After the sun goes down.
What you currently export.
50–60% is typical.
What you lose by storing instead.
Before any rebate.
Australian rebate — Cheaper Home Batteries
6.8 for May–Dec 2026. Steps down.
After admin costs. Market ~$40.
Advanced options
Usable share of nameplate.
Energy lost charging and discharging.
Fewer in winter. 365 is optimistic.
Virtual power plant credit, if joined.
Reserve held for outages.
This solar battery calculator sizes storage against both things that constrain it — your evening load and the daytime solar surplus available to charge it — then applies the tiered federal rebate, which changed on 1 May 2026 and now penalises larger batteries.
The short answer
Two limits govern battery size: what you use after dark, and how much surplus solar you have to fill it. The smaller wins. A 10 kWh evening load needs 12.4 kWh of nameplate — but if you only export 8 kWh a day, anything over 9.9 kWh never fills.
And a battery earns the gap between your rate and the feed-in tariff — 28c, not 33c — because storing a kilowatt hour means forgoing the export payment.
On this page
How to use this solar battery calculator
Three ways in:
- Evening load — what you use after dark, plus your daily solar surplus
- From my bill — total daily usage and roughly what share happens at night
- A battery size — you’ve been quoted something and want to check it
The daily solar surplus field is the one that matters most and the one you’re least likely to have to hand. It’s what you currently export — visible on your bill or in your inverter app as daily feed-in.
The rebate panel is open by default with the current STC factor and price, both editable, since they change twice a year.
The constraint nobody checks
Every battery calculator sizes to evening consumption. That’s half the problem.
A battery has to be filled, and the only free energy available to fill it is the solar you’re currently exporting. If you export 8 kWh a day, a battery that could store 15 kWh will never see more than 8 go in.
| Daily solar surplus | Largest battery that fills |
|---|---|
| 6 kWh | 7.4 kWh |
| 8 kWh | 9.9 kWh |
| 10 kWh | 12.3 kWh |
| 12 kWh | 14.8 kWh |
| 15 kWh | 18.5 kWh |
It isn’t a small effect. A household using 14 kWh after dark but exporting only 8 kWh has an evening load justifying a 17.3 kWh battery and a surplus supporting 9.9 kWh. Buying to the first figure means paying for over 7 kWh of capacity that never charges.
The fix is usually more panels before more battery — additional generation is cheaper per kWh than additional storage, and it lifts the ceiling on what storage can do. The calculator reports both numbers so you can see which one is binding.
Winter makes this worse. Surplus in June can be half the annual average, so a battery sized to summer export sits partly empty for months.
Two losses, and both apply
Nameplate capacity is not what you get out. Two separate reductions stack:
Loss one
Depth of discharge
Typically 90%. Batteries hold back a reserve to protect cell life, so a 13.5 kWh unit offers about 12.2 kWh usable.
Loss two
Round-trip efficiency
Also around 90%. Energy is lost converting DC to AC and back, and to heat. Put 10 kWh in, get 9 kWh out.
Combined
×1.23
To deliver 10 kWh you need 12.4 kWh of nameplate — both divisions, not one.
kWh = Evening load ÷ Depth of discharge ÷ Round-trip efficiency
10 ÷ 0.90 ÷ 0.90 = 12.35 kWh
Applying depth of discharge alone gives 11.1 kWh, which undersizes by 11%. It’s a common omission — round-trip efficiency is on the datasheet but rarely in the sizing calculation.
What a battery actually earns
This is where quotes tend to overstate.
Storing a kilowatt hour doesn’t earn you the retail rate. It earns the difference between the retail rate and the feed-in tariff — because that kilowatt hour was going to be exported for 5c anyway. You’re converting a 5c export into a 33c saving.
Annual value = kWh delivered × Cycles × (Retail rate − Feed-in tariff)
At 33c and 5c that’s 28c per kWh cycled, not 33c
The difference is 18%. A quote valuing storage at the full retail rate overstates the return by that much, and it compounds over a payback calculation.
It also means a generous feed-in tariff makes a battery worse, not better. If you’re on a legacy 20c tariff, the spread is only 13c and the arithmetic changes completely.
The federal rebate
Australia’s Cheaper Home Batteries Program began 1 July 2025 and runs to 2030. It’s delivered as an upfront discount through Small-scale Technology Certificates, claimed by your installer — there’s no separate application and no means test.
| Current position | Value |
|---|---|
| STC factor, May–Dec 2026 | 6.8 per usable kWh |
| STC price after admin costs | ~$37 |
| Rebate, first 14 kWh | ~$252/kWh |
| Minimum battery size | 5 kWh |
| Rebate cap | First 50 kWh usable |
| Next step down | 1 January 2027 |
The STC factor dropped from 8.4 to 6.8 — a 19% cut — and the scheme moved from annual to six-monthly reductions. It also became tiered by size, which it wasn’t before.
The factor is set on your installation date, not the date you sign. A quote based on today’s rate is not a guarantee if installation slips past a step-down. Both the factor and the STC price are editable in the calculator, and both should be confirmed with your installer.
Why bigger stops paying
Since 1 May 2026 the rebate tapers by size — and the taper is steep:
| Usable capacity | Share of STC factor | STCs per kWh | Rebate per kWh |
|---|---|---|---|
| 0–14 kWh | 100% | 6.80 | $252 |
| 14–28 kWh | 60% | 4.08 | $151 |
| 28–50 kWh | 15% | 1.02 | $38 |
| Above 50 kWh | — | 0 | Nil |
A kilowatt hour of capacity bought at the 30 kWh mark attracts 15% of the rebate of one bought at 10 kWh. The total keeps rising, but the effective rate falls hard:
| Usable capacity | Total rebate | Average per kWh |
|---|---|---|
| 5 kWh — the minimum | $1,258 | $252 |
| 10 kWh | $2,516 | $252 |
| 14 kWh — the sweet spot | $3,515 | $251 |
| 20 kWh | $4,403 | $220 |
| 28 kWh | $5,624 | $201 |
| 50 kWh — the cap | $6,438 | $129 |
Note that a battery under 5 kWh gets nothing at all, and the rebate is calculated on usable capacity, not nameplate — so a 13.5 kWh battery at 90% depth of discharge claims on 12.2 kWh.
State schemes
A few states add incentives on top of the federal program, and several have closed. As of mid-2026:
- NSW — a VPP connection incentive, stackable with the federal rebate, plus an income-tested interest-free loan
- ACT — Sustainable Household Scheme loan up to $15,000, rising to $20,000 from 1 July 2026. A loan, not a grant, and no longer interest-free for general applicants
- Victoria — no state battery rebate. The Solar Homes battery rebate and its loan both closed in late 2024. Older guides still list it
State schemes change more often than the federal one. Check your state’s current position rather than relying on any guide, including this one.
Backup power
Not every battery provides backup during an outage, and the ones that do usually need extra hardware and a dedicated circuit — the whole house rarely stays live.
Backup capacity is also reserved, meaning it’s held back rather than cycled daily. That reduces the energy earning you money, so it has a real cost. The calculator has a field for it.
If outages are your main reason for buying, that’s a legitimate purchase — but it’s a resilience decision, not a savings one, and the payback arithmetic shouldn’t be doing the persuading.
Is a battery worth it?
Honestly, it depends on numbers you can check rather than on general advice. The four that decide it:
- Your spread. Retail minus feed-in. Below about 15c, savings alone rarely justify it
- Your surplus. No spare solar means no charge, whatever the battery costs
- Your evening load. Low night usage means the battery cycles shallowly
- The rebate at your install date. Worth several thousand, and falling twice a year
Put your own figures in. If payback lands beyond the warranty period, the battery hasn’t paid for itself within the period the manufacturer stands behind it — which is the honest test, whatever a quote says about a 15-year life.
Battery sizing mistakes to avoid
- Sizing to evening load alone. Solar surplus is often the binding constraint.
- Applying depth of discharge but not round-trip efficiency. Undersizes by 11%.
- Valuing storage at the retail rate. It earns the spread — 28c, not 33c.
- Sizing to daily generation. It’s the evening load that matters.
- Assuming the rebate scales. Past 14 kWh usable it drops to 60%, then 15%.
- Confusing nameplate with usable. The rebate is calculated on usable.
- Locking in a rebate at quote date. It’s set on the installation date.
- Assuming 365 cycles. Winter surplus is much lower.
- Buying a big battery on a good feed-in tariff. A high FIT narrows the spread.
Frequently asked questions
What size solar battery do I need?
The smaller of two figures: evening load ÷ depth of discharge ÷ round-trip efficiency, and daily solar surplus ÷ the same two factors. A 10 kWh evening load needs 12.4 kWh of nameplate — but if you only export 8 kWh a day, anything above 9.9 kWh never fills.
Why does solar surplus limit battery size?
Because the battery has to be charged, and the free energy available is the solar you currently export. Export 8 kWh a day and a 15 kWh battery will never see more than 8 go in. Capacity above that line is paid for and idle. Adding panels is usually cheaper than adding storage you can’t fill.
How much does a battery actually save?
The difference between your retail rate and your feed-in tariff, per kWh cycled — around 28c at 33c retail and 5c feed-in, not the full 33c. Storing a kilowatt hour means giving up the export payment you’d otherwise have received. Quotes valuing storage at the retail rate overstate the return by about 18%.
What is the federal battery rebate in 2026?
The Cheaper Home Batteries Program gives 6.8 STCs per usable kWh from May to December 2026, worth about $252 per kWh at an STC price of $37 after admin costs. It applies to the first 14 kWh at full rate, tapers above that, and requires a minimum 5 kWh battery. No means test.
Does the rebate get smaller for bigger batteries?
Yes, since 1 May 2026. The first 14 kWh of usable capacity earns the full 6.8 STCs per kWh, 14–28 kWh earns 60% of that, and 28–50 kWh just 15%. A kilowatt hour bought at the 30 kWh mark attracts 15% of the rebate of one bought at 10 kWh.
When does the battery rebate reduce?
Every six months, with the next step down on 1 January 2027. The factor dropped from 8.4 to 6.8 on 1 May 2026 and the scheme moved from annual to six-monthly cuts. Critically, the rate is set on your installation date, not your quote date — a delayed install can cost you the difference.
What is depth of discharge?
The share of nameplate capacity you can actually use. Most home batteries allow around 90%, holding back a reserve to protect cell life — so a 13.5 kWh unit offers about 12.2 kWh. The federal rebate is calculated on usable capacity, not nameplate.
What is round-trip efficiency?
The share of energy you get back out after storing it — typically 88–90%. Losses come from DC-AC conversion and heat. It applies on top of depth of discharge, so delivering 10 kWh needs 12.4 kWh of nameplate, not 11.1. Applying only one of the two undersizes by 11%.
Is a solar battery worth it?
It depends on four numbers: your retail-to-feed-in spread, your daily solar surplus, your evening load, and the rebate at your install date. Below about a 15c spread, savings alone rarely justify it. If payback lands beyond the warranty period, the battery hasn’t paid for itself within the period the manufacturer stands behind.
Does a high feed-in tariff make a battery better?
No — worse. A battery earns the gap between retail and feed-in rates. On a legacy 20c tariff against a 33c retail rate the spread is only 13c, less than half the typical 28c. Good export rates make storage harder to justify, not easier.
Do batteries provide backup during a blackout?
Only some, and usually with extra hardware and a dedicated circuit rather than the whole house. Backup capacity is also reserved rather than cycled daily, which reduces the energy earning you money. Buying for resilience is legitimate, but it’s a different decision from buying for savings.
How many cycles a year will a battery do?
Around 300–340 full cycles is realistic. 365 assumes surplus every single day, which winter rules out — June surplus can be half the annual average, so the battery cycles shallowly or not at all for weeks. Using 365 overstates annual savings by roughly 11%.
Sources and method
- Cheaper Home Batteries Program, administered by DCCEEW and the Clean Energy Regulator through the Small-scale Renewable Energy Scheme. Began 1 July 2025, runs to 2030
- Tiered STC structure effective 1 May 2026: 100% of factor to 14 kWh, 60% for 14–28 kWh, 15% for 28–50 kWh, nil above 50 kWh
- STC factor of 6.8 per usable kWh applies May–December 2026; steps down six-monthly from 1 January 2027
- Rebate model reproduces published worked examples exactly — a 10 kWh battery returns $2,516 and a 21.5 kWh battery $4,625 at $37 per STC
- STC price varies with the market and administration costs; $37 is a common after-costs figure against a market price nearer $40
- State schemes verified as at mid-2026. Victoria’s state battery rebate closed in late 2024. Confirm your state’s current position before relying on any figure
Related calculators
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Cable Size CalculatorAmpacity and voltage drop
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Insulation CalculatorCut the load before storing it
Pool Volume CalculatorPool pumps soak up surplus
How this solar battery calculator works out its numbers
Two required sizes are computed independently. The first divides evening load — plus any reserved backup — by depth of discharge and then by round-trip efficiency. The second does the same to your daily solar surplus. The smaller is reported, along with which constraint bound it, because a battery is useless beyond either limit.
Energy delivered per cycle is capped three ways: by what the battery can hold, by what you actually use in the evening, and by what the surplus can charge. Annual value multiplies that by cycles and by the spread between retail and feed-in rates — never the full retail rate.
The rebate applies the tiered STC structure to usable capacity, sums the bands, floors the result to a whole number of certificates, and multiplies by the STC price. That method reproduces published worked examples to the dollar.
Rebate values change twice a year and this page will go stale. The STC factor is set on your installation date, not your quote date, and the STC price moves with the market. Both are editable in the calculator, and both should be confirmed with your installer or the Clean Energy Regulator before you commit. State schemes change more often still. Battery pricing, warranty terms, degradation rates and VPP payments all vary by product and retailer — get real quotes and put those numbers in rather than relying on defaults.