Transparency

Methodology and assumptions

Last technical review: 21 July 2026. Rebate figures verified as of July 2026.

This page explains how the solar battery cost calculator turns your inputs into size, rebate, net cost, savings and payback estimates. Every result on the site is guidance only - not a quote, rebate approval or financial advice.

We use the values you enter first. Everything else is a published model assumption listed below.

What you enter

State or territory - selects which state scheme (if any) stacks with the federal discount.
Usage - either a quarterly bill amount plus your import rate, or average daily kWh from your bill.
Import rate and feed-in tariff - used for bill-derived usage and for savings. Default feed-in is 0.05 AUD/kWh when left blank.
Tariff type - flat or time-of-use. TOU applies a simple uplift to modelled savings (see savings section).
Solar - whether you already have panels and, if known, system size in kW.
Backup preference and install year - affect suggested size and which federal STC factor is used.
Optional battery model - catalog systems use indicative installed midpoints; otherwise we use a generic $/kWh cost model.

Battery sizing

In Size my battery mode we estimate nightly load as about half of daily usage, then scale by backup level (essential 0.6x, partial 1.0x, whole home 1.5x). The result is clamped between 5 kWh and 100 kWh (the federal program usable-capacity band we model).

Best payback sweeps generic sizes and catalog models to find the fastest modelled payback under your inputs. Hit my target bill looks for a configuration that gets estimated annual grid energy cost near your target. Both modes use the same rebate and savings maths as the main mode.

Installed cost

Generic (non-catalog) systems assume about $1,000 per usable kWh plus a $1,500 base install allowance. Catalog models use an indicative installed midpoint for that product. Extra identical units are costed at 90% of the first unit price to reflect shared install overhead.

Real quotes vary with inverter work, switchboard upgrades, roof access, travel and brand. Always replace our indicative cost with your written quote when you have one.

Federal rebate (Cheaper Home Batteries Program)

We model the federal discount as STCs: factor 6.8 for installs in 2026 (May-Dec settings in our model) times about $37 per certificate, or roughly $252 per usable kWh on the first tier. From 1 January 2027 the modelled factor drops to 5.7.

Capacity tapering in the model:

100% of the per-kWh value for the first 0-14 kWh
60% for the next 14 kWh (14-28 kWh)
15% for the next 22 kWh (28-50 kWh)
Nothing modelled above 50 kWh of tapered capacity

Federal figures as of July 2026. Source: Clean Energy Regulator. Exact certificate value and eligibility are confirmed by your installer at sale.

State and territory schemes

Net cost adds any modelled state rebate or one-off VPP incentive on top of the federal discount unless a scheme note says stacking is not available. Interest-free loans are disclosed but not subtracted from net cost - they are financing, not a rebate.

Jurisdiction	How we model it	As of
NSW New South Wales	NSW state battery install rebate is suspended (the federal rebate is larger and cannot be stacked with it). A one-off VPP connection incentive is modelled. Source	July 2026
VIC Victoria	Victorian state battery rebate/loan currently closed; federal rebate applies. Source	July 2026
QLD Queensland	Queensland Battery Booster rebate is closed; federal rebate applies. Source	July 2026
SA South Australia	SA Home Battery Scheme is closed; federal rebate applies. Source	July 2026
WA Western Australia	WA Residential Battery Scheme: $1,300 for Synergy customers (modelled). Horizon Power customers may receive up to $3,800. VPP participation is mandatory. Stacks with the federal rebate. Source	July 2026
TAS Tasmania	Tasmania offers interest-free loans rather than an upfront rebate; federal rebate applies. Source	July 2026
ACT Australian Capital Territory	The ACT offers zero-interest loans rather than an upfront rebate; federal rebate applies. Source	July 2026
NT Northern Territory	NT Home and Business Battery Scheme is closed to new applications (funding fully allocated); federal rebate applies. Source	July 2026

Savings and payback

We approximate daily solar generation as system size (kW) times 4.2 peak-sun hours. About 40% of household usage is treated as daytime load; the rest is evening/night. Excess solar after daytime use can charge the battery. The battery can cycle about 85% of its usable capacity when excess solar and night load allow.

Annual savings use the gap between your import rate and feed-in tariff on energy shifted from export to overnight self-use. Time-of-use plans apply a 1.4x uplift to that savings figure as a simple peak displacement adjustment - not a full TOU period model.

If you do not have solar yet and do not enter a system size, savings assume a new 6.6 kW system installed with the battery. Supply charges are excluded from annual bill before/after figures. Payback is net cost divided by estimated annual savings.

Simple formula shown in results: payback (years) = net installed cost / estimated annual savings.

Limitations

We do not import smart-meter interval data, so usage timing is assumed rather than measured.
Weather, shading, inverter limits, export caps and retailer plan changes are not fully modelled.
Battery and solar degradation, maintenance, insurance and inverter replacement are not deducted from long-horizon savings totals.
Catalog prices and generic $/kWh costs are market indicators, not installer quotes.
Rebate eligibility, VPP rules and certificate prices are confirmed at installation - our dated sources can lag sudden program changes.

Keeping figures current

Rebate constants live in one configuration file with as-of dates and official source URLs. A monthly monitor checks those pages for content changes so we can update the model when programs move. If something looks wrong, contact us.