Battery and Solar Sizing: How to Get the Right System in Australia

Most households that regret their solar or battery purchase made the same mistake: they sized the battery against their total daily electricity consumption rather than against what their solar system can actually fill.

A family using 22 kWh/day buys a 20kWh battery. The solar system is 6.6kW. In Melbourne, that system generates roughly 22–25 kWh/day on average - and 7–8 kWh of that is consumed during the day while the sun is shining. The battery receives perhaps 15–17 kWh of surplus on a good day. On an average winter day, it might receive 8 kWh. The 20kWh battery sits perpetually half-charged and pays back over 15+ years, if ever.

The right approach starts with understanding what your solar system actually produces, how much of that production you consume during daylight hours, and what surplus remains to charge the battery. Everything else follows from those three numbers.

How Much Solar Surplus Does Your System Actually Produce?

Australian solar irradiance varies substantially by location. The figures below are annual averages for a north-facing 6.6kW system at typical roof pitch.

City	Solar Zone	6.6kW Daily Average	Notes
Perth	Zone 3	28–33 kWh/day	Highest solar resource of capital cities
Adelaide	Zone 3	28–32 kWh/day	Strong year-round production
Brisbane	Zone 3	28–30 kWh/day	Consistent but wet season variation
Sydney	Zone 3	28–30 kWh/day	Good year-round, mild winters
Melbourne	Zone 4	22–25 kWh/day	Significantly lower, particularly winter
Darwin	Zone 1	Highly variable	Wet season dramatically reduces output

Melbourne’s lower figure is not a minor rounding difference. In June and July, a 6.6kW system in Melbourne may generate 12–16 kWh on an overcast day. This has direct consequences for battery sizing - a 13.5kWh battery that fills reliably every day in Brisbane will frequently finish a Melbourne winter day at 40–50% charge.

For a 10kW system, multiply the per-kW production figures above by roughly 1.5. A 10kW system in Sydney generates approximately 42–46 kWh/day on annual average.

Understanding Your Load Profile: Day vs Evening Consumption

The second number you need is your daytime self-consumption - how much electricity your household uses during the hours when solar is generating (broadly 9am to 4pm).

For most Australian households, daytime consumption represents 30–40% of total daily usage. Appliances running during work hours - refrigerators, standby loads, hot water systems, and any appliances set to run on timers - account for this share. The remaining 60–70% is consumed in the evening and overnight: cooking, lighting, entertainment, air conditioning, and EV charging.

This means the battery needs to cover your evening load, not your total daily load.

Household Type	Daily Consumption	Daytime Usage (35%)	Evening Load	Appropriate Battery
Single person / couple	8–14 kWh	3–5 kWh	5–9 kWh	5–10 kWh
Family of 3–4	16–24 kWh	6–8 kWh	10–16 kWh	10–13.5 kWh
Family with pool + ducted AC	28–40 kWh	10–14 kWh	18–26 kWh	16–20 kWh

The battery size in the final column matches the evening load - the portion of daily consumption that solar cannot directly cover. This is the figure to size against, not the total daily consumption number.

Three Household Sizing Scenarios

Scenario 1: Single Person or Couple, 10 kWh/day - Sydney

A single-person household or couple in Sydney using 10 kWh/day has a modest evening load of roughly 6–7 kWh. A 6.6kW solar system in Sydney generates ~28–30 kWh/day, consuming perhaps 3–4 kWh during daylight hours, leaving 24–26 kWh of surplus available for export or storage.

A 5–10 kWh battery covers the evening load comfortably. A 13.5kWh battery would rarely fill - the evening load does not draw it down enough each night for the next day’s surplus to consistently top it back up.

Recommended system: 6.6kW solar + 5–10 kWh battery Good hardware matches: Sungrow SBR 9.6kWh (~$5,200 installed) or BYD HVS 10.2kWh Export note: This household will still export significant solar; a high feed-in tariff or VPP participation adds value.

Scenario 2: Family of Four, 22 kWh/day - Brisbane

A family of four in Brisbane using 22 kWh/day has an evening load of approximately 14–15 kWh. A 6.6kW system generates 28–30 kWh/day, with 7–8 kWh consumed during daylight, leaving roughly 20–22 kWh of surplus.

A 13.5kWh battery covers most evening loads and fills reliably from the solar surplus. A 16.6kWh battery is also a reasonable match if the household has variable heavy evening usage - air conditioning in summer, for example.

With a 10kW solar system, the surplus grows to ~35 kWh/day, and a 16.6kWh battery becomes the right anchor.

Recommended system (6.6kW): 6.6kW solar + 13.5kWh battery Recommended system (10kW): 10kW solar + 13.5–16.6kWh battery Good hardware matches: Tesla Powerwall 3 (13.5kWh), BYD HVM 16.6kWh, Sungrow SBR 16kWh

Scenario 3: Family of Four with EV, 32 kWh/day Total - Adelaide

Adding an EV driven approximately 15,000km/year to the family above adds roughly 10 kWh/day of charging demand, bringing total household consumption to around 32 kWh/day.

The most efficient approach is to charge the EV directly from solar during the day - not to route solar through a home battery and then into the EV. This avoids battery round-trip losses and avoids needing an enormous battery. A smart EV charger that reads solar surplus and adjusts charge rate accordingly handles this automatically.

With EV charging handled during the day from solar, the home battery is again sized against the evening household load of ~14–15 kWh. A 13.5kWh battery covers this well. The solar system, however, needs to be larger to cover both the household daytime load and the EV charging load.

Recommended system: 10–13.2kW solar + 13.5kWh battery + smart EV charger Rationale: A 10kW system in Adelaide generates ~45 kWh/day, covering the 8–10 kWh household daytime load and providing 10+ kWh for EV charging before the battery begins filling.

The Solar:Battery Ratio Rule of Thumb

The simplest way to check whether a proposed system makes sense is to apply the production-to-battery ratio.

Every 1kW of solar generates roughly 4–5 kWh/day in temperate Australian climates (annual average). So:

• 6.6kW solar generates ~26–30 kWh/day

• Minus 7–8 kWh daytime self-consumption

• Surplus available for battery: ~18–22 kWh

• Battery sweet spot: 10–13.5kWh (fills daily without wasted surplus)

• 10kW solar generates ~40–45 kWh/day

• Minus 8–10 kWh daytime self-consumption

• Surplus available for battery: ~30–35 kWh

• Battery sweet spot: 13.5–16.6kWh

If a salesperson recommends a 20kWh battery with a 6.6kW solar system in Melbourne, the numbers do not support it. The daily surplus in Melbourne averages 14–17 kWh - the battery will rarely charge fully, and the overnight load will not fully draw it down either.

Payback Comparison: Solar Only vs Solar and Battery

Battery storage extends payback periods relative to solar-only installations. The economics depend heavily on evening consumption and battery cost.

System	Approx. Installed Cost	Estimated Payback	Notes
6.6kW solar only	$5,500–$7,000	3–5 years	32c/kWh electricity, 5c/kWh FiT
6.6kW solar + 10kWh battery	$12,000–$15,000	7–10 years	Standard usage profile
6.6kW solar + 10kWh battery + VPP	$12,000–$15,000	6–8 years	Assumes $400+/year VPP earnings
10kW solar + 13.5kWh battery	$18,000–$22,000	7–10 years	High-consumption household

Battery payback improves most for households with high evening consumption - those who would otherwise be drawing significant grid electricity between 5pm and 10pm. If most of your consumption is during the day, solar alone delivers the best return.

State battery rebates - available in Victoria, South Australia, and Queensland at varying levels - can reduce upfront battery costs by $1,000–$4,000 and shorten payback by 1–3 years. Check current Australian battery rebates here.

What Changes When You Add an EV

An EV adds consumption that is large, predictable, and - crucially - schedulable. Unlike evening household loads, EV charging can be shifted to align precisely with solar production.

Key principles for solar + battery + EV systems:

Size the solar first. An EV adds 8–12 kWh/day. Your solar system needs to cover household consumption plus this EV load before surplus remains for the battery. For most families adding an EV, this means upgrading from 6.6kW to 10kW or larger.

Charge the EV directly from solar. Use a smart EV charger with solar divert capability. The charger reads your solar inverter’s output and adjusts charge rate in real time to consume surplus directly, without routing it through the battery.

Keep the battery focused on the household evening load. The battery does not need to be sized to charge the EV as well. A 13.5kWh battery covering the evening household load is still the right anchor; the EV handles itself during the day.

Grid charging as a fallback. If the EV has not charged fully from solar by 4pm, a scheduled top-up from the grid on a time-of-use tariff (off-peak rates typically 15–20c/kWh) is far cheaper than buying a larger battery to handle EV charging.

Common Sizing Mistakes

1. Buying a battery before sizing the solar system correctly. A 13.5kWh battery paired with a 5kW solar system in Melbourne cannot fill daily. Prioritise solar capacity before deciding on battery size.

2. Sizing battery against total daily consumption rather than evening load. A family using 24 kWh/day does not need a 24kWh battery. They need a battery sized to cover the 14–16 kWh consumed after sunset.

3. Treating Melbourne like Brisbane. A battery sized for Brisbane’s daily solar surplus will routinely sit half-empty through a Melbourne winter. Be specific about your location when modelling expected fill rates.

4. Ignoring seasonal variation. A system that looks excellent in a January simulation may underperform badly from May to August. Ask your installer to model winter production specifically, not just annual averages.

5. Buying a larger battery to prepare for future consumption. If you do not yet have an EV, do not buy an oversized battery to prepare for one. Buy a battery-ready solar system (appropriate inverter, cable runs to battery location) and add the battery when the load justifies it.

How to Get a Reliable Estimate

The most reliable starting point is your last 12 months of electricity bills, which will show your total annual consumption and often a seasonal breakdown. Divide annual consumption by 365 for your daily average.

If your retailer provides interval data (half-hourly usage), download it and look at when your consumption peaks. Most retailers provide this via their online portal. This tells you your actual evening load, which is the key input for battery sizing.

A reputable solar installer will ask for this data and use it to model your system. If an installer quotes a battery size without asking about your usage pattern, that is a red flag. Use a solar savings calculator as a cross-check before committing.

Solar calculators such as Gridly’s solar savings calculator allow you to input your consumption, location, and tariff to estimate the right system size and indicative payback.

Verdict

Solar battery sizing in Australia comes down to three numbers: how much surplus your solar produces after daytime self-consumption, how much electricity you consume in the evening, and what a battery of that size costs installed in your state.

For most Australian families with a 6.6kW solar system, a 10–13.5kWh battery is the right anchor. For larger systems or households with high consumption, 13.5–16.6kWh is well-matched. Going larger than these ranges with undersized solar, or in lower-irradiance climates like Melbourne, produces a battery that will spend years running below capacity.

If you are starting from scratch, size the solar first, then match the battery to the surplus. If you are adding a battery to existing solar, model your daily surplus before committing to a capacity - your installer should be able to pull 12 months of inverter production data to make this precise.

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Frequently Asked Questions

What size battery do I need for a 6.6kW solar system in Australia?

For a 6.6kW solar system, a battery between 10kWh and 13.5kWh is the right match for most Australian households. A 6.6kW system generates roughly 22–30 kWh/day depending on your city, and after accounting for daytime self-consumption, you typically have 15–20 kWh of surplus available to store. A 10kWh battery covers most evenings; 13.5kWh covers nearly all. Going larger than 13.5kWh with a 6.6kW system in a temperate climate like Melbourne means the battery will regularly sit partially empty.

Is it worth oversizing your battery relative to your solar system?

Generally no. A battery can only store surplus solar energy - what your panels produce beyond what you consume during daylight hours. If your solar system cannot reliably fill the battery each day, you are paying for capacity that sits idle. The exception is if you plan to charge the battery from the grid during off-peak periods using a time-of-use tariff, in which case a larger battery may be justified. For solar-only use, match battery size to your evening load and your solar surplus, not your total daily consumption.

How does adding an EV change your solar and battery sizing?

An EV driven approximately 15,000km per year adds roughly 8–12 kWh of daily charging demand. The most cost-effective approach is to charge the EV directly from solar during the day using a smart EV charger, rather than routing that energy through a home battery. This avoids battery round-trip losses and reduces the battery capacity you need. If you add an EV, prioritise sizing your solar system up to 10kW or higher, and use a smart charger that aligns EV charging with solar surplus. Your home battery can then focus on covering the evening household load.

What is the payback period for solar plus battery in Australia?

A 6.6kW solar-only system typically pays back in 3–5 years in most Australian capital cities at current electricity prices of around 32c/kWh and feed-in tariffs of 5c/kWh. Adding a 10kWh battery extends payback to roughly 7–10 years, depending on how much of your load falls in the evening and what the battery costs installed. Participating in a Virtual Power Plant can improve payback by 1–2 years if annual VPP earnings reach $400 or more. The economics of battery storage are most favourable for households with high evening consumption.

Does solar production vary significantly between Australian cities?

Yes, significantly. Perth and Adelaide are the strongest capital city solar markets, with a 6.6kW system generating 28–33 kWh/day on average. Sydney and Brisbane are close behind at 28–30 kWh/day. Melbourne produces considerably less - around 22–25 kWh/day annual average - due to lower solar irradiance and more overcast winter days. This means a battery sized correctly for Brisbane may regularly sit half-empty through a Melbourne winter. Always size your system based on your specific city’s solar zone, not national averages.

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Related reading:

• Is a home battery worth it in Australia?
• Best home batteries in Australia 2026
• How much does a home battery cost in Australia?
• Solar and battery rebates in Australia 2026
• Solar savings calculator