Our method
Know what the answer is built on.
We separate what your home tells us from the assumptions needed to compare solar and battery choices. Here is exactly how the current version calculates your results, and what it can’t yet tell you.
Reviewed 20 October 2026 · Australian residential focusHow it works
- Start with a real baseline
- Turn one bill into a typical year
- Simulate every hour
- Price it with your plan
- Compare the extra step
- Work out what's worth paying
- Rank every size at typical prices
- The 30-second size finder
- Adding a battery to existing solar
- Battery prices and the discount schedule
- Your roof, your car
- Use your smart-meter data when you have it
- Read a PDF bill, then let you check it
- Time-of-use batteries
- State incentives only when you choose
- Be clear about the limits
Start with a real baseline
Every option is compared with keeping your current equipment and electricity plan. For a home with solar, your existing system is the baseline for a battery. Grid imports are not your home's total use, because some solar is used directly, so we work out total use from your imports and exports together.
Turn one bill into a typical year
We scale a seasonal household pattern so that, over your billing period, it uses exactly the electricity on your bill. Because one bill can't show when during the day you use power, we run the whole calculation three times, for evening-heavy, mixed and daytime use, and show the range.
Simulate every hour
For 8,760 hours of a typical year we balance household use, solar generation, battery charging and discharging, grid imports and exports. Energy is conserved every hour, inverter limits are respected, and a battery can't charge and discharge at once or start with free energy. Batteries charge only from spare solar; if storing solar is worth less than the feed-in credit after losses, the model doesn't cycle the battery.
Price it with your plan
Imports, exports and supply charges are priced with the rates you entered, including a simple 3pm–9pm peak for time-of-use plans. Rates are held flat over time, which is the conservative case. Demand charges and wholesale or virtual-power-plant plans aren't supported yet, and we say so instead of approximating them.
Compare the extra step
Solar value and battery value are shown separately, and a larger battery is compared with a smaller one, not only with no battery. That's where the diminishing value of extra capacity shows up.
Work out what's worth paying
Your price limit is the total estimated saving over your chosen payback target, allowing solar benefit to fall 0.5% and battery benefit 2% a year, with battery value counted for 15 years. We also show lifetime value over the years you expect to stay, discounted at 5% a year. If you enter a quoted price, we show its payback and lifetime value on the same basis.
Rank every size at typical prices
We compare five solar sizes from 5 to 13.2 kW, each with no battery, 10 kWh and 20 kWh (or four battery sizes if you already have solar), using typical installed prices for your capital city from the Solar Choice price indexes, after the federal discount. The option that leaves you furthest ahead over the years you expect to stay, in today's dollars, is shown first, alongside the lowest-cost option that still pays off and the best option with a battery. State incentives aren't subtracted, so battery results are on the cautious side. If nothing pays off, we say so.
The 30-second size finder
The Battery Size Finder and the add-a-battery check run this same engine on your state's typical rates (or the time-of-use rates you enter) across battery sizes from 5 to 30 kWh. A bill amount is turned into kWh by finding the use that reproduces it, including the supply charge and any feed-in credits. The best-value size is the one with the shortest payback (a smaller battery wins a near-tie); the range is every size within three-quarters of a year of it; and the ceiling is the last size whose own extra 5 kWh still repays within 15 years.
Adding a battery to existing solar
Every kWh a battery stores is a kWh you would otherwise have exported. So we show both sides: the grid power you no longer buy, and the feed-in credit you give up, including WA's higher 3pm–9pm buyback rate. If you enter the solar you export from your bill, we work out how your panels actually perform (shade and direction included) instead of assuming they face north, and size the battery to the spare solar you really have, summer and winter.
Battery prices and the discount schedule
Typical battery prices are national averages including installation and the inverter, after the federal Cheaper Home Batteries discount, from Solar Choice (August 2026 data). The 5 kWh price is our estimate and is labelled as one. Brand prices are estimated ranges for one common setup from published price surveys, with the source shown. The federal discount is worked out from its published rules: 6.8 certificates per kWh until 31 December 2026, tapering above 14 kWh, falling every six months to 2030, at an assumed $38 a certificate. We show the next step-down for your size as a dated fact, not a deadline.
Your roof, your car
Output is adjusted for the way your panels face, using PVWatts modelling published by SolarQuotes (north 99%, north-east and north-west 94%, east and west 84%, south 65% of the best direction), and for shade you describe (88% for some shade, 70% for heavy shade, rough adjustments rather than a shade survey). East-facing panels make more in the morning and west-facing in the afternoon. A planned electric car adds charging at 17 kWh per 100 km in the window you choose. Exports are capped at 5 kW, a common single-phase limit.
Use your smart-meter data when you have it
If you load a NEM12 file (the standard Australian smart-meter format), we use your own hour-by-hour imports and exports from the latest 365 days instead of an assumed pattern, so no usage timing is guessed. Missing days are filled from the same month and the coverage is shown. For a home with solar, your measured exports are the spare solar a battery could store, so panel size and roof direction aren't needed. A full year of data gives a Detailed assessment; part of a year gives a Refined one. The file is read in your browser and never uploaded.
Read a PDF bill, then let you check it
You can choose a PDF bill instead of typing. It's read in your browser, and only charge lines that show both a quantity and a rate (or an amount we can divide) are used. Every figure is shown for you to check before it fills the form, and anything we can't find is left blank rather than guessed. Scanned bills can't be read.
Time-of-use batteries
On a time-of-use plan, the battery is also tested in a mode that holds charge for the dearer hours, judged only from the previous day's use (as a real battery controller would). We use whichever of the two modes saves more and say which. Charging from the grid at cheap times isn't modelled yet.
State incentives only when you choose
Typical prices exclude state battery incentives unless you tick the box for your state's scheme (currently WA, NSW and SA). Each scheme's rate, cap, VPP conditions and source are shown, and you can enter the amount on your own quote. Savings are modelled without VPP payments or limits.
Be clear about the limits
Solar output starts from Australian Government averages for your capital city, not a survey of your roof. Switchboard work, network approval, exact export limits and backup design need an installer's site check. Every result is labelled with its evidence level and the assumptions behind it, and the calculations are covered by automated tests, including the worked examples in our engineering specification.
Try it
Start with your own details.
See your options in a few minutes, with no account or installer contact.