VoltGarage
How is an electric car's cost per km calculated?
Last updated: 19 October 2026
Cost per km = kWh used per 100 km × 1.1 × price per kWh ÷ 100. The 1.1 covers a charging loss of about 10%: for every 100 kWh that reaches the battery you pay for about 110. A car that uses 18 kWh per 100 km and charges at home for €0.24 per kWh pays about €4.75 per 100 km, or €0.048 per km.
The formula
You need three numbers: how many kWh the car uses per 100 km, how much is lost while charging, and what you pay per kWh.
Cost per 100 km = kWh used per 100 km × (1 + charging loss) × price per kWh
Cost per km = cost per 100 km ÷ 100
The loss depends on how you charge. When VoltGarage does not know the billed kWh, it assumes about 10% for AC and home charging and about 5% for DC fast charging. The calculator on this site keeps it simple and uses 10% for everything.
Worked example
Assumptions: the electric car uses 18 kWh per 100 km, so with a 10% loss you pay for 19.8 kWh per 100 km. The petrol car uses 7 litres per 100 km. The prices are the calculator's starting values: home electricity is the median across euro countries from Eurostat (first half of 2026, taxes included), petrol the median from the EU Weekly Oil Bulletin (12 October 2026), and the fast-charging price is a typical example. Your local prices will change the result.
| Where you charge | Price | Per 100 km | Per km |
|---|---|---|---|
| Home | €0.24/kWh | €4.75 | €0.048 |
| Fast charger (DC) | €0.59/kWh | €11.68 | €0.117 |
| 70% home, 30% fast | €6.83 | €0.068 | |
| Petrol car (7 L/100 km) | €1.93/litre | €13.51 | €0.135 |
Charging 70% at home and 30% at fast chargers, the car costs €0.068 per km. That is about 44% more than charging only at home, and still about half what the petrol car costs.
Why the order of charges matters
The energy in your battery may have come at different prices. If you average them, one expensive fast charge also inflates the cost of trips that never used it. VoltGarage uses first in, first out instead: every charge becomes a layer in the battery, and the energy you use driving is taken from the oldest layer first. The battery does not really keep energy apart; this is an accounting method.
Example: your battery holds 30 kWh from home, for which you paid €9.60 (€0.32 per kWh). You then add 15 kWh at a fast charger and pay €9.30 (€0.62 per kWh). Then you drive 150 km and use 27 kWh.
| Method | Cost of the 27 kWh | Per km |
|---|---|---|
| Oldest layer first (VoltGarage) | €8.64 | €0.058 |
| Average of all charges | €11.34 | €0.076 |
Once all the energy is used, both methods add up to the same total. What changes is which trip carries the cost: the fast charge is paid for by the next trip, the one that actually used that energy.
What changes the result
- Where you charge. A kWh at a fast charger can cost several times what it costs at home. How much of your charging happens at home changes the result more than anything else.
- Your car's real consumption. 18 kWh per 100 km is an example. The model, speed, weather and climate control all change it.
- Charging loss. Part of the energy drawn from the socket never reaches the battery. If you know the billed kWh, there is nothing to estimate.
- Free charging at work. VoltGarage counts it as a layer that cost nothing, which brings your cost per km down.
How VoltGarage works it out
For each charge you enter what you paid and the start and end percentage. The app works out the kWh that went into the battery from your car's capacity and divides what you paid by it to get that layer's price per kWh. That way the charging loss is already inside the price.
How far the percentage drops between two charges gives the energy used in between. That energy is taken from the oldest layer first and divided by the distance between your odometer readings. A charge you did not enter a price for is not counted as free; it is kept separately as unknown.