How we calculate
How we pick sources, label and review figures, and round results. Then a link to the working behind every calculator.
Where our figures come from
The rates, prices and other figures behind the calculators are kept in data files. Each file records where its figures come from and the date it was last reviewed.
We prefer the official source. When we cannot read a number on the official page, we use a dated third-party page. When no source gives it at all, we work out our own estimate and say what it is based on. Any other figure we have not yet checked against the official page carries the “Not yet checked” label.
See every source and where we use it (50 topics · 290 sources)
What the labels mean
A label says how far a figure has been checked. Our sources page counts 739 figures that carry their own status label. Here are the labels, best first, with how many of those figures carry each one.
- Official source 407 figures
- Read on the official page: a regulator, a government scheme, a published tariff, or the maker’s own datasheet or spec page.
- Dated third-party source 119 figures
- Read on a dated third-party page, such as a price survey, a retailer or a product test.
- Estimate 196 figures
- Our own figure, worked out from other sources. The tool says what it is based on.
- Not yet checked 17 figures
- Taken from a source, but not yet checked against the official page.
How often we review
We review every data file at least every 90 days. A review means reading the values on their source pages again. The review date moves even when nothing changed, because it records the check.
A test fails when any review date in a data file is more than 90 days old. It also runs by itself every week, so an old date is caught even when nothing else changes.
Some figures change on a known date, such as regulated electricity rates. We aim to check those when the new ones are published.
Petrol and diesel prices change every week. An automated job fetches the new weekly averages, and each new week is checked before it goes on the site. The prices on the site now are for the week ending 27 September 2026.
Each calculator page shows the date its figures were last reviewed. Across the 34 data files behind our sources page, the oldest review date is .
Rounding and estimates
Results are estimates. A result is only as good as the figures it uses and the inputs you enter. Your own bill and real quotes come first.
Results are rounded when they are shown:
- Dollar amounts shown without cents, such as a yearly cost or a rebate, are rounded to the nearest dollar.
- Dollar amounts shown with cents, such as a cost per hour or per load, are rounded to the nearest cent.
- Electricity rates in cents per kWh show up to 2 decimal places.
- Payback times in years show up to 1 decimal place.
That rounding is for results. A table that lists the rates a calculator uses can show a rate in full, as its source publishes it, with more decimal places: 33.2621c/kWh for Western Australia, for example.
Who checks the numbers
Sithum Imarsha, a full-stack and AI engineer, builds and runs WattMaths and checks its figures. Each calculator page shows the author's name next to the date its figures were last reviewed.
Corrections
If a figure or a result looks wrong, tell us on the contact page. Say which page it is on and what you saw. A link to a source helps.
We check it against the source. When an error is confirmed, we fix it and update the review date.
How each calculator works
Each of our 31 calculators has a section called “How we calculated this”.
That section works through the result step by step and lists its sources. These links go straight to it.