What size home battery do I need?

It is the question we hear on almost every site visit across Sydney and the Illawarra: what size home battery do I need? The honest answer is that there is no single magic number, but there is a sensible range for your home. Getting it right means looking at four things together: how much power you use, how big your solar system is, how much of your load lands in the evening and overnight, and what your budget allows. This guide walks through each one using current 2026 figures so you can size with confidence rather than guesswork.

Start with your real daily usage

The first place to look is your electricity bill, not a generic calculator. Your bill shows your average daily consumption in kilowatt hours (kWh). The typical Australian household uses somewhere around 16 to 20 kWh per day, but yours could be well under or well over that depending on your home, your appliances and how many people live there.

As a rough guide, here is how usage tends to break down by household:

The number that matters most is not your total daily use. It is the slice of that energy you draw after the sun has stopped producing. That is the gap a battery is built to fill.

Factor in your solar system size

If you have solar, or you are installing it at the same time, you do not need a battery big enough to cover your whole day. Your panels handle the daytime load directly. The battery only needs to soak up the surplus your panels make during the day and hand it back at night.

Picture a home that uses 20 kWh a day. If solar covers around 12 kWh while the sun is up, you only need to store roughly 8 to 10 kWh to carry the evening and overnight period. Buying a 20 kWh battery in that situation would mean paying for capacity you can never fill on a normal day.

The relationship runs the other way too. A larger solar array produces more surplus, which means a larger battery can actually be charged most days and earns its keep. A small 5 kW system paired with a very large battery often leaves that battery half empty, especially in winter. Sizing solar and storage together is the whole point, which is why we look at both during a solar consultation rather than treating the battery as an afterthought.

Usable kWh vs nominal kWh

This is where a lot of confusion creeps in. Batteries are often advertised by their nominal (total) capacity, but you never get to use every last bit. Most quality home batteries give you around 95 to 97 percent usable capacity, and good practice is to size around the usable figure.

When you compare quotes, always check whether the number quoted is nominal or usable. A battery sold as "10 kWh" might deliver closer to 9.5 kWh of real, daily-cycling storage. This matters for two reasons: it changes how much evening load you can actually cover, and the federal rebate is calculated on usable kWh, not nominal. The systems we install, including Sigenergy SigenStor and ESY Sunhome, are modular, so usable capacity is straightforward to scale to your home.

The 2026 rebate sweet spot

Money shapes sizing as much as engineering does, and the federal Cheaper Home Batteries Program is the big lever in 2026. It works by discounting the upfront cost through small-scale technology certificates (STCs) tied to your battery's usable capacity, worth roughly 30 percent off for most homes.

From 1 May 2026, the value is tiered by size, and the tiering creates a clear sweet spot:

From 1 May 2026 the full-rate band is worth in the order of 6.8 STCs per usable kWh, which works out to roughly $252 per kWh off the upfront price (indicative, and it varies slightly by STC zone). The practical takeaway: the first 14 kWh of usable storage attracts the strongest support, so a battery in the 10 to 14 kWh usable range is often the best value-per-dollar choice for a typical NSW home. Going bigger still makes sense for high-use homes, just understand that capacity beyond 14 kWh is subsidised at a lower rate. All figures here are indicative and current at the time of writing.

One more timing point. The STC value steps down again from 1 January 2027 and continues tapering every six months until the program ends in 2030. The rate is locked in on your installation date, so installing sooner secures a higher rebate.

Oversize or undersize? The honest trade-off

No battery will be perfectly sized for every single day. The real question is which way you would rather miss. Here is how the two mistakes play out.

On balance, a slight oversize is usually the smarter error. An undersized battery hits empty most evenings and you keep buying grid power at the worst times. Adding a module later almost always costs more per kWh than buying that capacity in the original install. The catch is that oversizing badly, say a 25 kWh battery on a 5 kW solar system, wastes money on storage you cannot fill. The goal is a small, deliberate buffer above your real need, not a guess at the top of the range.

Don't forget backup and power, not just capacity

Capacity in kWh tells you how long a battery runs. It does not tell you how much it can run at once. That is power output, measured in kW. A battery with high capacity but modest output may not start a large air conditioner or run several heavy appliances together during a blackout.

If backup matters to you, ask two extra questions: how much continuous power can it deliver, and how fast does it switch over when the grid drops? Quality home batteries change over in roughly 5 to 20 milliseconds, fast enough that most appliances never notice. Decide early whether you want whole-home backup or just essential circuits like the fridge, lights, internet and a power point or two, because that choice changes both the size and the wiring. If you are also planning to charge a car at home, our EV charger setup is worth factoring into the same conversation.

Putting it together: a worked example

Take a four-person home in the Illawarra using 20 kWh a day with an 8 kW solar system. Solar covers most daytime load and leaves a healthy surplus. The evening and overnight draw is about 11 kWh. We would point this home toward roughly 13 to 16 kWh of usable storage: enough to cover a normal night with a buffer for winter and cloudy runs, while keeping most of the capacity inside the full-rate rebate band. Add an EV charged overnight and we would look harder at the upper end or a modular battery that can grow. If you are weighing specific systems, our Sigenergy vs ESY comparison walks through how each scales.

That is the kind of judgement that is hard to make from a calculator alone, because it depends on your roof, your tariff, your appliances and your goals. The reliable path is to size from real bill data and a quote that clearly states usable kWh. If you want a straight answer for your specific home, our in-house Master Electricians, never subcontractors, will model it with you and recommend a size based on your home, not on what we happen to have in the warehouse. Book a consultation and we will put real numbers against your usage before you commit to anything.

This guide is general information for Australian homeowners and reflects publicly available information at the time of writing (June 2026). Specifications, warranty terms, pricing and rebates change, and the right system depends on your home. Pricing figures are indicative only. Always confirm current details and rebate eligibility for your specific configuration at consultation.