What size solar battery do you need in Australia?

A home battery mainly shifts daytime solar into the evening and overnight. The right size is therefore driven by how many kilowatt-hours you import after sunset, not by the largest marketing number on a product sheet.

If your household uses about 8 to 12 kWh between late afternoon and morning, a 10 to 13.5 kWh usable battery often covers a large share of that load once solar has charged it. If evenings are lighter - for example a small home with efficient appliances - 8 kWh can be enough. Heavy evening cooking, heating, EV charging at home, or large families push the range upward.

Sizing is a planning exercise, not a guarantee. Weather, behaviour and tariff changes move real cycling day to day. Aim for a size that fills most sunny days and discharges most evenings rather than one that chases every winter peak.

Use this sequence before you treat any single kWh number as final. Each step takes a few minutes with a recent bill or your solar app.

Product lines cluster around familiar usable capacities. Treat these as planning bands, not prescriptions. Gross installed bands below come from our solar battery cost guide; net cost drops after the federal Cheaper Home Batteries Program discount.

These sketches help you narrow the conversation before brand shopping. Run the calculator with your actual bill - profiles are not prescriptions.

A battery that never fills is wasted capital. If you have a modest 5 to 6.6 kW solar system and cloudy winters, a 20 kWh battery may sit half empty for much of the year. Conversely, a large north-facing array that exports a lot of daytime energy can justify more storage - as long as evening demand exists to discharge into.

Worked example: a 6.6 kW system on a medium-use home might export 12-18 kWh on a spring weekday while importing 8-10 kWh overnight. A 10 kWh battery can capture a useful share of that surplus without oversizing. The same battery on a 3 kW system in winter may only see 4-6 kWh of chargeable surplus - 8 kWh might be the smarter cap.

Also check inverter limits. Some hybrid inverters cap charge and discharge power (kW). A big battery that can only charge slowly may not refill on short winter days, which hurts both self-consumption and backup readiness. Ask installers for continuous charge and discharge kW on your shortlist, not just kWh capacity.

Capacity is how much energy the pack holds. Power is how fast it can charge or discharge. A 10 kWh battery with a 5 kW discharge limit can run a 2 kW oven and lights comfortably but may struggle if the air conditioner, oven and kettle start together during backup.

For bill optimisation, power limits rarely block savings - the battery trickle-charges from solar through the day and discharges over several evening hours. For backup or whole-home resilience, power limits matter as much as kWh.

Modular stacks (Pylontech, BYD, Sungrow SBR) let you add modules later if evening load grows. All-in-one units (Tesla Powerwall 3, sonnen, Alpha ESS SMILE) fix capacity upfront - compare expansion paths before you assume you can add kWh in two years without a new inverter conversation.

The Cheaper Home Batteries Program pays full federal value on the first 14 kWh usable, then tapers. In May to December 2026 that full rate models to about $252 per usable kWh. Capacity from 14 to 28 kWh receives about 60% of that rate.

That means 8, 10 and 13.5 kWh packs sit entirely inside the best rebate tier. A 16 kWh all-in-one receives full support on 14 kWh and reduced support on the last 2 kWh. A 20 kWh system does not receive double the discount of a 10 kWh system - the extra kWh cost more net dollars per kWh saved.

If your evening load genuinely needs 16-20 kWh, the larger pack can still make sense. If an installer pushes 20 kWh when 10 kWh covers your imports, you are paying for tapered capacity you rarely cycle. Read our Cheaper Home Batteries Program guide for tier rules and our 10 kWh battery cost guide for a worked net-cost example at the most searched size.

Daily cycling depends on your tariff and usage, but net cost differs by jurisdiction. A smaller battery in Western Australia can net less than a larger battery elsewhere because of stacked state support.

Open your state page (/nsw, /vic, /qld, /sa, /wa, /tas, /act or /nt) for local rebate context and a calculator pre-set to that state. Our state rebate comparison guide has the full stackable-rebate picture.

A home EV adds predictable evening or overnight demand. If you charge at home after solar production ends, those kilowatt-hours count toward sizing the same way as cooking and heating.

Rough planning: a typical commute might add 8-15 kWh per charge session depending on vehicle and driving. A 10 kWh battery will not cover a full EV charge plus household evening use in one night - but it can still offset a portion if you charge on off-peak rates or split charging across solar and grid.

If EV charging is your main reason to buy storage, model the tariff you will use for the car separately from household peaks. Sometimes a right-sized battery plus a controlled EV charger beats an oversized pack you cannot fill from solar alone.

Backup during an outage depends on which circuits are covered, continuous and surge power limits, and how long you need critical loads to run. A battery sized perfectly for bill savings can still be too small - or wrongly configured - for whole-home backup.

Decide early whether you want bill optimisation only, essential-circuits backup, or something closer to whole-home resilience. That choice often matters more than picking 13.5 versus 16 kWh on paper.

Essential-circuits backup (fridge, lights, modem, one power point) often works on 10 kWh with correct gateway wiring. Whole-home backup during summer peaks needs both enough kWh and enough kW discharge headroom - and may cost extra for switchboard work.

Modular DC stacks (Pylontech US5000, BYD Battery-Box, Sungrow SBR) let accredited installers add modules in 2-4 kWh steps. That suits households unsure between 8 and 10 kWh, or planning to expand when an EV arrives.

All-in-one AC units (Tesla Powerwall 3 at 13.5 kWh, sonnen at 10 kWh, Alpha ESS SMILE at 8 or 10 kWh) ship as fixed blocks. Retrofit-friendly AC coupling is common; expansion means another unit or a new quote tier.

Neither approach is automatically better. Modular paths can reduce upfront net cost at 8-10 kWh. Integrated units can simplify install and backup on one wall. Compare net cost at the same usable kWh on our solar battery cost guide before you pay a premium for brand alone.

Federal rebate taper means the last kilowatt-hours of a large pack get less subsidy support per kWh than the first 14. If those extra kWh only cover rare high-use nights, payback can stretch out.

Oversizing also hurts when the battery sits partially empty most days. Empty kWh earn no savings but still cost capital. Our battery payback guide shows how import rates and feed-in tariffs move payback more than brand - an oversized pack with weak cycling often lands past 12 years simple payback even after 2026 rebates.

Use our calculator in size and payback modes, then compare quotes for two neighbouring sizes (for example 10 vs 13.5 kWh). Choose the smaller one if the larger pack only shaves a little more grid import for a lot more capital.

Install timing changes net cost, not the physics of your evening load. From 1 January 2027 the STC factor steps down - on a 10 kWh battery the modelled federal discount drops by roughly $400.

That does not mean you should upsize in 2026 to capture more rebate dollars. A 13.5 kWh pack you cannot fill still cycles poorly. If 10 kWh is the right size for your home, installing in 2026 captures the better rebate on the right capacity rather than buying extra kWh you rarely use.

If you are torn between 10 and 13.5 kWh and planning to install before the step-down, run both sizes in the calculator with install year set to 2026 and 2027. Sometimes the 2026 rebate on 10 kWh nets close to what 8 kWh would cost in 2027 - but only if evening load supports the larger pack.

When quotes arrive, check these fields before you pick a size or brand. Mismatched assumptions are a common reason two households choose different kWh for the same evening load.

Run the calculator with your state, quarterly bill or daily kWh, tariff and solar size. Switch to Best payback mode to see suggested size, net cost and payback at neighbouring kWh bands.

If 10 kWh looks like the best fit, read our 10 kWh battery cost guide for brand-level net bands, then our is a home battery worth it guide for a yes/no screen before you request quotes.

Cross-check gross and net tables on the solar battery cost guide. Match installer quotes to usable kWh, not marketing names like Powerwall or SMILE model numbers alone.