Honest answer: as accurate as the numbers you put in. Here's exactly how they work.
Every tool on this site uses simple, published arithmetic — no black boxes, no "proprietary models". This page sets out the formulas, the assumptions behind the defaults, and the things we deliberately leave out so you can judge the results for yourself.
Countries that use miles, MPG or dollars-per-gallon are converted internally at exact factors (1 mile = 1.609344 km; US gallon = 3.785 L; imperial gallon = 4.546 L), so the maths is identical everywhere.
Defaults are drawn from public national data for the country you select — energy regulators, government emissions factors and motoring-body fuel monitors (AER, ACCC and the National Greenhouse Accounts in Australia; MBIE and the AA in New Zealand; Ofgem and RAC in the UK; EIA and AAA in the US, and so on). Each calculator links its sources. They're national averages from 2026 data — a sensible starting point, not your bill.
Purchase price, depreciation, insurance, registration, servicing and tyres. These vary so much by model and driver that including "averages" would create false precision. Most calculators answer one question well — what does the energy cost? — and leave whole-of-life comparisons to you. The one exception is the break-even tool, which exists precisely to weigh the purchase premium against those energy savings (it still excludes depreciation, and says so). One known gap worth flagging: time-of-use tariffs mean your effective rate depends on when you charge. New Zealand Road User Charges are included: on the NZ version, $76 per 1,000 km (the NZTA light-EV rate) is added to EV running costs in the EV vs Fuel, break-even and road trip calculators (rates: NZTA — RUC for electric vehicles and NZTA — RUC rates and transaction fees, checked July 2026) — and to diesel comparisons too, since light diesels pay the same rate. Petrol vehicles pay fuel excise at the pump instead, which is already part of the pump price. PHEV RUC ($38 per 1,000 km) and per-purchase admin fees (~$12–14) are not modelled. Flat yearly EV road fees are handled by an editable “Extra EV road fees” field on the EV vs Fuel and break-even calculators: the US version defaults to $150/year, the median of the EV registration fees now charged by 41 states (ranging roughly $75–$270 — set your own state's figure; see Tax Foundation). Canada defaults to $0 because most provinces charge nothing, but Alberta ($200), Saskatchewan ($300) and Quebec (announced ~$125) drivers should enter theirs. The UK defaults to $0 because standard VED now applies at a similar rate to both electric and combustion cars, so it roughly cancels in a comparison (the announced per-mile eVED, due 2028, will be added when it takes effect).
If you use the defaults, treat results as a solid ballpark — typically within 10–20% for the energy-cost comparisons, driven mostly by how much electricity and fuel prices vary around the national average. If you enter your own tariff, your car's real consumption and your local fuel price, the arithmetic itself is exact; the remaining uncertainty is just how your driving varies month to month.
This site is built and run by one person, with AI-assisted development used for coding and drafting. Every formula it uses is published on this page precisely so you don't have to take anything on trust — the maths is simple enough to check with a calculator, and we'd rather you did. All figures and assumptions are reviewed by a human before they go live.
Prices move — especially fuel. We review the defaults periodically against the sources above and bump the figures when they drift. If you spot a number that looks out of date, we genuinely want to know: email [email protected] and we’ll check it against the sources.
