Methodology
How we calculate EV vs petrol running costs — the formulas, assumptions, data sources, and limitations.
Overview
The calculator compares the annual fuel and electricity running costs of a petrol or diesel car against an equivalent electric vehicle. It does not include purchase price, depreciation, insurance, servicing, road tax, or finance costs — these vary too much between vehicles and circumstances to be included reliably.
All calculations run entirely in your browser. No personal data, no postcode, and no inputs are transmitted to or stored on any server beyond a postcode lookup (which is handled via the free postcodes.io API).
Petrol / diesel cost calculation
We convert MPG to litres per 100 km, then calculate annual litres consumed and multiply by your local fuel price.
Litres per year
litresPerYear = (annualMiles ÷ MPG) × 4.54609
4.54609 is the number of litres in one Imperial gallon (UK standard).
Annual fuel cost (£)
fuelCostPerYear = litresPerYear × (fuelPricePencePerLitre ÷ 100)
Petrol cost per mile (£)
petrolCostPerMile = fuelCostPerYear ÷ annualMiles
EV electricity cost calculation
We calculate kWh consumed per year from your EV efficiency, then apply a blended electricity rate based on your home vs public charging split.
kWh per year
kWhPerYear = annualMiles ÷ evMilesPerKwh
Blended electricity rate (p/kWh)
blendedRate = (homeChargePercent ÷ 100 × homeRate) + ((100 − homeChargePercent) ÷ 100 × publicRate)
Annual EV electricity cost (£)
evCostPerYear = kWhPerYear × (blendedRate ÷ 100)
EV cost per mile (£)
evCostPerMile = evCostPerYear ÷ annualMiles
The home vs public charging split has a large impact on results. Home charging at 28p/kWh gives roughly 8p per mile for a typical EV. Public rapid charging at 75p/kWh pushes that to around 21p per mile — more expensive than many petrol cars.
Savings and break-even
Annual saving (£)
annualSaving = fuelCostPerYear − evCostPerYear
A positive value means EV is cheaper. A negative value means petrol / diesel is cheaper with your current assumptions.
Break-even (years)
breakEvenYears = evPricePremium ÷ annualSaving
Break-even is only shown when the annual saving is positive (EV is cheaper). The EV price premium defaults to £2,500 — an approximate difference between a popular mid-range EV and its petrol equivalent, after government grants where applicable. Adjust this to reflect your own comparison.
Default values
| Input | Default | Source |
|---|---|---|
| Annual mileage | 10,000 miles | DfT average UK driver annual mileage |
| Petrol MPG | 45 MPG | Typical mid-range petrol hatchback official combined |
| Petrol price | 143.9p/litre | BEIS / RAC national average (updated manually) |
| Diesel price | 151.9p/litre | BEIS / RAC national average (updated manually) |
| Home electricity | 28.0p/kWh | Ofgem price cap unit rate (standard variable) |
| Public charging | 60.0p/kWh | UK network average fast charging rate |
| Home charge % | 80% | Typical EV owner with home charger |
| EV efficiency | 3.5 miles/kWh | Mid-range EV real-world efficiency |
| EV price premium | £2,500 | Approximate EV vs petrol equivalent premium |
Limitations and caveats
- ⚠Results cover fuel and electricity running costs only. Insurance, depreciation, servicing, road tax, and finance can add £2,000–£5,000+ per year and are not included.
- ⚠Official MPG figures are typically 10–25% higher than real-world fuel economy. Consider reducing the MPG figure to reflect your driving style.
- ⚠EV real-world efficiency varies significantly with speed, temperature, load, and driving style. Winter efficiency can be 20–30% lower.
- ⚠Public charging prices change frequently and vary by network, location, and subscription. Check Zap-Map or individual network apps for current pricing.
- ⚠Home electricity rates depend on your tariff. EV-specific overnight tariffs (e.g. Octopus Go) can reduce home charging costs to 7–10p/kWh — dramatically improving EV economics.
- ⚠Fuel prices are national averages. Local prices vary by up to 20p/litre. Enter your postcode for live local pricing from nearby forecourts.
- ⚠This tool does not constitute financial advice. Always verify costs with your own research before making purchasing decisions.
Data sources
- Fuel prices: BEIS Weekly Road Fuel Prices, RAC Fuel Watch — updated manually in fallback data. Live local prices are fetched via the CMA Fuel Finder API when a postcode is entered.
- Electricity rates: Ofgem price cap quarterly updates for standard variable tariff rate.
- Postcode lookup: postcodes.io (free, open source).
- EV efficiency: Real-world data from Spritmonitor, WLTP test data, and EV Database.
- Gallons conversion: 1 UK Imperial gallon = 4.54609 litres (BIPM definition).
Something not right? Spotted an error in our methodology? Get in touch. We update defaults when official data changes.