The formula
cost per km = consumption ÷ 100 × price of the unit
Where it comes from
The honest comparison is per kilometre, not per tank or per charge. Electric is measured in kilowatt hours per hundred kilometres, and that is where it pulls ahead: electrical energy costs far less per unit of work than a fossil fuel.
How to work it out by hand
- Divide each option's consumption by a hundred
- Multiply by the price of its unit: a litre or a kilowatt hour
- That gives the cost per kilometre
- Multiply by the kilometres you do in a year
What is worth knowing
This comparison leaves out what usually decides the purchase: the price of the vehicle, the servicing — an electric car has no oil, filters or clutch — and the depreciation. It also shifts a great deal with where you charge: at home overnight a kWh can cost a third of what a motorway rapid charger asks, and there the electric advantage narrows considerably.