The assumptions, the formulas and the limits. If a number on your report looks wrong, this page tells you where it came from.
Run the free checkA free, independent solar and battery calculator for Australian homeowners.
You answer ten questions about your home and power use. It returns an estimated system size, an installed price range before and after the incentives you personally qualify for, an estimated annual saving, a payback period and a ten-year cost projection. It takes about a minute and there is no sign-up before you see your results.
It is run by Oliver Hill, who previously worked in Victorian solar sales. It is not an installer, and no company pays to appear on it.
Homeowners deciding whether solar, a battery, or both are worth it for their specific house, who want their own numbers before a salesperson gives them someone else's. Renters can use it, but solar generally needs the owner's approval.
A comparison site's product is your phone number: you fill in a form, your details go to three installers, and three salespeople call you. This gives you the assessment itself first. Nobody contacts you unless you ask to be introduced.
It does not give you a quote, and it is not a substitute for one. It cannot see your roof, your switchboard, your meter data or your shading at particular times of day. It does not read your actual electricity bill or your retailer's specific tariff. Every figure it produces is an estimate built from the answers you give it, and an installer still needs a site inspection to turn that into a real price.
Four steps, in this order. Every later number depends on the earlier ones.
Array size is set by your quarterly bill bracket. Annual generation is that size multiplied by your state's average peak sun hours, then by a performance ratio of 0.80, which covers inverter losses, heat, soiling and wiring. Roof shading then derates it: no shading applies no reduction, some shading applies 10 per cent, heavy shading applies 28 per cent.
This is set by whether someone is home during the day, because it decides how much of your consumption happens while the panels are producing. Home most days assumes 45 per cent of daily use falls in daylight hours, sometimes assumes 34 per cent, and out for work or school assumes 25 per cent. Shading reduces this too, because shade removes particular hours rather than trimming output evenly across the day.
A battery can only move whatever is left after direct use, and only as much as it can physically cycle, and only as much as you still need after dark. The smallest of those three limits is what it actually shifts. Usable depth of discharge is taken as 90 per cent and round-trip efficiency as 90 per cent.
Power you use on site avoids the grid import rate. Power you export earns the feed-in tariff. Both come from your state's figures. The saving is held below your estimated annual bill less its daily supply charge, currently taken as $1.10 a day, because the supply charge is the part of a bill no system removes. It approaches that limit with diminishing returns rather than being clipped to it, so a genuinely better setup always shows a better number.
The single number that decides whether a battery pays for itself.
Charging a battery with solar that would otherwise have been exported converts cheap exported energy into avoided expensive imported energy. The value of one kilowatt-hour delivered by the battery is the grid rate it avoids, minus the feed-in income given up on the solar spent charging it. Because charging is lossy, delivering one kilowatt-hour consumes rather more than one:
Those rates are a Victorian example. Your report uses your own state's figures and prints all four numbers, so you can check the arithmetic on your own page.
Daytime occupancy pushes in opposite directions depending on what you are buying.
| Pathway | What is valued | Incentives applied |
|---|---|---|
| Solar only | Direct daytime use plus exports. Being home during the day matters most here, because unused generation exports for a fraction of its value. | Federal STC, plus any state panel rebate. No battery rebate. |
| Solar + battery | Direct use, stored energy and exports. Occupancy matters less, because the battery absorbs the daytime surplus either way. | Federal STC and any state panel rebate, plus the federal battery rebate and any state battery scheme. |
| Battery only | Only the energy the battery shifts from export to evening use. The panels are already paid for, so their savings are already in your bill. | Battery incentives only. No new-panel rebate, because no new panels are being installed. |
The federal STC is applied as an upfront discount and scales with system size and location. The federal Cheaper Home Batteries Program is calculated at roughly $252 per usable kilowatt-hour, counted on the first 14kWh, so a 10kWh battery claims on 10kWh and anything larger claims on 14. State schemes are applied only where that state actually runs one.
Rebate levels, feed-in tariffs and retail electricity prices all change, and federal rebates step down over time. The figures in the tool reflect the settings it was last updated with, and it says so on every report. Confirm anything time-sensitive with your installer on the day you sign.
Which battery is suggested is a separate calculation from the money. It scores brands on compatibility with the inverter you already have, whether the system is new or a retrofit, and the one or two priorities you selected. It cannot change your cost, sizing or payback figures.
The calculator works for every Australian state and territory, using that state's own sun hours, retail rate, feed-in tariff and incentive settings. There are dedicated pages for capital cities and major regional centres, including Brisbane, the Gold Coast, the Sunshine Coast, Gympie, Byron Bay and Melbourne.
Aussie Solar Info is an online tool. It has no offices, branches or staff in any of those places. Vetted installers are independent businesses that operate in their own service areas.
Free. About a minute. No one calls you unless you ask.
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