What Does the Petrol vs EV Cost Calculator Do?
This calculator compares energy spending for the same annual distance using petrol MPG and EV miles per kWh. It translates both into annual pounds and pence per mile, while showing charging losses separately through an efficiency input. It answers whether the entered electricity arrangement costs less than the entered petrol arrangement for moving the car. Buying the car and maintaining ownership are separate questions.
How to Use This Calculator
- Choose a common annual mileage. If comparing two household cars for the same driving task, both sides should cover that distance rather than one being assigned a shorter journey pattern.
- Enter the petrol car's imperial MPG and pump price in pence per litre. Select values representative of the routes and conditions you are considering.
- Enter EV miles per kWh, the average price of purchased electricity and charging efficiency. A blended electricity price should be weighted by energy purchased at each rate, not by the number of charging stops.
- Calculate and compare annual energy bills and pence per mile. Recalculate using a higher public-charging share or lower winter efficiency before treating the difference as a stable yearly allowance.
The Maths & Assumptions Behind the Tool
Petrol pence per mile = 4.54609 ÷ MPG × fuel pence per litre. EV pence per mile = electricity pence per kWh ÷ miles per battery kWh ÷ charging-efficiency fraction. Annual pounds for each side equal pence per mile × annual miles ÷ 100. Subtracting EV cost from petrol cost gives the energy saving; a negative result means the entered EV charging arrangement costs more.
Worked example
For 10,000 miles/year, petrol at 45 UK MPG and 145p/litre costs £1,464.85. An EV at 3.5 miles/kWh and 90% charging efficiency, paying 25p/kWh, costs £793.65. The EV energy saving is £671.20/year.
Fuel is 14.65p/mile and electricity is 7.94p/mile. If your efficiency is already measured from electricity bought at the meter, enter 100% charging efficiency to avoid counting losses twice. This tool compares energy only.
Why This Matters for UK Drivers
The EV electricity price can make a large difference. A driver with dependable low-cost home charging has a different cost pattern from someone relying mainly on public rapid chargers. Use the charging-mix tool if a single price would conceal that variation. For the petrol side, use the pump price and measured economy relevant to the intended routes. Comparing brochure efficiency for one car with a difficult real-world journey for the other is not a fair test.
A positive annual energy saving does not automatically justify replacing a car. The replacement's additional depreciation or insurance can consume that saving. For a vehicle you already own, however, the marginal energy cost can be useful when planning which car to take on a journey. Keep the distinction clear when presenting the result to somebody else. It is an energy comparison under stated assumptions, not a current-market recommendation or a prediction of future fuel and electricity prices.
Interpreting the result in practice
The comparison is most useful when the measurement boundaries match. A petrol MPG figure normally describes fuel consumed by the vehicle. An EV dashboard figure may describe battery energy, while the bill is based on purchased electricity. The charging-efficiency field bridges that gap. If your EV figure already comes from total miles divided by meter kWh, applying an additional loss would overstate the electricity bill.
For a household deciding which existing car to use on a trip, energy cost can be one relevant marginal cost. For a household deciding whether to buy another car, annual energy is only a component. Keep purchase price, resale, insurance, repairs and any charging installation costs alongside it. A favourable energy result does not establish that a particular replacement saves money overall.
Frequently asked questions
Should I use home or public charging prices?
Use a blended rate that reflects where you actually charge, or run the calculation twice.
Does this prove an EV is cheaper overall?
No. It compares energy cost only, not total ownership cost.
Comparing energy cost on equal mileage
What the number tells you
The cleanest petrol-versus-EV energy comparison puts both vehicles over the same annual mileage. Petrol cost depends on MPG and pump price; EV cost depends on miles per kWh and the blended electricity price you actually pay. Using the same mileage prevents usage from distorting the comparison.
Use realistic inputs
EV charging price deserves particular attention. A driver charging mostly at home overnight can have a very different cost per mile from someone reliant on rapid chargers. If your pattern is mixed, use the Charging Mix Calculator first and bring the blended price into this comparison.
What is outside the calculation
Energy cost is not total ownership cost. Purchase price, depreciation, insurance, tax, servicing and finance may outweigh the energy difference. Use this as the operating-energy comparison, then broaden the analysis if you are making a vehicle-purchase decision.