What size battery do I need?
By Sithum Imarsha Last reviewed
A 3-person home in Sydney with 10 kW of solar uses about 11.1 kWh a day in the evening and overnight (2019 benchmark). That points to a battery of about 12.4 kWh usable, limited by your evening use. Bigger batteries sit part empty and get less rebate per kWh.
Your result
Suggested usable size
12.4kWh
Limited by your evening use: 11.1 kWh a day to shift, plus the reserve.
- Evening use
- 11.1kWh/day
- Solar surplus
- 31.6kWh/day
- Rebate on this size
- $3,108
- Simple payback
- 7.2 years
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Daily use17.4 kWh: AER benchmark, 2019 grid use.
Off-peak hot waterThe benchmarks include off-peak hot water, which a battery rarely runs. The measured median home with 10 kW of solar shifts about 7.9 kWh a day (Ausgrid 2012-13). If your hot water is on controlled load, type your use without it.
How we calculated this
| Usable size | Rebate | Rebate per kWh | Used a day (kWh) | Saving a year | Payback |
|---|---|---|---|---|---|
| 5 kWh | $1,258 | $252 | 4.5 | $452 | 7.2 years |
| 10 kWh | $2,516 | $252 | 9 | $905 | 7.2 years |
| 13.5 kWh | $3,367 | $249 | 11.1 | $1,120 | 7.8 years |
| 16 kWh | $3,811 | $238 | 11.1 | $1,120 | 9.5 years |
| 20 kWh | $4,403 | $220 | 11.1 | $1,120 | 12.1 years |
| 28 kWh | $5,624 | $201 | 11.1 | $1,120 | 17.5 years |
| 40 kWh | $6,068 | $152 | 11.1 | $1,120 | 26.7 years |
Rebate per kWh falls above 14 kWh and again above 28 kWh because of the capacity bands. A battery bigger than your evening use or solar surplus is not used every day, so the saving stops rising.
Daily use
Your figure, or the AER benchmark for your city and household. The benchmark is 2019 grid use only, so a home with solar used more than it shows. It also includes controlled load such as off-peak hot water, which the evening share leaves out.
17.4 kWh a day
Evening and overnight use
Daily use × the share used from 4pm to 7am, × the part you want covered.
11.1 kWh, cover 11.1 kWh
Solar surplus
Panels kW × solar yield, minus daytime use. The yield is the CER deemed value for your zone. Real surplus is lower on cloudy days and in winter.
37.9 − 6.3 = 31.6 kWh
What the battery can shift
The smaller of the evening use to cover and the surplus × round-trip efficiency.
11.1 kWh a day, limited by your evening use
Suggested usable size
Shiftable kWh ÷ (1 − reserve), rounded to 0.1 kWh.
12.4 kWh usable
Rebate and payback for each size
Rebate: usable kWh weighted in bands (0–14 kWh at 100%, 14–28 kWh at 60%, 28–50 kWh at 15%) × the STC factor, rounded down, × the STC price. Saving a year = 365 × (kWh used a day × import rate − kWh charged × feed-in rate). kWh used a day = the smaller of the size less the reserve and the shiftable kWh. Payback = (size × price per kWh − rebate) ÷ saving.
$3,108, 7.2 years
| City | 1 | 2 | 3 | 4 | 5+ |
|---|---|---|---|---|---|
| Sydney | 8.5 | 14.3 | 17.4 | 20.0 | 24.7 |
| Melbourne | 8.1 | 13.3 | 13.9 | 15.9 | 20.1 |
| Brisbane | 9.3 | 14.0 | 16.9 | 21.0 | 24.1 |
| Adelaide | 8.0 | 13.8 | 16.7 | 19.3 | 23.9 |
| Perth | 8.9 | 12.8 | 16.1 | 16.6 | 16.6 |
| Hobart | 16.4 | 24.1 | 26.0 | 29.6 | 31.7 |
| Canberra | 11.9 | 16.7 | 21.2 | 26.1 | 27.8 |
The AER has no benchmark for WA or the NT. For Perth we use Synergy's average use by household type (A1 customers, 2023-24): singles for 1 person and couples for 2. For 3, 4 and 5 or more people we map its household types to people, which is an estimate. Synergy has no group bigger than a family, so 5 or more uses the family figure. Every other figure is the four AER seasonal benchmarks for the capital's postcode, added up and ÷ 365.
| City | Zone | MWh per kW a year | kWh per kW a day |
|---|---|---|---|
| Sydney | Zone 3 | 1.382 | 3.79 |
| Melbourne | Zone 4 | 1.185 | 3.25 |
| Brisbane | Zone 3 | 1.382 | 3.79 |
| Adelaide | Zone 3 | 1.382 | 3.79 |
| Perth | Zone 3 | 1.382 | 3.79 |
| Hobart | Zone 4 | 1.185 | 3.25 |
| Canberra | Zone 3 | 1.382 | 3.79 |
| Value | Default | Source | Status |
|---|---|---|---|
| Daily use | By city and people | AER electricity consumption benchmarks 2020 (Frontier Economics), 2019 grid use, summed by season (AER workbook, Wayback copy, 19 Nov 2025) | Official source |
| Daily use, Perth, 1 and 2 people | By household type | Derived from Synergy's average 60-day bills by household type, A1 customers FY2023-24 (Synergy blog, Wayback copy, 2 Apr 2025): singles for 1 person, couples for 2 | Official source |
| Daily use, Perth, 3 to 5+ people | By household type | Synergy's more than two adults for 3 people and families for 4 and 5+, from the same bills. Our mapping of household types to people | Estimate |
| Used 4pm to 7am | 64% | Derived from Ausgrid solar home half-hour data, 300 Sydney homes, 2012-13 (Wayback copy, 26 Dec 2025) | Official source |
| Solar yield | By CER zone | CER postcode zone ratings, as of 1 January 2020 | Official source |
| Reserve | 10% | WattMaths estimate | Estimate |
| Check: median shift, 10 kW solar | 7.9 kWh a day | Derived from Ausgrid solar home half-hour data, 300 Sydney homes, 2012-13 (Wayback copy, 26 Dec 2025). General use only, no controlled load | Official source |
Sources
- Clean Energy Regulator: Postcode zone ratings for solar panel systems
- Clean Energy Regulator: Small-scale installation postcode data
- Clean Energy Regulator: Quarterly Carbon Market Report, June quarter 2026
- AER: Electricity and gas consumption benchmarks for residential customers 2020
- Ausgrid: Solar home electricity data 2012-13 (archived copy)
- Synergy: average electricity bill by household type (read via the Wayback Machine, 2 April 2025)
- Clean Energy Regulator: Solar batteries
- Clean Energy Regulator: Battery rebates are changing 1 May 2026
- DCCEEW: Cheaper Home Batteries Program
- DCCEEW: Small-scale technology certificates for batteries
- Canstar: electricity cost per kWh
- EnergyPlans: solar feed-in tariffs July 2026
- OTTER: Tasmanian feed-in tariffs
- WA legislation: Synergy A1 tariff, charges amendment by-laws 2026 (SL 2026/140)
Two limits decide the size
- Your evening and overnight use. A battery earns its keep on the power you would otherwise buy after the sun goes down. In Ausgrid’s data for 300 Sydney solar homes, the median home used 64% of its power between 4pm and 7am.
- Your solar surplus. The battery can only store what your panels make beyond your daytime use. A small or old system may not fill a big battery.
The suggested size is the smaller of the two, plus a reserve the battery keeps back.
Bigger is not always better
A battery larger than both limits is not used every day, so its saving stops rising while the price keeps going up. The federal rebate also pays less per kWh above 14 kWh and again above 28 kWh. The common sizes table shows both effects at your inputs.
Where the figures come from
- Daily use: AER consumption benchmarks, built from 2019 bills. Grid use only, so solar homes used more than they show. No WA or NT figures.
- Evening share: Ausgrid solar home data, 2012-13. Before most EVs, heat pumps and home batteries.
- Solar yield: the Clean Energy Regulator’s deemed rating for your zone. Real rooftops often make a little less.
All of these are editable. Your own bill and smart meter data beat any benchmark.
Before you get quotes
- Check your total daily use, not just what you buy from the grid.
- Ask for the usable capacity. Brochures often show nominal.
- Think about winter. Solar is lower and heating raises evening use, so a battery sized for an average day will not fill on many winter days.
Questions
Why not buy the biggest battery the rebate allows?
A battery only saves money on the kWh it moves each day. Above your evening use or your solar surplus, the extra capacity sits unused. The rebate also shrinks per kWh: the first 14 kWh get the full rate, 14 to 28 kWh get 60%, and 28 to 50 kWh get 15%.
How old are the usage figures?
The daily use benchmarks are the AER 2020 set, built from 2019 bills. Perth has none, so it uses Synergy's averages by household type (2023-24). The evening share comes from Ausgrid data for 300 Sydney solar homes in 2012-13. They are older than most EVs and heat pumps, so type your own figures if you have them.
My bill is lower because I have solar. Which figure do I use?
Your total use, not just what you buy from the grid. A solar home uses some of its own solar during the day, and the bill does not show it. The AER benchmarks are grid use only too, so they read low for solar homes.
Does the benchmark include my off-peak hot water?
Yes. The AER benchmarks count everything on the meter, including controlled load such as off-peak hot water. The 64% evening share is from general use only, so a benchmark can oversize the battery. As a check, the median home in Ausgrid's data, with its solar scaled to 10 kW, could shift about 7.9 kWh a day. If your hot water is on controlled load, type your daily use without it.
What about winter?
Solar output is lower in winter while heating raises evening use, most of all in Canberra and Hobart. The suggested size is for an average day. On many winter days it will not fill.
Usable or nominal capacity?
Usable. The rebate and this calculator both use usable kWh. Brochures often show the larger nominal figure.