Solar battery calculator estimating battery size and usage for an Australian home

Solar Battery Calculator: Size Your Battery to Your Usage

Q: Does a bigger battery mean better blackout backup?

Only partly. Backup runtime also depends on which circuits are wired for backup and your inverter's output rating, not capacity alone.

Most oversized batteries never earn their extra cost back, and most undersized ones leave you buying peak power anyway. Our calculator estimates usable capacity, evening coverage, runtime and blackout backup in about 30 seconds — here's how to read the numbers it gives you. For pricing once you know your size, see our Solar Battery Cost Australia 2026 guide.

Quick answer: The calculator matches your evening usage against battery capacity to show how many hours it will actually cover, plus rough blackout runtime for essentials. Most homes size correctly at 10–13.5 kWh usable — bigger only helps if you genuinely use more after dark.

What the calculator actually estimates

Scenario	Draw	Runtime
Essentials only in a blackout	~300 W	2+ days
Normal evening household use	~1.5 kW	10–12 hours
Evening use plus aircon	~2.5 kW	6–7 hours
EV charging at 7 kW	7 kW	2–2.5 hours

Runtimes assume steady draw on a 10–13.5 kWh usable battery. Real consumption fluctuates, and your inverter's output limit can cap how fast the battery discharges regardless of capacity.

The three numbers that matter

Usable capacity, not nameplate capacity

A battery's advertised size and its usable capacity aren't the same — most units reserve a slice for longevity. The calculator works off usable kWh, since that's the number that actually determines how many hours you're covered.

Evening coverage

This is the percentage of your after-dark usage the battery can supply before it's depleted. A home using 20 kWh a day with 60% falling after 4pm needs roughly 12 kWh of usable storage to cover that window — sizing against the full daily total means paying for capacity that never gets cycled.

Blackout backup time

Separate from daily cycling, this estimates how long the battery keeps essentials running — fridge, lights, wifi, some power points — during a grid outage, based on the backup circuits wired at install.

Output matters as much as Need Help Access all Pro widgets. Upgrade Now capacity

A large battery on an undersized inverter is a bottleneck. Capacity determines how long you can run something; the inverter's continuous output rating determines what you can run at once. Running an EV charger and aircon simultaneously from a battery rated for 5 kW output simply won't work, no matter how much energy is stored. If you're unsure whether your existing setup can support this, check whether you need a hybrid inverter before sizing your battery.

How to use the calculator in 3 steps

Enter your average evening usage — check a recent bill or your inverter app for a rough kWh figure after 4pm.
Add any big evening loads — EV charging, aircon, or pool pumps skew the numbers significantly.
Compare battery sizes against the runtime shown — then check the price for that size in our cost guide.

Check your battery size against your usage

Our calculator shows how much of your evening load a given battery would cover, plus runtime and blackout backup time.

Open the Calculator

Once you know your size

Compare specific brands in our best solar battery guide, check sizing costs for 10kW or 20kW systems, or see what installation actually costs on top of the hardware.

Frequently asked questions

How accurate is a solar battery calculator?

It gives a reliable estimate based on the usage figures you enter, but real household consumption fluctuates day to day, and your inverter's output limit can affect actual runtime regardless of what the calculator shows.

What size battery do I actually need?

Most homes get the best payback matching capacity to evening usage rather than total daily usage — typically landing around 10–13.5 kWh usable for an average household.

Does a bigger battery mean better blackout backup?

Only partly — backup runtime also depends on which circuits are wired for backup and your inverter's output rating, not capacity alone.

Can the calculator account for EV charging?

Yes, but EV charging draws far more power than typical evening use, so it significantly shortens runtime and is worth entering as its own load rather than folding into general usage.

Use our free battery sizing calculator → to find the right battery size for your home.

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