EV Battery Size Calculator: The Formula, a Worked Example and Tables
There is no single number that answers ev battery size calculator, because the answer moves with four inputs: the price of petrol, the petrol car's MPG, the price of electricity and the EV's miles per kWh. Change any one and the result changes, sometimes by enough to flip the conclusion. That is why this page shows its working instead of handing you a verdict: the formula, a worked example with stated assumptions, and tables at different mileages.
On this page, ev battery size calculator is the case in point: 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 $3.85 per US gallon, 35 US MPG, electricity at 25 cents per kWh and 3.8 miles per kWh — the petrol car costs about $0.11 per mile and the EV about $0.07 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.
Assumptions used on this page for ev battery size calculator
For ev battery size calculator, 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 | $3.85 per US gallon | Example price |
| Petrol efficiency | 35 US MPG | Assumption |
| Electricity price | 25 cents per kWh | Example price |
| EV efficiency | 3.8 miles per kWh | Assumption |
| Annual mileage | 12,000 miles | Example |
ev battery size calculator: How the EV versus petrol comparison works
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. That is the point this page works through for ev battery size calculator.
ev battery size calculator: Worked example: the same distance, two energy bills
Take a driver covering 12,000 miles a year. The petrol car does 35 MPG and petrol costs $3.85 per gallon as an example price, so its cost per mile is $3.85 ÷ 35 = $0.11, and 12,000 miles cost $1,320.00 a year. The EV does 3.8 miles per kWh and electricity costs 25 cents per kWh as an example price, so its cost per mile is $0.25 ÷ 3.8 = $0.07, and 12,000 miles cost $789.47 a year. The difference on fuel and charging alone is $530.53 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.
ev battery size calculator: Cost at different annual mileages
On this page, ev battery size calculator is the case in point: The table uses the worked-example assumptions above — petrol at $3.85 per gallon (example), 35 MPG, electricity at 25 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 | $660.00 | $394.74 | $265.26 |
| 8,000 | $880.00 | $526.32 | $353.68 |
| 10,000 | $1,100.00 | $657.89 | $442.11 |
| 12,000 | $1,320.00 | $789.47 | $530.53 |
| 15,000 | $1,650.00 | $986.84 | $663.16 |
| 20,000 | $2,200.00 | $1,315.79 | $884.21 |
ev battery size calculator: Cost per mile and per 100 miles
Taking ev battery size calculator as the case in point: At the example assumptions, the petrol car costs $0.11 per mile ($11.00 per 100 miles) and the EV costs $0.07 per mile ($6.58 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.
ev battery size calculator: Charging costs: home, public and rapid
Charging cost depends on where the electricity comes from, and the spread is wide. Home charging at the example rate of 25 cents per kWh is the cheapest common option; public rapid charging can cost two to three times as much per kWh, and that difference matters more than the car's efficiency. As an example, filling a 60 kWh battery from empty at the home rate used here costs 60 × $0.25 = $15.00; the same charge at double the rate costs $30.00. In practice a battery is rarely charged from empty, and charging losses mean slightly more electricity is drawn from the plug than the battery stores — typically a few percent — so treat the fill-up figure as approximate. Work out your own mix of home and public charging, price each part separately, and only then compare the totals. A driver who charges at home overnight will beat the figures on this page; a driver who relies on rapid chargers may not.
ev battery size calculator: What the comparison leaves out — and why it matters
On this page, ev battery size calculator is the case in point: Treat the saving on this page as a fuel-and-charging figure, not a total-cost verdict. It leaves out the price difference between the cars, depreciation, insurance, servicing, tyres and tax, and it assumes one electricity price when real charging is a blend of cheap home charging and expensive public rapid charging. A driver who charges at home will do better than these figures; a driver who lives on rapid chargers may do worse. Add the excluded items with stated assumptions if you want the full comparison.
Common mistakes for ev battery size calculator
- 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. That is the point this page works through for ev battery size calculator.
- Taking ev battery size calculator as the case in point: Forgetting charging losses is another: slightly more electricity is drawn from the plug than the battery stores, so dividing the battery size by the miles driven understates the real cost per mile by a few percent.
- On this page, ev battery size calculator is the case in point: Comparing a laboratory test figure against a real-world figure tells you nothing about the car and everything about the methods. Keep the method constant — real-world against real-world — and the comparisons become meaningful.
- On this page, ev battery size calculator is the case in point: Forgetting that winter changes EV efficiency is easy to miss. Cold weather reduces an EV's miles per kWh noticeably, so a saving calculated on summer figures will overstate the winter result.
ev battery size calculator: Related calculations
On this page, ev battery size calculator is the case in point: If this page answered your question, these pages take the next step — the same method applied to a closely related question, or the calculator that does the arithmetic for you.
- EV Depreciation vs Petrol
- EV vs Petrol Cost Over 5 Years
- EV vs Diesel Cost Calculator
- EV Charging Cost vs Petrol Calculator
- Use the calculator
- Browse all ev vs petrol pages
Putting your figure to work for ev battery size calculator
Use the result as a tool, not a verdict. Re-run the two divisions with your own prices and efficiencies whenever they change, and the comparison stays honest. A monthly log — miles, energy used, price paid — is enough to spot the patterns that matter: colder months cost more per mile for an EV, a new charging habit shifts the average, and a petrol car driven more in town drifts the other way. Measured figures beat remembered ones, and the method on this page is built for measured figures. That is the point this page works through for ev battery size calculator.
Frequently asked questions
What affects an EV's miles per kWh?
Driving speed, temperature, heating and air conditioning, tyre pressures and how much town versus motorway driving you do. Cold weather and high speeds reduce miles per kWh, while gentle town driving usually improves it. Measure yours over several weeks rather than trusting a single trip. That is the point this page works through for ev battery size calculator.
Does an EV cost more to insure?
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. That is the point this page works through for ev battery size calculator.
How sensitive is the saving to the electricity price?
On this page, ev battery size calculator is the case in point: Very sensitive. Doubling the electricity price doubles the EV's charging cost per mile, while the petrol side is unchanged. That is why this page states its example prices openly: change the electricity price to the one you actually pay and the saving changes with it, sometimes by enough to change the conclusion.
Can I use the same method for a diesel car?
On this page, ev battery size calculator is the case in point: 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.
Does charging at home really make that much difference?
On this page, ev battery size calculator is the case in point: Yes, usually more than any other single factor. Home charging is typically far cheaper per kWh than public rapid charging, so a driver who charges at home overnight will see a much bigger saving than one who relies on rapid chargers. Work out your own mix of home and public charging and price each part separately for an honest figure.