EV vs petrol: what it really costs per km
Enter your consumption, your tariffs and how much you charge at home, on public AC and on fast chargers: you get the cost per 100 km, the yearly bill and the break-even mileage.
The figures already filled in are only a starting point: replace them with your own consumption and with the tariffs you actually pay, because that is what decides the comparison.
Electric car
The trip computer counts the energy leaving the battery, while the charger and your house meter count what goes in through the cable: the gap between them is the charging loss, usually 8 to 12% on alternating current. If you took the figure from the charging app, pick at the plug: the losses are already in it.
Where you charge (the three shares must add up to 100)
Petrol or diesel car
If you think in miles per gallon, divide 235.2 by US mpg (282.5 for imperial mpg) to get litres/100 km: 30 US mpg is 7.8 litres/100 km. Use the figure you really get, not the official one.
How much you drive, and how much more the electric costs
How much more the electric car costs than the equivalent petrol one, after any incentives. Leave it empty if you only care about the running cost.
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How the cost per 100 km is worked out
The electric side needs two numbers: how many kWh you pay for every 100 km, and what a kWh costs you. The kWh you pay for are consumption ÷ (1 − losses): 17 kWh/100 km read on the trip computer with 10% losses becomes 17 ÷ 0.90 = 18.9 kWh at the meter. The price per kWh is the weighted average of your mix: 70% at 0.25 + 20% at 0.45 + 10% at 0.79 gives 0.344 per kWh. Multiply them: 18.9 × 0.344 = 6.50 per 100 km. The petrol side is just litres × price: 6.5 × 1.75 = 11.38 per 100 km. Over 15,000 km a year that is 975 against 1,706.
Units: kilometres, litres, and how to think in miles and gallons
Both sides use the same units on purpose, so nothing can quietly diverge between the two cars: kilometres for distance and litres for fuel. Converting is quick. Litres/100 km = 235.2 ÷ US mpg, or 282.5 ÷ imperial mpg — so 30 US mpg is 7.8 litres/100 km and 40 US mpg is 5.9. For distance, 1 mile is 1.609 km: multiply any cost per km by 1.609 to get the cost per mile, and multiply your yearly mileage by 1.609 before typing it in. Electric consumption in miles/kWh converts as kWh/100 km = 62.14 ÷ miles per kWh.
The charging mix is what really decides the answer
With identical consumption, those 100 km cost 4.72 charging only at home and 14.92 charging only on rapid chargers — more than petrol. That is why comparisons quoting a single tariff, almost always the cheap overnight home rate, are misleading for anyone without a driveway. If you do not know your own split, look at a year of history in your charging app: most drivers overestimate how much they fast-charge, while people without home charging discover the opposite and find the electric car far less attractive than the advertising suggests.
Charging losses: why the meter shows more than the battery takes
Energy is lost between the socket and the cells in the on-board charger, the cables and the battery cooling: typically 8 to 12% on AC, somewhat less on DC charging itself, though in winter battery preconditioning adds to it. The figure on your dashboard is measured at the battery and does not include those losses, but you pay for them all the same. If instead you take the number from the charger or from your electricity meter, the loss is already inside it: pick at the plug, otherwise you count it twice and inflate the cost by a good 10%.
Break-even, and what is deliberately left out
The break-even mileage is price difference ÷ saving per kilometre: a 8,000 premium with 0.049 saved per km is about 164,000 km, nearly eleven years at 15,000 km a year. Compare that with how long you actually keep a car: if you change every five years the break-even never arrives and resale value matters far more. Only energy is counted here — road tax, insurance, servicing, tyres, tolls, charging subscriptions, the home wallbox and depreciation are all outside. And if the electric car costs more per kilometre, break-even simply does not exist: the tool says so instead of printing a meaningless negative number. Treat the whole thing as an estimate that is worth exactly as much as the numbers you typed.