Fuel Versus Electricity: What a Trip Actually Costs
Comparing running costs between a petrol car and an EV is one formula with different units, which makes the comparison cleaner than the debate around it usually is.
The same arithmetic, twice
The trip cost calculator multiplies distance by consumption per 100 km by price per unit. For 600 km:
- Petrol at 7 L/100 km and £1.60/L → 42 litres, £67.20 (£11.20 per 100 km)
- EV at 18 kWh/100 km and £0.30/kWh → 108 kWh, £32.40 (£5.40 per 100 km)
Roughly half, on those inputs. But the inputs are doing all the work, and the energy price is the one that varies most.
The price input is the whole argument
Home overnight electricity and motorway rapid charging can differ by a factor of three or four. Run the same EV trip at a rapid-charging rate and the advantage narrows sharply or disappears; run it on a cheap overnight tariff and it widens.
Which means "EVs are cheaper to run" is not a property of the vehicle so much as a property of how it is charged. A car charged almost entirely at home and one charged almost entirely on the motorway are financially different cars.
Consumption is not a constant either
Both figures move with how you drive, and they move in opposite directions. A combustion car is least efficient in stop-start town driving; an EV is most efficient there, because regenerative braking recovers energy and there is no idling.
On a motorway the positions reverse: aerodynamic drag rises with the square of speed and hits the EV harder in proportional terms, which is why EV range estimates look pessimistic in traffic and optimistic at 70 mph.
So a comparison built on a single consumption figure describes one kind of driving. Running it twice — once with town figures, once with motorway — is more informative than arguing about the average.
What the calculator deliberately excludes
Fuel or energy only. Not depreciation, insurance, tax, servicing, tyres or the purchase price — all of which differ between the two and several of which are larger than the energy line over a year.
It is a per-trip operating cost, and it is honest about being that. Total cost of ownership is a different calculation with far more assumptions in it.
Using it well
The most useful application is a specific trip you actually take. A regular commute, priced at your real consumption and your real energy price, gives a number that means something — unlike a generic comparison built on national averages that match nobody.
Measure your own consumption over a few tanks or charges rather than using the manufacturer's figure. It is usually worse than the official number, and it is the input the whole calculation rests on.
Run it for the trip you actually take
A generic comparison built on national averages describes nobody. The same calculator run on your real commute, at your real consumption and your real energy price, produces a number you can act on.
Two runs are usually worth doing: one at home-charging or supermarket-fuel prices, and one at motorway rates. The spread between them is frequently larger than the gap between the two fuel types.
Where the bigger costs sit
Energy is rarely the largest line in running a car. Depreciation usually is, followed by insurance and servicing, and none of those appear here.
That is not an argument against the calculation — a per-trip energy cost is exactly what you want when deciding whether to drive somewhere — but it is worth remembering before extrapolating a trip figure into a claim about ownership.