"What size battery do I need?" is the first question most people ask — and the honest answer is that it depends on three things: how much energy your home uses, how much solar you have (or are adding), and what you actually want the battery to do. There's no single right answer, and any installer who quotes a fixed system before looking at your usage is selling a package, not sizing a solution. Here's how to think it through.
Start with your daily energy use
The single most useful number is how many kilowatt-hours (kWh) your home uses per day. You'll find it on your electricity bill, usually shown as an average daily usage figure. Most Victorian homes land somewhere between 15 and 25 kWh a day, but it varies enormously — a smaller home with gas heating and cooking might use 8–10 kWh, while a large all-electric home with ducted air conditioning, a pool, or an EV can push past 40 kWh.
What matters for sizing isn't just the total, though — it's how much of that energy you use after the sun goes down. A battery's main job is to store your daytime solar and hand it back in the evening and overnight, when your panels aren't producing and grid power is most expensive. So the real question is: how much energy do you need to carry your household from late afternoon through to the next morning?
Factor in your solar
A battery can only store what your panels produce. If you have a 6.6 kW system generating plenty of surplus on a sunny day, you can comfortably fill a larger battery. If you have a smaller array — or heavy daytime usage that already soaks up most of your generation — there may be less surplus left to store, and a smaller battery makes more sense.
If you don't have solar yet and are adding panels and a battery together, we size the two as a matched system, so the array is big enough to keep the battery useful all year round — not just in midsummer.
Decide what you want the battery to do
Three goals tend to pull sizing in different directions:
- Cut your bills. If the aim is simply to stop buying expensive peak-rate power in the evening, you want enough capacity to cover your typical overnight usage — often a single battery module is plenty for an average home.
- Backup through blackouts. If keeping essential circuits running during an outage matters, size needs to account for how long you'd want to stay powered and which loads you're backing up (more on this below).
- Maximum grid independence. If you want to lean on the grid as little as possible — including through cloudy stretches — you'll want more storage, often several modules, plus enough solar to refill it.
Translating that into a real system
We build our residential systems around the Dyness Powerbox G2, which is modular — it scales in increments of roughly 10.24 kWh, up to 51.2 kWh. That modularity is the point: you add capacity to match your home rather than buying a fixed box.
As a rough guide:
| Household profile | Typical starting point |
|---|---|
| Smaller home, modest evening usage, focused on bill savings | ~10 kWh (single module) |
| Average family home, good solar, wants most evenings covered | ~10–20 kWh |
| Large all-electric home, EV charging, or chasing grid independence | ~20–51 kWh |
These are starting points, not quotes — your actual figure depends on your bill, your solar, and your goals. But they give you a sense of the ballpark.
A note on backup sizing
If blackout protection is a priority, sizing shifts slightly. The question becomes: which circuits do you want to keep alive — just the essentials (lights, fridge, internet, a few power points), or most of the house — and for how long? Backing up essentials for an evening needs far less capacity than running ducted air conditioning through a multi-day outage. We design the backup side around what actually matters to you, so you're not paying for capacity you'll never use.
Don't over-size, and don't under-size
Both cost you. An oversized battery is money spent on capacity that sits half-empty most nights and never pays itself back. An undersized one runs flat partway through the evening and sends you back to the grid at peak rates — undermining the whole reason you bought it. The sweet spot is enough to cover your real overnight usage, with a sensible margin, and room to expand later if your needs grow (an EV, an extension, going all-electric).
The honest way to get it right
The only way to size a battery properly is to look at your actual energy use, your solar, and your goals together — which is exactly what we do in a free site assessment. We'll look at your bill, talk through what you want the system to achieve, and recommend a size that fits your home rather than the biggest one we can sell. That's why most of our work comes from referrals.
Learn more about our home battery systems, or get a free quote and we'll size it for you. You can also call Rhys directly on 0428 660 510.