EV energy cost per 100 km:
formula and worked example

Why running cost is quoted per 100 km

Electric car efficiency is usually stated as kWh used per 100 km, which is the reverse of the miles-per-gallon habit many drivers know. Turning that figure into money gives a number that sits directly beside a petrol or diesel car's cost for the same distance, and it is the cleanest way to compare running costs between different cars.

The value changes with where and how you charge. Home charging, a workplace socket and a public fast charger can differ greatly in price per kWh, so the same car can have very different per-100-km costs depending on your routine.

There is a second reason the metric is useful: it separates the car from the price. You can hold consumption fixed and ask how a change of tariff plan alters the bill, or hold the tariff fixed and compare two cars. Either way you are changing one number at a time.

The formula and each input

• Consumption: energy used per 100 km. Check whether the figure is measured at the battery or at the wall.
• Tariff: the price you pay for each kWh you buy, averaged across your typical charging mix.
• Result: currency per 100 km. Divide by 100 to get cost per kilometre.

Note that a car showing 160 Wh per km is the same as 16 kWh per 100 km. Convert before multiplying.

If your tariff has time bands, use a weighted average. For example, if three quarters of your charging is overnight at a lower rate and a quarter is during the day at a higher rate, weight accordingly rather than quoting the cheapest band.

Worked example: 16 kWh/100 km at 9 per kWh

An owner drives a car that averages 16 kWh per 100 km and charges at home at 9 currency units per kWh.

1. 1Consumption: 16 kWh per 100 km.
2. 2Tariff: 9 per kWh.
3. 3Multiply: 16 × 9 = 144.
4. 4The cost is 144 per 100 km, or 1.44 per kilometre.

For a petrol car doing 15 km per litre with fuel at 105 per litre, 100 km costs about 700. Against that, the electric figure is roughly a fifth, so the saving per 100 km is substantial, even before maintenance differences.

At 1.44 per kilometre, a 12,000 km year costs about 17,280 in electricity in this example. The same distance on petrol at 7 per kilometre is 84,000, so the gap is large enough that even a heavier tariff or a rougher winter leaves electric ahead.

Reading the result in context

The per-100-km cost is only a running figure. It leaves out the purchase price, insurance, battery ageing and any home charger installation. It is also sensitive to which tariff applies at the time of charging.

• Home off-peak charging gives the lowest figure for most owners.
• Public fast charging often costs several times more per kWh, so trips that rely on it cost far more than the home-based number.
• Cold weather, high speeds and heavy loads raise consumption, so measure your own average over several months.

If you want an all-in comparison, add the cost of ownership items per kilometre and compare the totals. For running cost alone, the per-100-km figure is a fair, easy comparison.

How consumption and tariff move the cost

A 10% increase in consumption raises the cost from 144 to 158.4. Tariff has the same proportional effect: every extra 1 per kWh adds 16 to the cost per 100 km at this consumption. Your habits and your price plan are the two levers worth working on.

Interestingly, driving more gently often helps more than shopping for a cheaper tariff. Motorway speeds drive consumption up sharply because air resistance rises with the square of speed, so a calmer pace on long runs can move the figure by more than a small tariff change would.

Common errors with EV cost figures

• Using the rated consumption from the brochure instead of your own long-term average.
• Ignoring charging losses between the wall and the battery, which can add a noticeable share to the energy bought.
• Applying the home tariff to every trip when part of your charging is on a public network.
• Mixing Wh per km with kWh per 100 km, which creates a tenfold error.
• Leaving out fixed charges and demand components of your electricity plan.

Most of these errors make the electric option look better than it will turn out to be, so a cautious figure is the safer one to plan with.

A quick way to check your own figure

1. 1Note the car's consumption reading over a few hundred kilometres.
2. 2Look at your actual rate per kWh, including any taxes.
3. 3Multiply, then cross-check against a month of charging bills.
4. 4Re-run it for public charging to see the worst case.

Repeat the calculation for each charging situation you have: home, workplace and public. Together they give a realistic blended figure, which is the one worth comparing with petrol.