One line, compared properly
Comparisons between electric and petrol vehicles usually collapse into slogans. The problem is that the two are billed in different units. Petrol is sold by the litre and consumed at litres per hundred kilometres. Electricity is sold by the kilowatt hour and consumed at kilowatt hours per hundred kilometres. Prices for both vary by country, by supplier, by time of day and by where you fill up.
The way through is to convert both to a single unit, cost per hundred kilometres, and then to build the calculation with your own numbers rather than someone else’s. In this article we deal only with the energy line. Depreciation, insurance, maintenance and taxes are separate lines with their own logic, and they matter, but mixing them into an energy comparison is what makes most published comparisons unusable.
The formulas
Petrol cost per 100 km = litres per 100 km multiplied by the price per litre.
Electric cost per 100 km = kilowatt hours per 100 km at the wheels, divided by charging efficiency, multiplied by the blended price per kilowatt hour.
Two elements of the second formula are routinely omitted, and both push the electric figure upward.
Charging efficiency. Energy billed at the meter is more than energy stored in the battery. Losses occur in the charger, the onboard converter, the cables and in battery thermal management. Alternating current home charging is typically the least efficient, rapid direct current charging somewhat better on the conversion step. If your car reports consumption from the battery rather than from the wall, the meter reading will be higher.
Blended tariff. Almost nobody charges exclusively at one price.
Blended price = share charged at home multiplied by the home rate, plus share charged in public multiplied by the public rate.
A hypothetical worked example
Every figure below is invented for illustration. None of them is a price in any country. Replace each one with your own before drawing conclusions.
Suppose you drive 15,000 kilometres a year.
Petrol. Suppose real world consumption is 7.2 litres per 100 kilometres and petrol costs 1.70 per litre. Cost per 100 kilometres is 7.2 multiplied by 1.70, which is 12.24. Annual cost is 150 multiplied by 12.24, which is 1,836.
Electric. Suppose real world consumption is 17 kilowatt hours per 100 kilometres measured at the battery, and charging efficiency is 88 percent. Energy billed is 17 divided by 0.88, which is 19.32 kilowatt hours per 100 kilometres. Suppose 80 percent of charging happens at home at 0.22 per kilowatt hour and 20 percent in public at 0.55. The blended price is 0.8 multiplied by 0.22, which is 0.176, plus 0.2 multiplied by 0.55, which is 0.11, giving 0.286. Cost per 100 kilometres is 19.32 multiplied by 0.286, which is 5.53. Annual cost is 150 multiplied by 5.53, which is approximately 829.
| Input | Petrol (illustrative) | Electric (illustrative) | Where to get your own figure |
|---|---|---|---|
| Consumption | 7.2 litres per 100 km | 17 kWh per 100 km | Your own trip computer over several months, not the official test figure |
| Efficiency adjustment | Not applicable | Divide by 0.88 charging efficiency | Compare meter readings against the car’s reported energy added |
| Energy actually paid for | 7.2 litres | 19.32 kWh | Calculated |
| Unit price | 1.70 per litre | 0.286 blended per kWh | Pump receipts, your electricity bill, public charging network tariff pages |
| Cost per 100 km | 12.24 | 5.53 | Calculated |
| Annual at 15,000 km | 1,836 | 829 | Calculated |
In this invented scenario the annual energy saving is 1,836 minus 829, that is 1,007.
Break even, the most useful number
Rather than a single answer, calculate the point at which the two are equal. Using the same invented consumption figures, the electricity price at which electric energy costs the same as petrol energy is the petrol cost per 100 kilometres divided by the billed kilowatt hours per 100 kilometres, that is 12.24 divided by 19.32, which is 0.63 per kilowatt hour. Below that blended rate, electric is cheaper on energy. Above it, petrol is.
Run it the other way as well. The petrol price at which petrol matches the electric cost is 5.53 divided by 7.2, which is about 0.77 per litre. Two break even numbers tell you how much headroom your conclusion has, which is far more robust than a single point estimate that becomes wrong the next time tariffs change.
Where the comparison usually goes wrong
- Using official test consumption. Laboratory cycles understate real world consumption for both drivetrains, and not by the same margin.
- Ignoring charging losses. This inflates the electric advantage by roughly the loss percentage.
- Assuming all home charging. The blend is what you pay. A driver without off street parking may charge mostly in public at multiples of the home rate.
- Ignoring standing charges and session fees. Public charging often adds a connection fee, a minimum charge or idle fees after the session ends. Home electricity has a daily standing charge, although that is usually payable whether or not you own a car.
- Ignoring the tariff structure. Time of use tariffs, night rates and demand charges can change the home rate substantially, and some require a compatible meter.
- Ignoring temperature. Cold weather raises electric consumption more than petrol consumption, because cabin heating cannot use waste engine heat.
- Ignoring how energy is taxed. Fuel duty is embedded in the pump price. Electricity is taxed differently, and several jurisdictions are introducing distance based charges for electric vehicles specifically to replace lost fuel duty. That is a policy risk to your forecast.
What is deliberately outside this calculation
Energy is one line. Purchase price differences, depreciation patterns, insurance, tyre wear, servicing intervals, battery warranty and home charger installation all differ between drivetrains and all belong in a full ownership comparison. An electric vehicle can be cheaper per kilometre on energy and more expensive overall, or the reverse. We would run the energy line first because it is the easiest to calculate accurately, then place the result into the wider total cost model rather than treating it as the answer.
Energy prices, tariff structures, taxation of fuel and electricity, and any distance based charges vary by country and change frequently. Confirm current rates with your electricity supplier, the charging networks you use and the relevant national authority before relying on any figure. This article explains a calculation method and is general information, not personalised financial advice, and it does not recommend any particular vehicle.
How to Use This Guide
Every worked example on this page is illustrative. The figures are chosen to show how the calculation behaves, not to report current market rates. Costs, tax rules and eligibility criteria differ by country and change over time, so take the method from this page and put your own current figures into it. Browse the rest of our guides, or see how we source and check our comparison figures on the about page.
Information only. This article explains how costs and rules are structured. It is not financial, tax, legal or immigration advice, and it is not a recommendation to buy any property, vehicle or investment. Confirm anything that affects a decision with the relevant local authority or a qualified professional before you act on it.
Last updated: August 14, 2026