How Many kWh to Charge an EV: A Step-by-Step Worked Example

If you want a straight answer on how many kwh to charge an ev, 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 $3.60 per US gallon, 30 US MPG, electricity at 25 cents per kWh and 3.5 miles per kWh — the petrol car costs about $0.12 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.

how many kwh to charge an ev: Assumptions used on this page

On this page, how many kwh to charge an ev is the case in point: 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 $3.60 per US gallon Example price
Petrol efficiency 30 US MPG Assumption
Electricity price 25 cents per kWh Example price
EV efficiency 3.5 miles per kWh Assumption
Annual mileage 12,000 miles Example

how many kwh to charge an ev: 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 how many kwh to charge an ev.

how many kwh to charge an ev: Worked example: the same distance, two energy bills

Take a driver covering 12,000 miles a year. The petrol car does 30 MPG and petrol costs $3.60 per gallon as an example price, so its cost per mile is $3.60 ÷ 30 = $0.12, and 12,000 miles cost $1,440.00 a year. The EV does 3.5 miles per kWh and electricity costs 25 cents per kWh as an example price, so its cost per mile is $0.25 ÷ 3.5 = $0.07, and 12,000 miles cost $857.14 a year. The difference on fuel and charging alone is $582.86 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 how many kwh to charge an ev

The table uses the worked-example assumptions above — petrol at $3.60 per gallon (example), 30 MPG, electricity at 25 cents per kWh (example), 3.5 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 $720.00 $428.57 $291.43
8,000 $960.00 $571.43 $388.57
10,000 $1,200.00 $714.29 $485.71
12,000 $1,440.00 $857.14 $582.86
15,000 $1,800.00 $1,071.43 $728.57
20,000 $2,400.00 $1,428.57 $971.43

how many kwh to charge an ev: Cost per mile and per 100 miles

At the example assumptions, the petrol car costs $0.12 per mile ($12.00 per 100 miles) and the EV costs $0.07 per mile ($7.14 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.

Charging costs: home, public and rapid for how many kwh to charge an ev

On this page, how many kwh to charge an ev is the case in point: 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 75 kWh battery from empty at the home rate used here costs 75 × $0.25 = $18.75; the same charge at double the rate costs $37.50. 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.

What the comparison leaves out — and why it matters for how many kwh to charge an ev

On this page, how many kwh to charge an ev is 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.

Common mistakes for how many kwh to charge an ev

  • For how many kwh to charge an ev, 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.
  • 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 how many kwh to charge an ev.
  • 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. That is the point this page works through for how many kwh to charge an ev.
  • Taking how many kwh to charge an ev as 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.

how many kwh to charge an ev: Related calculations

For how many kwh to charge an ev, the point is this: For the next step, these pages apply the same method to a closely related question — or hand the arithmetic to the calculator.

how many kwh to charge an ev: Putting your figure to work

Taking how many kwh to charge an ev as the case in point: Putting the figure to work is where the method pays off. Once you know your own cost per mile for each car, measured with your own prices and efficiencies, you can price any journey before you set off and compare cars honestly, because you are comparing measured figures rather than brochure claims. Keep a simple log — date, miles, energy used, price paid — and review it monthly. Patterns show up quickly: a winter dip in EV efficiency, a motorway improvement, a slow rise in your charging cost that points to more public charging than you planned. The log turns a vague feeling about fuel use into a number you can act on, and that is the whole point of measuring in the first place.

Frequently asked questions

Is an EV always cheaper to run than a petrol car?

On this page, how many kwh to charge an ev is the case in point: Not always. On fuel and charging alone, an EV is usually cheaper per mile when it charges mostly at home at an example electricity price, but the answer flips if it relies on expensive public rapid charging or if the petrol car is very efficient. Purchase price, depreciation, insurance and servicing are not in the fuel-cost maths, so a full comparison has to include them before calling either car cheaper overall.

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

On this page, how many kwh to charge an ev is the case in point: 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 does miles per kWh mean?

For how many kwh to charge an ev, the point is this: It is the EV equivalent of MPG: how many miles the car travels on one kilowatt-hour of electricity. Higher is more efficient. Divide the electricity price per kWh by miles per kWh to get the EV's cost per mile, in the same way you divide the fuel price per gallon by MPG for a petrol car.

How sensitive is the saving to the electricity price?

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. That is the point this page works through for how many kwh to charge an ev.

Does an EV cost more to insure?

For how many kwh to charge an ev, the point is this: 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.