What Size Solar Battery Do I Need? A 2026 Sizing Method

The solar battery size an Australian home needs is the smaller of two numbers: the kilowatt hours (kWh) you use after sunset and overnight, and the spare solar you export on an average day. For most homes that is 5 to 15 kWh of usable capacity, and 10 to 13.5 kWh is the common range as at November 2026. This guide shows how to find both numbers from your own bill and inverter app.

This is independent desk research. We did not test any battery or collect quotes, and the examples are arithmetic on stated assumptions, not measurements.

Key facts

• A solar battery should be sized to the smaller of a home’s evening and overnight use and its average spare solar export, both in kWh per day.
• The federal Cheaper Home Batteries Program calculates its discount on usable capacity, not nominal capacity.
• Since 1 May 2026 the Cheaper Home Batteries Program applies its certificate factor at 100% for the first 14 kWh of usable capacity, 60% from 14 to 28 kWh and 15% from 28 to 50 kWh, according to the Clean Energy Regulator (as at November 2026).
• For installations from 1 May to 31 December 2026 a 10 kWh battery earns about 68 certificates and a 13.5 kWh battery about 91, and the factor falls from 6.8 to 5.7 per usable kWh on 1 January 2027.
• A 5 kW battery takes about 2.7 hours to deliver a full 13.5 kWh, so power in kW matters as well as storage in kWh.

How do I work out what size solar battery I need?

Work out your night use, work out your spare solar, and size to the smaller one with a margin. You need only your bill and your inverter app.

Step 1: evening and overnight use. Your bill prints average daily usage in kWh, and a smart meter portal shows hourly use. Add up use from about 5 pm to 9 am. If you only have the daily total, assume half of it falls in those hours. That 50% is our assumption, not a published figure; check it against your smart meter data, and raise it if you run air conditioning or charge an electric vehicle (EV) overnight.

If you do not know your daily use, a published benchmark gives a starting point. The Australian Energy Regulator’s residential consumption benchmarks (study by Frontier Economics, 9 December 2020, the latest we could open; the Australian Energy Market Commission’s 2023 rule change repealed the AER’s duty to keep updating them) give seasonal kWh by household size and climate zone. The table converts the Climate Zone 5 New South Wales figures (which cover urban Sydney and urban Adelaide climates) to kWh per day. The conversion is our arithmetic, dividing the four seasonal totals by 365 days, and the data is from 2020, so many homes now use more or less.

Table: AER residential electricity benchmarks, Climate Zone 5, New South Wales (Frontier Economics for the AER, December 2020), converted to kWh per day by Solar Cobber. Benchmarks for other climate zones and states differ and are in the same report.

Step 2: spare solar. Read the daily export in your inverter app for a typical summer week and winter week. Without an app, estimate system kW times daily output per kW, minus daytime use. Our guide to what size solar system you need gives Clean Energy Council output per kW by city, such as 3.9 kWh for Sydney and 3.6 for Melbourne.

Step 3: smaller number plus 10 to 20%. A battery cannot discharge what it never stored, so the smaller of the two figures is the most you will use on a normal day. If winter spare solar is well below night use, pick a size between them and accept some grid power on dull days.

Solar battery size lookup table: daily usage by solar system size

The table gives usable battery kWh for a typical home as the smaller of night use and spare solar, before margin. All of the following are our assumptions, not sourced figures: 50% of daily use falls outside solar hours, an average of 3.5 kWh of solar per kW per day (a round figure between the Clean Energy Council city values of 3.6 for Melbourne and 3.9 for Sydney quoted in our solar sizing guide), no EV, and no allowance for the roughly 5 to 10% round-trip losses. The method for each cell is: night use = daily usage x 0.5; spare solar = system kW x 3.5 minus daytime use (daily usage x 0.5); the cell is the smaller of the two. It is our arithmetic, checked November 2026.

A home using 30 kWh a day with 6.6 kW of panels has only about 8 kWh of spare solar, so a bigger battery would sit part empty. Round to a sold size, and a margin usually tips you to the larger one.

What size battery for 6.6 kW solar?

A 6.6 kW system suits about 5 to 10 kWh of usable storage, and the top of that range only makes sense above roughly 20 kWh of daily use.

Worked example one, a small household. Assumptions: 12 kWh a day, 6 kWh of it after dark, summer output of 4.5 kWh per kW and winter 2.5.

• Spare solar is 6.6 x 4.5 = 29.7 kWh minus 6 kWh of daytime use, about 23.7 kWh in summer, and 16.5 minus 6, about 10.5 kWh in winter.
• Night use of 6 kWh is the smaller figure in both seasons.
• Adding 15% gives about 7 kWh, so a 6 to 8 kWh battery.

What size battery for 10 kW solar?

A 10 kW system usually pairs with 10 to 13.5 kWh, because it can fill that much in all but the dullest winter weeks.

Worked example two, a family with an EV. Assumptions: 22 kWh a day for the house, 11 kWh after dark, an EV driving 40 km a day at 0.2 kWh per km (8 kWh), summer 4.5 and winter 2.5 kWh per kW.

• Summer spare solar is 45 kWh minus 11 kWh of daytime use and 8 kWh of midday EV charging, about 26 kWh.
• Winter spare solar is 25 minus 11 and 8, about 6 kWh.
• House night use is 11 kWh.

A 10 kWh battery is a sound middle choice if the car charges at midday or on a cheap off-peak window. Choose 13.5 kWh if you want the battery to carry the evening and part of the car’s charge. Charging an EV from the battery is a costly use of stored energy, because 8 kWh a night nearly empties a 10 kWh unit.

What size battery for an all-electric home with 13 kW of solar?

A large all-electric home with 13 kW of panels typically needs 15 to 20 kWh. Heat pump hot water, reverse-cycle heating and induction cooking push evening use up.

Worked example three. Assumptions: 35 kWh a day, 17.5 kWh after dark, summer 4.5 and winter 2.5 kWh per kW.

• Summer spare solar is 58.5 minus 17.5, about 41 kWh. Winter is 32.5 minus 17.5, about 15 kWh.
• Night use is 17.5 kWh.
• Winter spare and night use bracket the answer, and a 10% margin gives 16 to 19 kWh.

At this size the inverter and phase limits below matter as much as storage.

Should I buy a 10 kWh or a 13.5 kWh battery?

Buy 10 kWh if your night use is under about 9 kWh, and 13.5 kWh if it is 11 kWh or more. Between those, the extra 3.5 kWh is worth it mainly if your summer export is large.

Both sizes sit in the first 14 kWh tier, where the federal discount is worth most per kWh. The certificate counts below follow our battery rebates by state guide, and prices are the indicative ranges in our battery cost guide, checked November 2026.

The extra 3.5 kWh costs about A$2,000 before discount and earns 23 more certificates, worth roughly A$850 to A$920 at the A$37 to A$40 per certificate used in our large battery price guide. The net extra is about A$1,100. Our battery payback guide covers the savings side.

What is the difference between usable and nominal capacity?

Nominal capacity is the total energy a battery’s cells hold, and usable capacity is the part the battery lets you draw. Usable is never larger than nominal, and it is the figure the federal discount counts.

The Clean Energy Regulator defines usable capacity as the amount of energy that can be discharged and nominal capacity as the maximum amount a battery can store at full charge. Makers hold back some cells to protect battery life, so a hypothetical pack of 10 kWh nominal and 90% usable delivers 9 kWh (an illustration, not a real product). Some brands quote only usable capacity and some list both: Tesla’s Australian Powerwall 3 listing gives 13.5 kWh of usable energy as a single figure (we could not machine-read the datasheet PDF, which is image-only, so this rests on Tesla’s published listings). Ask for “usable kWh” in writing. The Clean Energy Regulator says each battery needs nominal capacity of 5 to 100 kWh to qualify. We list models in our reviewed home batteries and best solar batteries guide.

What is the difference between kW and kWh, and does phase matter?

A kilowatt hour (kWh) is how much energy a battery stores, and a kilowatt (kW) is how fast it can deliver it. Single or three phase mostly limits inverter power, not storage. See our kW vs kWh guide.

A 5 kW battery cannot run a 7 kW load, such as an oven, dryer and heat pump together, and the grid covers the rest. Divide kWh by kW for runtime: 13.5 kWh at 5 kW is 2.7 hours, and 20 kWh at 5 kW is 4 hours. Our large battery guide cites Energex guidance (February 2025) of 10 kVA of inverter capacity and 5 kW of export on single phase, and Ausgrid’s streamlined 10 kW per phase (checked November 2026). Other networks differ, so ask for yours.

How do I adjust for backup, an EV, time-of-use tariffs and a VPP?

Add storage for backup and for any EV charged from the battery, and size to your peak window on a time-of-use plan. A virtual power plant (VPP) is an arrangement where a retailer can draw on home batteries to support the grid in return for payments.

• Backup. A reserve is held back from daily use. Reserving 20% of 13.5 kWh leaves 10.8 kWh for savings. The 20% is an example setting.
• EV. Add its nightly kWh only if the battery charges it.
• Time-of-use. Size to use in the peak window, since savings come from avoiding peak rates. See peak and off-peak times.
• VPP. Check minimum capacity and reserve rules in the VPP guide.

How does the federal discount change oversizing?

Oversizing is now penalised, because certificates taper above 14 kWh. Using the Clean Energy Regulator factor of 6.8 (May to December 2026) and A$37 to A$40 a certificate, our arithmetic checked November 2026 is below.

Size to need, since each kWh beyond 14 earns a smaller discount, and the step down on 1 January 2027 trims every size.

What to check on your quote

Compare at least three quotes from the Solar Cobber directory, and check accreditation yourself on the Solar Accreditation Australia register.

• Usable kWh, nominal kWh and certificate count as separate lines.
• Continuous kW of battery and inverter, and single or three phase.
• Backup scope and reserve percentage.
• Which usage data the installer used for sizing.
• Location compliant with AS/NZS 5139, as the Clean Energy Regulator requires.

FAQ

What size solar battery do I need for an average Australian home?

Most homes need 10 to 13.5 kWh of usable capacity, but your night use decides it. A home using 15 to 20 kWh a day typically needs 7.5 to 10 kWh under our assumptions.

How many kWh battery do I need for a 6.6 kW solar system?

About 5 to 10 kWh of usable storage. A home using 12 kWh a day needs roughly 7 kWh under our assumptions.

How many kWh battery do I need for 10 kW of solar?

About 10 to 13.5 kWh. A home using 20 kWh a day needs about 10 kWh, and 25 kWh a day needs about 12.5 kWh. An EV charged from the battery adds its nightly kWh.

Is there a solar battery size calculator?

We do not have one, but the three steps above do the same job by hand. Some installers and retailers offer online tools that use your usage data.

Is a 13.5 kWh battery better than a 10 kWh battery?

Only if you use and export enough to fill it. Under about 9 kWh of night use, the extra 3.5 kWh sits idle.

Does a bigger battery earn a bigger federal discount?

Yes, but the extra is small above 14 kWh. Clean Energy Regulator tiers pay 100% of the factor to 14 kWh, 60% to 28 kWh and 15% to 50 kWh.

Sources

• Clean Energy Regulator, solar batteries (STC factors by period, 14/28/50 kWh tiers, usable and nominal capacity definitions), read November 2026
• Cheaper Home Batteries Program, DCCEEW: program page; we could not open it, so tiers and factors above were checked on the Clean Energy Regulator page instead.
• AER, Residential energy consumption benchmarks, Frontier Economics, 9 December 2020: household consumption by size (read via an archived copy).
• Tesla Energy Library, Powerwall 3 datasheet (Australia): usable energy.
• Postcode zone ratings, Clean Energy Regulator: solar yield zones.
• Solar Choice and SolarQuotes: price and power context, via our large battery guide.
• Clean Energy Council: daily output by city, via our solar sizing guide.
• Energex and Ausgrid: single-phase inverter guidance, via our large battery guide.

Battery program factors and tiers were confirmed on the Clean Energy Regulator page in November 2026. Installed prices and the A$37 to A$40 certificate value are from our own guides and were not re-checked here. Confirm program figures with your installer before you sign.

About this guide

Written by the Solar Cobber editorial team from independent desk research checked in November 2026. Solar Cobber is a free directory that may earn referral or advertising fees from installers, and commercial relationships never affect our content. This is general information, not financial advice. To report a correction, use our contact page.