Should I Buy an EV or Petrol Car: Worked Example With Stated Assumptions

If you want a straight answer on should i buy an ev or petrol car, start with the maths, not a headline figure. This page works through the formula step by step, runs a worked example with every assumption stated in full, and gives you tables you can read your own numbers off. Nothing here is presented as a current price or a measured efficiency: every figure is labelled as an assumption or an example, and the method works with whatever numbers you substitute.

The short answer is that the comparison comes down to cost per mile: the petrol car burns US gallons, so its cost per mile is the price per gallon divided by its MPG, while the EV uses kilowatt-hours, so its cost per mile is the electricity price per kWh divided by its miles per kWh. With the example assumptions on this page — petrol at $3.25 per US gallon, 32 US MPG, electricity at 22 cents per kWh and 3.8 miles per kWh — the petrol car costs about $0.10 per mile and the EV about $0.06 per mile. Those are example figures, not prices you should quote, and the rest of this page shows how they were reached and what would change them.

should i buy an ev or petrol car: Assumptions used on this page

For should i buy an ev or petrol car, the point is this: Every figure in the tables below comes from this assumptions table. Each one is an assumption or an example, not a measured fact about any particular car or a current price. Substitute your own numbers and the method still works.

Input Value used Status
Petrol price $3.25 per US gallon Example price
Petrol efficiency 32 US MPG Assumption
Electricity price 22 cents per kWh Example price
EV efficiency 3.8 miles per kWh Assumption
Annual mileage 10,000 miles Example

should i buy an ev or petrol car: How the EV versus petrol comparison works

For should i buy an ev or petrol car, the point is this: Hold the mileage constant and price the energy each car actually uses. For the petrol car, cost per mile equals the price per gallon divided by its MPG. For the EV, cost per mile equals the price per kWh divided by its miles per kWh. Multiply each cost per mile by the annual mileage and you have the annual fuel and charging bill for each car; subtract one from the other and you have the saving, or the extra cost, under those assumptions. The method is deliberately simple, because a simple method with honest inputs beats a complicated model built on guessed ones. The two numbers that matter most are the electricity price and the petrol car's MPG: a driver who charges at home on a cheap overnight rate will see a very different answer from a driver who relies on public rapid chargers, and a thirsty petrol car widens the gap while an efficient one narrows it.

Worked example: the same distance, two energy bills for should i buy an ev or petrol car

Take a driver covering 10,000 miles a year. The petrol car does 32 MPG and petrol costs $3.25 per gallon as an example price, so its cost per mile is $3.25 ÷ 32 = $0.10, and 10,000 miles cost $1,015.62 a year. The EV does 3.8 miles per kWh and electricity costs 22 cents per kWh as an example price, so its cost per mile is $0.22 ÷ 3.8 = $0.06, and 10,000 miles cost $578.95 a year. The difference on fuel and charging alone is $436.68 a year in the EV’s favour under these assumptions. Now change one input and watch the answer move: if the EV is charged mostly at a public rapid charger at, say, double the home electricity price used here, its annual charging cost doubles and most of the saving disappears. If the petrol car is driven mainly in town at lower MPG, the petrol bill rises and the saving grows. That sensitivity is the real lesson — the answer is only as solid as the assumptions, so write yours down before you believe the result.

Cost at different annual mileages for should i buy an ev or petrol car

The table uses the worked-example assumptions above — petrol at $3.25 per gallon (example), 32 MPG, electricity at 22 cents per kWh (example), 3.8 miles per kWh. The gap grows with mileage, which is why high-mileage drivers see the biggest fuel-cost difference and low-mileage drivers see the smallest.

Annual miles Petrol cost per year EV charging cost per year Difference
6,000 $609.38 $347.37 $262.01
8,000 $812.50 $463.16 $349.34
10,000 $1,015.62 $578.95 $436.68
12,000 $1,218.75 $694.74 $524.01
15,000 $1,523.44 $868.42 $655.02
20,000 $2,031.25 $1,157.89 $873.36

Cost per mile and per 100 miles for should i buy an ev or petrol car

At the example assumptions, the petrol car costs $0.10 per mile ($10.16 per 100 miles) and the EV costs $0.06 per mile ($5.79 per 100 miles). Per 100 miles is often the easier figure to compare with a fuel receipt or a charging bill, because both are usually priced in round numbers per hundred miles of driving. If you know your own MPG and your own prices, the same two divisions give your own figures: fuel price per gallon divided by MPG, and electricity price per kWh divided by miles per kWh.

should i buy an ev or petrol car: Break-even: how many miles before the EV pays back the price gap

A break-even figure needs one more assumption: the price difference between the two cars. Suppose, as an example only, the EV costs $12,000.00 more than the comparable petrol car. The fuel and charging saving under this page's assumptions is $0.04 per mile, so the break-even distance is $12,000.00 ÷ $0.04 = 274,802 miles. At 10,000 miles a year that is about 27.5 years of driving. That figure counts fuel and charging only. It excludes insurance, depreciation, servicing differences, tyre wear, tax and any grants or incentives — name them, do not hide them — and each one moves the real break-even point. Depreciation alone can outweigh years of fuel savings, and insurance can differ enough to matter. A break-even number that hides those costs is not a like-for-like comparison, so use this page's figure as a starting point and add the excluded items with stated assumptions before deciding anything.

What the comparison leaves out — and why it matters for should i buy an ev or petrol car

Taking should i buy an ev or petrol car as the case in point: The fuel and charging maths on this page is only one slice of the real cost. Left out, by design, are the purchase-price gap between the two cars, depreciation, insurance, servicing, tyre wear and tax — every one of them differs between an EV and a petrol car and every one of them moves the true answer. The page also prices electricity at a single rate, while real life mixes cheap overnight home charging with dear public rapid charging; that mix usually matters more than the headline rate. Price your own mix honestly before deciding which car is cheaper for you.

should i buy an ev or petrol car: Common mistakes

  • On this page, should i buy an ev or petrol car is the case in point: Mixing US and imperial gallons is the biggest, because it shifts the answer by about 20 percent with no warning. A UK imperial gallon is about 4.55 litres; a US gallon is about 3.79 litres, so the same car has a higher MPG figure in imperial gallons than in US gallons.
  • Taking should i buy an ev or petrol car as the case in point: Treating the example prices on this page as current prices is the last mistake. Fuel and electricity prices move constantly, so always substitute the prices you actually pay before trusting any saving.
  • On this page, should i buy an ev or petrol car is the case in point: Using the same electricity price for every charge hides the real cost. A driver who mixes home and public charging has two different prices, and averaging them into one number without weighting by how much energy comes from each is a common error.
  • For should i buy an ev or petrol car, the point is this: Using one tank or one charge instead of an average is next — a single fill is noisy, and a pump that clicks off early can add a phantom gallon, while a single charge says nothing about your usual mix of home and public charging.

should i buy an ev or petrol car: Related calculations

For should i buy an ev or petrol car, the point is this: These pages continue from here: the same method on a neighbouring question, or the calculator that runs the arithmetic for you.

Putting your figure to work for should i buy an ev or petrol car

On this page, should i buy an ev or petrol car is the case in point: The method earns its keep when you run it on your own numbers. Measure your real cost per mile for each car — your prices, your efficiency, your charging mix — and you can price a journey before you leave and compare two cars on equal terms instead of brochure claims. Keep a small log of date, miles, energy and price, and look at it monthly: a winter efficiency dip, a change in your charging mix and a slow cost creep all show up fast, and each one is something you can act on.

Frequently asked questions

Does an EV cost more to insure?

On this page, should i buy an ev or petrol car is the case in point: Insurance is not part of the fuel-and-charging maths on this page, and it can differ between an EV and a petrol car for reasons that have nothing to do with energy: repair costs, parts availability and the car's value all play a part. Get a quote for both cars before comparing total costs.

How many kWh does it take to charge an EV?

For should i buy an ev or petrol car, the point is this: It depends on the battery size and how empty it is. A charge from near-empty to full takes roughly the battery's capacity in kWh, plus a few percent for charging losses. Divide the kWh drawn by the miles driven afterwards and you have the car's real miles per kWh for that period.

How do I work out my own EV versus petrol saving?

For should i buy an ev or petrol car, the point is this: Use the same mileage for both cars. For the petrol car, divide the fuel price per gallon by its MPG to get cost per mile. For the EV, divide the electricity price per kWh by its miles per kWh. Multiply each by your annual miles and subtract. Use your own prices — especially your real charging mix of home and public charging — rather than the example prices on this page.

What is excluded from the EV versus petrol comparison?

On this page, should i buy an ev or petrol car is the case in point: Purchase price, depreciation, insurance, servicing, tyre wear and tax. This page compares fuel and charging costs only, and says so, because a break-even figure that hides those costs is not a like-for-like comparison. Add them with stated assumptions if you want a total-cost answer.

Can I use the same method for a diesel car?

For should i buy an ev or petrol car, the point is this: Yes. A diesel car uses the same petrol-side formula — fuel price per gallon divided by MPG — with diesel prices and diesel MPG. The EV side is unchanged. Compare the two cost-per-mile figures the same way, and remember the same exclusions apply.