A transparent UK worked example
The table below keeps the inputs visible so the result can be challenged or replaced. The petrol figure is an example rather than a claim about today's cheapest station. The electricity inputs use current official reference values explained below.
| Input or result | Petrol | Electric |
|---|---|---|
| Energy price | 150p/litre | 80% at 26.11p/kWh; 20% at 54p/kWh |
| Efficiency | 45 UK MPG | 3.59 miles/kWh |
| Energy cost per mile | 15.15p | 8.83p |
| Energy cost at 10,000 miles | £1,515 | £883 |
The petrol-versus-electric calculator lets you replace every input and immediately shows which assumption changed the result.
How to calculate petrol cost per mile
Use the UK imperial-gallon conversion:
At 150p/litre and 45 UK MPG, the result is 15.15p per mile. Real-world MPG matters more than the official laboratory figure when you are comparing your actual household cost.
How to calculate electric-car cost per mile
For a single charging price, divide pence per kWh by miles per kWh. For mixed charging, calculate a weighted price first.
EV pence per mile = blended p/kWh ÷ miles per kWh.
In the example, the blended rate is 31.69p/kWh. Dividing by 3.59 miles/kWh produces 8.83p per mile.
What the official 2026 benchmarks say
HMRC's advisory rates from 1 June 2026 are 7p per mile for home charging and 15p per mile for public charging. HMRC's underlying table uses 3.59 miles/kWh, 26.90p/kWh for domestic charging and 54p/kWh for public slow or fast charging below 50kW. These are company-car reimbursement benchmarks, not a guarantee that every driver will pay those amounts.
Ofgem's average electricity unit rate for a direct-debit customer on the default-tariff price cap in England, Scotland and Wales is 26.11p/kWh from 1 July to 30 September 2026. Actual rates vary by region, tariff, payment method and time of use. The calculator uses Ofgem's current unit rate as its home-charging starting point and leaves the field editable.
For petrol, DESNZ publishes accredited weekly national averages. A nearby pump price is still the better input for an individual journey or household comparison.
When public charging changes the answer
Public charging can cost much more than home charging, particularly on rapid and ultra-rapid networks. The percentage charged away from home therefore matters. A driver who cannot charge at home should enter the prices they regularly pay rather than borrowing a home-heavy national scenario.
Charging losses are not added as a separate percentage in this calculator because many vehicle efficiency figures measured from the wall already include them while dashboard efficiency often does not. Use a wall-to-wheels figure where possible, or reduce the miles/kWh input to reflect losses.
Energy cost is not total ownership cost
The result deliberately excludes purchase price, finance interest, depreciation, insurance, servicing, tyres, road tax, repairs, home-charger installation and electricity standing charges. For many drivers, depreciation or finance can outweigh the annual energy difference.
That distinction prevents a common SEO error: saying one powertrain is categorically “cheaper” based only on fuel or electricity. The defensible statement is narrower—one has the lower energy cost under the inputs shown.
How to make the comparison fair
- Use the MPG and miles/kWh you actually achieve over similar seasons and routes.
- Use the petrol station and charging prices you normally pay, not the cheapest possible promotional rates.
- Estimate the real home-versus-public charging split.
- Use the same annual mileage for both vehicles.
- Compare purchase and ownership costs separately before making a buying decision.