SnapEnergyLab
EV charging

kWh/100 km, mi/kWh and Wh/mi

Three units, three regions, one physical quantity — and one of them runs the wrong way round.

Published 23 August 2026 · Published by SnapEnergyLab. Methodology and default assumptions are documented separately.

An electric estate car driving along a wet motorway through bleak hills on a cold grey afternoon

Quick answer

Three units describe one quantity — energy per distance — and one of them runs backwards. kWh/100 km and Wh/mi are consumption: lower is better. Miles per kWh is efficiency: higher is better.

Mixing them is how people conclude that a car is 20% more efficient when it is 20% worse. Convert to a single unit before comparing anything.

How to read the numbersYour inputCalculated result
18 kWh/100 km
equals
Your input
miles per kWh
3.45 mi/kWh
Calculated result
watt-hours per mile
290 Wh/mi
Calculated result

The conversions

  1. 1 mile = 1.609344 km
  3. Wh/mi = kWh/100 km × 10 × 1.609344
  4. mi/kWh = 1000 ÷ Wh/mi
  5. kWh/100 km = Wh/mi ÷ 16.09344
  6. kWh/100 mi = Wh/mi ÷ 10

The inversion is the trap: doubling kWh/100 km halves mi/kWh. The two scales cannot be compared by subtracting percentages.

Equivalent values across EV efficiency units
kWh/100 kmWh/mimi/kWhTypical of
121935.18Small efficient EV, gentle driving
142254.44Compact EV, mixed use
162573.88Compact EV, typical
182903.45Mid-size EV, typical
203223.11Mid-size EV, motorway
223542.82Large SUV EV
254022.49Large SUV, motorway or winter
304832.07Towing, or severe cold

Pack energy, not wall energy

Every figure above describes energy leaving the battery. The energy you buy is larger, because charging losses of 8–12% sit between your meter and the pack. A car consuming 18 kWh/100 km costs you about 20 kWh/100 km at the wall.

This is why a manufacturer’s consumption figure and your own metered cost per kilometre never quite agree even when driving conditions match the test perfectly.

Worked example: cost per 100 km, honestly

  1. Vehicle consumption18 kWh/100 kmYour input
  2. Charging efficiency90%Site default
  3. Energy at the wall18 ÷ 0.90 = 20 kWhCalculated result
  4. Electricity price0.30 per kWhSite default
  5. Cost per 100 km20 × 0.30 = 6.00Calculated result
  6. Cost per km0.060Calculated result
  7. Cost per mile0.060 × 1.609 = 0.097Calculated result
Cost per 100 km driven6.00

What moves real consumption

Speed. Aerodynamic drag rises with the square of speed and the power to overcome it with the cube. Sustained motorway driving can add 25–40% over the same car in town — the opposite of a combustion car’s pattern.

Temperature. Cabin heating draws from the same pack. In sub-zero conditions, consumption 20–40% above the rating is normal, and short trips are worst because the car heats up repeatedly.

Terrain and load. Climbing costs energy that regeneration returns only partly, typically 60–70% of it. Roof boxes and trailers hit aerodynamics hard.

Tyres and pressure. Winter tyres and under-inflation each cost a few percent, and they coincide with the coldest months.

Turn your own efficiency into a monthly figure

Enter your consumption, mileage, charging efficiency and electricity price to get cost per charge, per distance and per month with the wall-versus-pack distinction handled explicitly.

Open the home charging cost calculator

Assumptions and limitations

  • The 'typical of' column describes broad vehicle classes, not measurements of specific models. Use your own trip computer's long-term average where you have one.
  • Official WLTP and EPA figures are measured on standardised cycles that no real journey matches. Treat them as a comparison basis between cars, not as a prediction of your consumption.
  • Conversions in the table are exact arithmetic; the uncertainty lies entirely in which consumption figure you feed them.

Sources

  1. Worldwide Harmonised Light Vehicle Test Procedure (WLTP) — UNECE

    Supports: Standardised measurement of electric energy consumption in Wh/km for light vehicles.

  2. Fuel economy and environment labels for electric vehicles — U.S. Environmental Protection Agency

    Supports: US labelling uses kWh per 100 miles and MPGe rather than kWh per 100 km.

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