How the calculation works
The cost of a charge is not the energy stored in the battery; it is the energy pulled from the meter, which is always higher because conversion and thermal losses happen along the way.
Home AC charging typically loses around 10%, split between the onboard charger and battery heating. Trickle charging from a household outlet is worse in percentage terms, because the vehicle keeps its electronics and cooling awake for far longer while charging slowly. DC fast charging is more efficient at the vehicle, though public pricing usually erases that advantage many times over.
Worked example
A 60 kWh car charged 60% at a home rate of 0.15 per kWh:
At 18 kWh per 100 km, that 36 kWh carries the car roughly 200 km, giving about 0.03 per kilometre. The same charge at a public fast charger priced three times higher would cost around 18, which is the entire economic argument for home charging.
What actually moves the number
- Tariff, more than anything. Off-peak or EV-specific night tariffs are often less than half the standard rate. Shifting charging to those hours is the single largest saving available, and it requires nothing but a timer.
- Public versus home. Fast charging convenience is priced accordingly. Used only on trips it barely affects the annual figure; used daily it can double running costs.
- Consumption, not battery size. A big battery costs more to fill but also goes further. Cost per kilometre depends on how efficiently the car uses energy, which is why winter driving and towing raise costs sharply.
- Solar self-consumption. Energy you generate yourself is charged at zero marginal cost, though the true comparison is against what you would have earned exporting it.
Frequently asked questions
Is charging an EV cheaper than fuel?
At home rates, usually by a wide margin: a typical EV costs a few cents per kilometre against a combustion car burning six to eight litres per 100 km. The gap narrows or disappears if you rely on premium-priced public fast charging, which is why home or workplace charging matters more to running costs than the car itself.
Should I charge from my solar array instead?
If you have surplus generation, yes. A 6 kW array on a sunny day can supply 25 to 35 kWh, most of a typical charge. The practical challenge is timing, since the car is often away during peak production; solutions are workplace charging, a home battery, or a diverter that charges only from surplus.
Why does my meter show more than the car reports?
The car reports energy that reached the battery; the meter counts everything drawn from the wall, including conversion losses, cooling and cabin preconditioning. A gap of 10 to 15% is normal and is exactly what the efficiency setting in this calculator represents.