacity you never cycle earns nothing, and since 1 May 2026 the federal discount applies its full rate only to the first 14 kWh of usable capacity, so the marginal cost per kWh rises above that point.
Size to your evening load first, then check the incentive.
Will my battery run my whole house in a blackout?
Usually not, and this is sized on power rather than capacity. Fridge, lights, internet and general power outlets are straightforward. Air conditioning, induction cooking, EV charging and pool pumps can each exceed a typical home battery's continuous output on their own.
Backup also requires a dedicated circuit and correct isolation.
Can my battery charge from the grid?
Most hybrid systems can, and it can make sense on a time-of-use or wholesale-exposed tariff where off-peak import is cheap. On a flat tariff it usually does not. Check whether grid charging affects your incentive eligibility or your warranty's cycle counting.
Do I need a bigger battery if I get an electric vehicle?
Usually the better first move is more panels, not more storage. An EV adds a large overnight load, which makes solar surplus the binding constraint rather than capacity.
Charging the car during daylight from direct generation is almost always cheaper than routing that energy through a battery.
Sources and further reading
- Australian Energy Market Operator — consumer interval data access and metering arrangements
- AS/NZS 5139 — safety requirements for battery energy storage systems
- Clean Energy Council — battery installation guidelines and approved products list
- Australian Government — Solar Consumer Guide
Revision 2.4 · 3 October 2026. This revision rewrote the guide so it is easier to skim. No figures or rules changed.
Figures current at 9 August 2026. Set Energy technical team. NSW Electrical Contractor Licence 467699C.