Methodology
Data sources
Each page lists its exact sources. Our main sources are:
| Topic | United States | Australia |
|---|---|---|
| Income tax rates and thresholds | Internal Revenue Service; state revenue agencies | Australian Taxation Office |
| Retirement and savings limits | Internal Revenue Service | Australian Taxation Office |
| Electricity prices | U.S. Energy Information Administration | Australian Energy Regulator; Essential Services Commission (Victoria) |
| Solar output estimates | National Renewable Energy Laboratory (PVWatts) | PVWatts international data |
| Solar and battery rebates | Federal and state program pages | Clean Energy Regulator; Department of Climate Change, Energy, the Environment and Water |
| Fuel prices | U.S. Energy Information Administration | State fuel price schemes (e.g. NSW FuelCheck, Victoria Servo Saver, WA FuelWatch) |
| Consumer prices | Bureau of Labor Statistics | Australian Bureau of Statistics |
Data licences and required attributions are shown at the bottom of each page that uses the data.
How calculators work
- Calculators apply the official rules for the period you select. Rules are stored with their start and end dates, so results match the year or purchase date you choose.
- Calculations run in your browser; your inputs are not sent to us.
- Each result lists the rules and assumptions used and anything not included (for example, offsets, credits or local charges that don’t apply to everyone).
- Calculators are tested against worked examples or tables published by the official agencies. A calculator is not published unless all tests pass, and we don’t build calculators for rules without an official example to test against.
- Rule changes are applied only after the responsible agency publishes them on its own website.
Averages and estimates
- Average electricity prices from the EIA are calculated by the EIA as revenue divided by sales; they are averages, not the rate on any particular plan.
- Solar output estimates carry uncertainty (roughly ±10% for most locations, per the model’s publisher) and are shown as a range.
- Recent installation counts may be revised upward by the source and are labelled as preliminary.
Updates
Data is checked automatically on a schedule (daily to monthly, depending on the source). Each page shows when its data was last updated. If an expected update is late, the page says so.