Convert Wh/km to Miles per kWh
An electric car using 150 Wh/km is doing 4.14 miles per kWh. Both numbers describe the same car; they are simply the two conventions the world uses for the same fact. European dashboards show watt-hours per kilometre, British and American drivers talk in miles per kilowatt-hour, and — as with fuel economy — one goes up as the other goes down.
To convert Wh/km to miles per kWh, divide 621.371 by the Wh/km figure. So 150 Wh/km is 4.14 mi/kWh and 200 Wh/km is 3.11 mi/kWh. The relationship is its own reverse: 4 mi/kWh is 155 Wh/km.
mi/kWh = 621.371 ÷ (Wh/km), and equally Wh/km = 621.371 ÷ (mi/kWh). The constant is a kilowatt-hour’s 1,000 watt-hours divided by the 1.609344 kilometres in a mile — both exact by definition, so the conversion carries no uncertainty. Because it is a division, the same formula works in both directions.
| Wh/km | mi/kWh | Roughly |
|---|---|---|
| 120 | 5.18 | Very efficient small EV |
| 150 | 4.14 | Efficient family EV |
| 180 | 3.45 | Typical mid-size |
| 200 | 3.11 | Larger EV or cold weather |
| 250 | 2.49 | Large SUV, towing, motorway |
| 300 | 2.07 | Van, or severe conditions |
How to convert between the two
Divide 621.371 by whichever figure you have. The useful mental shortcut is that a car at 200 Wh/km does about 3.1 miles per kWh, and everything scales inversely from there — halve the consumption and you double the range per kilowatt-hour.
The direction check is the same one that catches fuel-economy mistakes: a lower Wh/km must give a higher mi/kWh. If both numbers move the same way, the calculation has been inverted.
There are three conventions, not two
This is the part that causes genuine confusion, because the third one is the reverse of the reverse.
| Convention | Form | Where | Better is |
|---|---|---|---|
| Wh/km | Energy per distance | Most of Europe; the car’s own display | Lower |
| mi/kWh | Distance per energy | UK and US drivers, reviews, forums | Higher |
| kWh/100 mi | Energy per distance | US EPA window sticker | Lower |
Wh/km and kWh/100mi are linear in energy: a ten per cent improvement saves ten per cent of the electricity, wherever you start. Miles per kilowatt-hour is not — going from 2 to 3 mi/kWh saves far more energy than going from 5 to 6, even though both look like a gain of one. This is the same asymmetry that makes L/100km a better metric than MPG, and it is why regulators and engineers use the energy-per-distance forms while drivers prefer the other one.
What actually moves the number
- Speed — the dominant factor. Motorway cruising can push consumption fifty per cent above an urban figure, because drag rises with the square of speed.
- Temperature — cold weather hurts twice, through battery chemistry and cabin heating. A winter figure 30% worse than a summer one is unremarkable.
- Charging losses — the car’s display usually reports energy drawn from the battery, while your electricity bill counts energy put into it. The gap is roughly a tenth, and it is the most common reason a calculated cost per mile comes out low.
If you are converting an efficiency figure in order to work out running costs or emissions, use the figure from the charger or the meter, not the one on the dashboard. Charging is around 85–95% efficient depending on the charger and conditions, so the electricity you pay for is meaningfully more than the electricity the car reports using. Neither number is wrong; they are measuring at different points.
An EV’s emissions depend entirely on the grid it charges from — the same car is far cleaner in France than in Poland. For that, see kWh to kg CO₂ by country. For a full comparison against a combustion car including manufacturing, the EV versus petrol lifecycle calculator and its methodology.
For the combustion equivalent of this page, MPG to L/100km. For fuel economy straight to emissions, MPG to gCO₂e per mile. For the electricity itself, kWh to MJ.
Frequently Asked Questions
Divide 621.371 by the Wh/km figure. So 150 Wh/km is 4.14 mi/kWh. Because it is a division, the same formula works in reverse: 621.371 divided by a mi/kWh figure gives Wh/km.
Around 150 Wh/km is efficient for a family car, which is about 4.1 miles per kWh. Small efficient EVs manage nearer 120, larger SUVs and vans sit at 250 to 300. Motorway driving and cold weather both push the figure up substantially.
Because they are reciprocals — one measures energy per distance and the other distance per energy. Lower Wh/km always means higher mi/kWh. If both move the same way in your calculation, it has been inverted.
It is the third convention, used on US window stickers, and it is energy per distance like Wh/km rather than distance per energy. Because it faces the same way as Wh/km, lower is better on both. Divide 100 by a mi/kWh figure to get it — 4 mi/kWh is 25 kWh per 100 miles.
Energy per distance — Wh/km or kWh/100mi — if you are doing arithmetic with it, because equal improvements represent equal energy savings. Miles per kilowatt-hour is more intuitive for judging range, which is why drivers prefer it. Just state which you are using: a number without its convention is ambiguous.
Mostly speed and temperature. Drag rises with the square of speed, so motorway cruising is far more demanding than the test cycle, and cold weather costs energy twice over through battery chemistry and cabin heating. A winter motorway figure thirty per cent worse than the official one is entirely normal.
No — the display usually reports energy taken from the battery, while your bill counts energy put into it, and charging is around 85 to 95 per cent efficient. If you are working out cost or emissions per mile, use the charger or meter reading rather than the dashboard, or you will understate both by roughly a tenth.
No — it converts efficiency between two conventions. An EV’s emissions depend on the grid it charges from, which varies by more than tenfold between countries, so the same car can be very clean or fairly ordinary depending entirely on where it is plugged in.