Start with overnight consumption, not daily total
A battery's job is to carry you from sunset to sunrise. During daylight your panels cover the house directly — the battery is not involved. So the number that determines the right size is how much you consume between sunset and sunrise, not your total daily usage.
For a lot of Melbourne households, overnight and evening consumption is somewhere in the range of 8 to 15 kWh. Homes with electric hot water, ducted heating running overnight, or an EV charging at home sit well above that.
How to find your real number
Your retailer can provide interval data — usually half-hourly readings — through their app or on request. That shows exactly when you use power, not just how much.
Add up consumption from roughly sunset to sunrise across a few typical days in different seasons. Winter evenings will be your peak if you heat electrically. That figure, not a rule of thumb, is your sizing target.
Usable versus nameplate capacity
Batteries are advertised at nameplate capacity but you cannot use all of it. Manufacturers reserve a buffer because discharging lithium cells to empty shortens their life. The difference between nameplate and usable is typically around 10 percent, though it varies.
This matters twice: for sizing, because usable is the real figure, and for the federal battery discount, which is calculated on usable capacity. Your quote should state which it is quoting.
Why bigger is not better
A battery only earns its keep when it cycles. Capacity that sits unused most nights has cost you money and returns nothing. Oversizing is the most common and most expensive mistake in home storage.
There is a second constraint: your solar array has to be able to fill it. A battery sized beyond your daily surplus generation will regularly sit partly empty, or will charge from the grid — which defeats the purpose entirely.
Rough sanity check: if your battery capacity exceeds your typical daily solar surplus, it is too big for your array.
Seasonal reality
Melbourne winters are the constraint. A clear January day might generate four times what an overcast July day produces, while your evening consumption is at its highest in winter.
No sensibly sized battery will carry a Melbourne household through winter on solar alone. That is fine — the goal is to shave the expensive evening peak most of the year, not to leave the grid. Anyone promising winter independence from a domestic battery is selling something.
When backup changes the sizing
If blackout protection is a priority, sizing shifts. You are then designing for a worst-case outage rather than a typical night, and you need to decide what stays on.
A backup circuit covering lights, fridge, internet and a few power points is modest and will run a long time. Adding ducted heating or air conditioning changes the requirement dramatically and is rarely worth it.
A practical approach
- Pull your interval data and work out real overnight consumption.
- Check it against your array's realistic daily surplus.
- Size to the smaller of the two, in usable kWh.
- Decide separately whether you want backup, and what it needs to run.
- Check whether the product is modular, in case your usage grows.