Inputs for Each Car
Electric car • Consumption (kWh/100 km, Wh/km or Wh/mi) • Whether that figure is battery-side or includes charging losses • Charging efficiency, if it is battery-side • Home and public electricity prices, and the share of charging at each
Petrol car • Fuel economy (L/100 km or mpg) • Fuel price per litre or per gallon
Both • The same distance and the same driving conditions
The Electric Vehicles hub collects the related tools.
Put Both Cars on the Same Basis
EV: cost per 100 km = kWh/100 km ÷ Charging efficiency × Price per kWh (If the consumption figure is measured from the mains — as EPA label and WLTP figures are — do not divide by efficiency again.)
Petrol: cost per 100 km = L/100 km × Price per litre
Unit conversions (exact definitions): • L/100 km = 235.215 ÷ US mpg, or 282.481 ÷ UK mpg — the MPG to L/100 km guide explains why the two differ. • Wh/mi → Wh/km: divide by 1.609344 — use the Wh/mi to Wh/km converter. 1 kWh/100 km = 10 Wh/km. • Cost per mile = Cost per km × 1.609344 • Annual cost = Cost per 100 km × Annual km ÷ 100; monthly = annual ÷ 12.
Base Example
Every input is illustrative — replace each with your own.
Running Cost — Illustrative Base Case
The Break-Even Electricity Price
Setting the two costs equal gives the electricity price at which the EV stops being cheaper per km:
Break-even price per kWh = Petrol cost per 100 km × Charging efficiency ÷ EV kWh/100 km
Base case: 12.25 × 0.90 ÷ 17 = €0.65/kWh. With a more efficient petrol car at 4.5 L/100 km (€7.88 per 100 km), it falls to 7.875 × 0.90 ÷ 17 = €0.42/kWh. Compare the break-even with the price you actually pay — a blended price if you charge in more than one place.
Scenarios
All scenarios use 15,000 km a year and the same 90% charging efficiency for simplicity. Prices and consumptions are illustrative, not market figures.
What the scenarios show
| Scenario (illustrative) | EV €/100 km | Petrol €/100 km | Annual difference |
|---|---|---|---|
| 1. Home flat rate €0.28; petrol 7 L at €1.75 | €5.29 | €12.25 | EV €1,044 less |
| 2. Home off-peak €0.10; petrol as 1 | €1.89 | €12.25 | EV €1,554 less |
| 3. Winter, EV 21 kWh/100 km at €0.28; petrol 7.5 L | €6.53 | €13.13 | EV €989 less |
| 4. 50% home €0.28 / 50% public €0.65; petrol as 1 | €8.78 | €12.25 | EV €520 less |
| 5. All public €0.65; efficient petrol 4.5 L at €1.75 | €12.28 | €7.88 | Petrol €660 less |
| 6. All public €0.45; efficient petrol 4.5 L at €1.75 | €8.50 | €7.88 | Petrol €94 less |
What Moves the Result
- Vehicle efficiency — of both cars, on your own driving. Official test figures (WLTP, EPA) use different procedures; compare cars using one source.
- Driving conditions — speed, traffic, climate and trip length affect both cars, not always equally.
- Home vs public charging — usually the largest single factor for the EV.
- Tariffs — time-of-use rates, fuel price changes and public network pricing.
- Distance — the difference per km scales with distance; at low mileage, absolute amounts are small.
Ownership Cost Is a Different Comparison
This guide stops at energy running cost. A total-cost-of-ownership comparison adds, for the same ownership period and distance:
- Purchase price and any grants you qualify for
- Depreciation — purchase price minus expected resale value
- Insurance — from real quotes
- Tax and registration — from your local rules
- Maintenance — from each manufacturer's service schedule and local quotes
Add these as separate lines next to the energy cost, and say clearly when a total includes them. Do not fold them into a "fuel cost" figure.
Calculate With Your Own Figures
The EV Charging Cost Calculator applies this method: enter consumption, charging efficiency, home and public prices, home share and annual distance, plus the petrol car's consumption and fuel price, to compare annual running cost. The methodology page documents the formulas.
Related questions with their own guides:
- Monthly charging budget — EV monthly charging cost guide.
- Time and cost of one charging session — EV charging time and cost guide.
- Charger power needed at home — Home Charger Sizing Tool.
- Emissions rather than cost — the electricity CO₂ guide explains grid factors.
❓Frequently Asked Questions
Related energy tools & guides
Part of the Electric Vehicles hub — explore the connected calculators, guides and topics.
Free calculator — Electric Vehicles
EV Charging Cost Calculator
Cost per session, per 100 km, per year — plus savings vs petrol and CO₂ avoided, with editable home/public tariff split.
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