One of the most common mistakes homeowners make when adding battery storage is choosing size based on a package deal rather than their actual usage. Choosing too small leaves you drawing from the grid well before midnight. Choosing too large means you are paying for capacity you charge and discharge only partially each day.
Start with Your Evening and Overnight Usage
The right battery size is primarily determined by how much electricity your household consumes after the sun sets. This is because the battery's job is to store energy captured during the day and release it when solar generation stops.
To estimate this, look at your electricity bill for your total daily consumption, then think about how much of that happens in the evening. A rough rule of thumb: most Australian households consume around 50–60% of their daily electricity after 3pm.
- Daily usage 15 kWh → evening load roughly 7–9 kWh → a 10 kWh battery is likely sufficient
- Daily usage 25 kWh → evening load roughly 12–15 kWh → a 13.5 kWh battery covers most nights
- Daily usage 35+ kWh → consider 15–20 kWh capacity or staged installation
Common Battery Sizes Available in Australia
Residential battery systems are typically available in these capacity brackets:
- 5–7 kWh: suitable for smaller households or as a supplementary unit alongside existing storage
- 10 kWh: the entry point for most Melbourne family homes; covers moderate evening usage
- 13.5 kWh: the most commonly installed size; balances coverage with cost
- 15–20 kWh: suited to larger homes, high consumption households, or properties planning EV charging
- Modular systems: some brands allow stacking units to reach 20–30+ kWh for larger properties
Usable vs. Nameplate Capacity
Battery products list a nameplate capacity (e.g., 13.5 kWh), but the usable capacity is typically slightly lower once depth-of-discharge limits and system reserves are accounted for. Most quality residential batteries offer 90–95% usable capacity, but it is worth checking the product specifications rather than assuming the full nameplate figure.
Should You Plan for Future Loads?
If you are planning to buy an electric vehicle in the next few years, or if you are about to install a heat pump hot water system or pool heat pump, it is worth factoring those loads into your battery sizing decision now. Adding storage capacity later is possible with modular systems, but it is usually more expensive per kWh than sizing correctly from the start.
- EV home charging (overnight): adds 8–20 kWh of demand depending on vehicle and charge schedule
- Heat pump hot water: adds 2–4 kWh per day depending on household size
- Pool heat pump: adds 3–6 kWh on running days
Does Bigger Always Mean Better?
Not necessarily. A battery that is significantly oversized for your load rarely discharges fully each day, which can affect both the financial return and, in some battery chemistries, long-term cycle health. The goal is a battery that is reliably well-utilised — typically reaching 80–100% discharge on most evenings — rather than one that sits at 60% capacity because the household simply does not consume that much.
Blackout Protection and Battery Sizing
If backup power during grid outages is a priority, sizing logic changes slightly. You need to account not just for evening load, but for how long you want to run essential circuits without grid power. Essential loads (lights, fridge, router, phone charging) typically draw 1–2 kW continuously. A 10 kWh battery could run these loads for 5–10 hours depending on configuration.
Getting the Sizing Right
The most reliable way to size a battery correctly is to have an installer model it against your actual bill data, including your seasonal consumption patterns and the proportion of usage that occurs in the evening. A proposal built on real data will be more defensible than one based on a standard package — and it will give you a more accurate payback calculation.