How every figure on this site is calculated
Every charging cost on ChargeCost is the output of three published formulas applied to two verified inputs: the car’s manufacturer specification and the July 2026 Ofgem price cap unit rate (26.11p/kWh). This page shows the arithmetic in full so any figure can be checked with a calculator.
The three formulas
- Energy drawn from the meter. Home AC charging loses energy between the meter and the battery, so you pay for more than the battery stores:
grid kWh = usable battery kWh ÷ (1 − 12% loss) - Cost of a charge.
cost = grid kWh × unit rate (p/kWh)Partial charges (say 20–80%) use the same formula on the kWh that window represents. - Cost per mile.
pence per mile = unit rate ÷ (efficiency mi/kWh × (1 − 12% loss))
Charging time is the kWh needed divided by the effective charger power, where effective power is the lower of the charger’s rating and the car’s own AC limit — which is why a 22 kW wallbox charges most cars no faster than a 7 kW one.
Worked example at the July 2026 cap
A car with a 60 kWh usable battery and an efficiency of 3.5 mi/kWh, charged at home on the July 2026 cap rate of 26.11p/kWh:
- Energy from the meter: 60 ÷ (1 − 0.12) = 68.2 kWh
- Full charge: 68.2 × 26.11p = £17.80
- Per mile: 26.11 ÷ (3.5 × 0.88) = 8.5p
These are the same functions the calculator and every model page run — this example recalculates automatically whenever the cap rate in use changes.
Prices based on the Ofgem price cap of 26.11 p/kWh, effective 1 July 2026.Source: Ofgem
The inputs, and their rules
- Unit rate: the Ofgem price cap (GB average, Direct Debit), effective 2026-07-01, checked against Ofgem’s published figures each quarter. Off-peak examples come from the public Octopus Energy API, refreshed daily and always labelled.
- Vehicle specs: compiled by hand, checked against the manufacturer’s spec sheet, and published only once verified — unverified rows never appear on any page. Each model page names its source and its caveats.
- Charging loss: home AC charging typically loses 10–15% between meter and battery; we assume 12% and state it wherever it is used. Our charging losses guide explains where the energy goes.
- Efficiency: WLTP-derived miles per kWh. WLTP is a lab figure; real-world costs run higher, especially in winter, and every page says so.
What we refuse to do
No figure on this site is estimated by a model or copied from another website. If a number has no verifiable source it is shown as a labelled assumption or a range, or it is not shown at all. Found one that fails that test? Tell me — corrections ship fast, because being checkable is the whole product. More background on the site itself is on the about page.