The Nissan Ariya carries 87 kWh of usable battery and covers about 402 km. The Tesla Model 3 carries 79 kWh and covers about 555 km. Eight kilowatt-hours less battery, a hundred and fifty kilometres more range. The whole difference is efficiency — 216 Wh/km against 142 — and it is the least-shopped figure on a spec sheet.
The three units, and how to stop being confused by them
Efficiency is quoted three ways and one of them points the wrong direction, which is a reliable source of mistakes.
| Unit | Used in | Better is | Example: Model 3 |
|---|---|---|---|
| Wh/km — watt-hours per kilometre | Europe | Lower | 142 Wh/km |
| kWh/100 mi — kilowatt-hours per hundred miles | United States (EPA) | Lower | 26.4 kWh/100 mi |
| mi/kWh or km/kWh | Informal, car displays | Higher | ≈3.8 mi/kWh |
To convert roughly: divide Wh/km by 6.2 to get kWh/100 mi, or multiply kWh/100 mi by 6.2 to go back. Precisely, one mile is 1.60934 km, so kWh/100 mi = Wh/km × 0.160934.
Or type any figure into the converter below. It converts units only: the Model 3 rows above are a WLTP result (142 Wh/km) and an EPA result (26.4 kWh/100 mi), two different tests, so they do not convert into each other. For the same cars on both tests side by side, see EPA vs WLTP range.
| Wh/km | kWh/100 km | mi/kWh | kWh/100 mi | Wh/mi |
|---|---|---|---|---|
| 160 | 16 | 3.88 | 25.7 | 257 |
Why efficiency beats capacity
Range is capacity multiplied by efficiency. There are only two ways to build a long-range electric car — fit more battery, or use less energy — and they are not equally good.
Adding battery adds weight, cost, charging time to reach a given number of kilometres, embedded carbon, and eventual replacement cost. Improving efficiency adds none of those. It also compounds: a more efficient car needs a smaller pack for the same range, and a smaller pack is lighter, which makes it more efficient again.
The pattern is visible right through our efficiency ranking: the cars near the top are not the ones with the biggest batteries.
What it costs you, in money
The Ariya-versus-Model 3 gap is 74 Wh/km. At 15,000 km a year that is 1,110 kWh — about 1.1 MWh of electricity, every year, for the life of the car. Whatever your tariff, multiply by it and then by the number of years you expect to keep the car.
A smaller and more typical gap makes the point better. Twenty Wh/km at 15,000 km a year is 300 kWh — not dramatic in one year, and a substantial sum across seven or eight. The running cost calculator on this site takes your own mileage and tariff and shows the total; every assumption in it is visible and editable, because a calculator with hidden multipliers is not a calculator.
What actually makes a car efficient
Shape, above about 80 km/h. Aerodynamic drag dominates at motorway speed and scales with frontal area and drag coefficient. This is the biggest single factor and the one buyers have least intuition for.
The clearest illustration in our database is a pair that shares everything mechanical. The Hyundai Ioniq 6 and Ioniq 5 use the same 800-volt platform, the same 168 kW motor and near-identical packs. The Ioniq 6 is a low saloon and returns 149 Wh/km. The Ioniq 5 is a tall hatchback and returns 178. Twenty-nine watt-hours per kilometre, bought entirely with a roofline.
Weight, below about 60 km/h. In town you spend energy accelerating mass and recover some of it braking. Regeneration is good but not free — a round trip through the motor and battery loses something each way.
Tyres and wheels. Rolling resistance and the aerodynamic penalty of a large wheel are why the same car is often rated differently on 18-inch and 21-inch wheels. Where we hold a figure for a specific wheel size, the source credit says so.
The heat pump. In cold weather, cabin heating is drawn from the traction battery — there is no waste engine heat to use. A heat pump moves heat rather than generating it and cuts that penalty substantially, which is why it matters far more in Oslo than in Seville.
Where the published figures come from
The Wh/km figures on this site are EV Database’s real-world consumption estimates, not the WLTP laboratory value — the observed range we publish for each car is simply its usable capacity divided by this number. Your own figure will still differ, because speed, temperature and terrain move consumption more than the car does. The WLTP rating is the lower, kinder number, and it is the one that appears in manufacturer brochures.
One thing to watch: some manufacturers quote consumption including charging losses and some do not, which can shift a figure by several per cent. Where the source distinguishes them, ours does too.
Frequently asked questions
Is a lower Wh/km always better?
For cost and range, yes. But efficiency is largely bought with a low, narrow, slippery body, so the most efficient cars are also the least practical for carrying people and things. That is a real trade rather than a flaw in the metric — decide which you are optimising for.
Why is my car’s display better than the official figure?
Usually because the display measures energy leaving the battery while the official figure includes charging losses, and because dashboard trip computers often reset in ways that flatter short, warm, gentle journeys. Compare like with like over several thousand kilometres before concluding anything.
Does efficiency change as the battery ages?
Consumption per kilometre stays broadly the same; what falls is the capacity available, so range drops while Wh/km does not. Our vehicle pages carry an eight-year capacity projection, clearly labelled as an estimate because no manufacturer publishes per-model degradation data.
Which is the most efficient car you hold data for?
The rear-drive Tesla Model 3 at 133 Wh/km, with the Long Range AWD at 142 and the Hyundai Ioniq 6 at 149 close behind. The full order, updated whenever a figure changes, is on most efficient electric cars.
Sources
- Consumption and range figures with their individual sources: the vehicle pages on this site, each credited with the source and the date read.
- US EPA / fueleconomy.gov — electric vehicle consumption ratings and the kWh/100 mi convention