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What do you need it for? That’s my first question when someone asks whether to buy a bigger battery.
If you’re adding an electric car, changing the hot water or moving away from gas heating, extra storage can make sense. If the bigger battery is just the biggest one in the package, I’d want a better reason to pay for it.
Start with the night
Average daily consumption is a useful place to start. It doesn’t tell me when you use the electricity, though. Some of that demand may already be covered directly by solar, without going through a battery.
I want to see the hours after sunset. Cooking, cooling, hot water and a car arriving home can move a fair bit of use into the evening. A mild night and a hot one with the air conditioning running are different sizing questions.
Look at interval consumption data across the year, alongside solar generation if you already have panels. A solar production graph tells you what the panels made. It won’t tell you what the home used.
Try keeping the household the same below and changing the storage. Then change the electricity use or spare solar. The battery gets larger in each row, but the extra energy it supplies may not.
The same home.
Three storage sizes.
Change the day, then see how much of the night each battery could cover. These are illustrative capacities, not three products or a recommendation.
10 kWh
2 kWh held in reserve0 kWh of capacity above reserve is not cycled for this night.
20 kWh
4 kWh held in reserve3.4 kWh of capacity above reserve is not cycled for this night.
30 kWh
6 kWh held in reserve11.4 kWh of capacity above reserve is not cycled for this night.
Going from 20 to 30 kWh does not cover any more of this night.
Change overnight use or spare solar to see when that changes.One example day, starting at the selected backup reserve. It assumes 90% round-trip efficiency and sufficient charging and output power. It does not simulate a blackout, weather, tariffs, VPP operation, standby use or ageing.
See the numbers and method
| Usable capacity | Added from solar | Supplied to home | Grid needed |
|---|---|---|---|
| 10 | 8 | 7.6 | 4.4 |
| 20 | 16 | 12 | 0 |
| 30 | 19 | 12 | 0 |
Usable capacity is the starting capacity, before the user-selected backup reserve. Storage quantities are battery-side; spare solar and energy supplied to the home are AC-side in this simplified model. The example splits the assumed 90% round-trip efficiency equally between charging and discharging: each stage uses √0.9. Solar added to storage = the smaller of capacity above reserve and spare solar × √0.9. Energy supplied to the home = the smaller of overnight demand and solar added × √0.9.
Capacity not cycled = capacity above reserve minus energy discharged from storage. It includes both unfilled space and charged energy left over. Reserve is assumed to be present at the start of the day and is not discharged. Actual equipment, operating limits and a time-based design can produce different results.
The 90% assumption is an illustration, not a specification for every battery. See the Australian Government’s battery guide for capacity, power and efficiency definitions.
Give the battery something to store
A bigger battery does not make more solar. The charging question is how much generation is left after the home has used what it needs during the day.
With plenty of spare solar and a substantial overnight load, more storage has a job to do. If either is small, check what the larger unit is adding. Winter matters here. I wouldn’t size a system only around a generous summer day.
Charging from the grid is a separate calculation. The electricity plan, charging cost and time available all matter. The example above uses spare solar only; it is one day, not an annual performance forecast.
Keep backup as its own decision
Bill savings aren’t the only reason to buy storage. Think about a business with a cool room. If losing power means losing stock or stopping work, longer backup can have value even when the extra capacity isn’t used every evening.
At home, write down what you want to keep running. The fridge, lights and internet are a different brief from running the whole house as usual.
Then check the inverter. Battery capacity, measured in kWh, is stored energy. Output, measured in kW, is how much power the system can supply at once. A larger battery does not, by itself, mean it can run more appliances together.
Backup needs compatible equipment and the right circuits included in the installation. It also depends on the charge available when the outage starts. A reserve percentage in an app doesn’t promise that every appliance will stay on.
Ask what the next size buys you
For a home planning to add more electric appliances, my rule of thumb is to start around average daily consumption and consider another 10–20% for those additions. I’m comfortable allowing some room for the next stage of electrification.
That is a starting point for a design, not a formula to apply to every house. Check when the extra electricity will be needed, how the battery will be charged and what you want from backup. A car charged from daytime solar asks something different of the battery from the same car charged after sunset.
Twice the size needs a much stronger reason. I wouldn’t recommend doubling the capacity just because it is available.
If a virtual power plant (VPP) is part of the plan, assess that separately too. Check the programme’s requirements, how much control it has over charging and discharge, and the inverter’s power limits. More stored energy and more power are different things.
Ask for the next size as a complete installed option: the extra cost, the extra energy it is expected to supply to your home and the assumptions behind that estimate. If you’re relying on expansion later, check the manufacturer’s rules and the installation work required. Today’s expansion option doesn’t guarantee the same modules will be available years from now.
If there’s a clear use for it and you value that use, the larger battery may be worth it. I want that decision made around your home, not the size of the package.