Electric car range:
how far a battery really takes you

Range is energy divided by appetite

A battery is a tank, and the car's consumption is how fast it drains. If you know how many kilowatt-hours the car can actually use and how many it takes to cover a kilometre, the distance is a single division.

Two inputs are commonly mixed up. The headline battery capacity includes a reserve the car keeps back to protect the cells and the buffer at the very top and bottom, so the usable figure is smaller. A nominal 54 kWh pack with 4 kWh held back gives 50 kWh to drive on.

The same division works in reverse for planning. To cover 200 km at 0.16 kWh per km you need 32 kWh in the battery, so on a 50 kWh pack you could start the journey at about 64% charge and still arrive with nothing in hand. Add your reserve on top of that.

The range formula

Some cars display Wh/km, which is the same number multiplied by 1,000. A reading of 160 Wh/km is 0.16 kWh/km.

Estimates are only as good as the consumption figure you choose. Use the trip computer average from a similar drive in similar weather if you can. A value from a calm 40 km/h commute is very different from one from a 110 km/h motorway run, and the same car can show both in a single week.

Worked example: a 50 kWh car

Planners rarely drive to zero. If you keep a 10% reserve on arrival, the usable stretch between charges is about 281 km from a full pack, which is 312.5 × 0.9.

Reading consumption figures without confusion

Cars quote consumption in several ways, and mixing them is the commonest source of error. Kilowatt-hours per 100 km is the usual dashboard unit in many markets. Wh per km is the same number times ten, and miles per kWh runs the other way round, where a bigger number means better efficiency.

The three entries in the table describe the same car. The final row can be used directly: with 50 kWh and 6.25 km per kWh, the range is 50 × 6.25 = 312.5 km, which is the identical answer from a multiplication instead of a division.

Conditions that move consumption

Aerodynamic drag grows with the square of speed, so the jump from 80 to 120 km/h costs far more than the 50% extra speed suggests. Heating and cooling the cabin, carrying heavy loads, low tyre pressure and steep climbs all push consumption up. Regenerative braking in stop-and-go traffic can pull it down.

Tyres, wheels and roof boxes matter more than people expect. A roof carrier can add a noticeable amount of drag at highway speed, so if you rarely use it, take it off. Regenerative braking settings change how much energy returns on descents and in traffic, which you may notice as a more favourable figure on hilly roads that end lower than they began.

Battery age and charge limits

Over years of use a battery loses some capacity, so the usable energy is below the new-car figure. A pack that has settled to 90% of its original capacity turns a 50 kWh car into a 45 kWh one: 45 ÷ 0.16 ≈ 281 km. Where a car shows a state-of-health figure, use it in the formula.

Daily charge limits of 80% protect the cells, and they also scale the range proportionally. Treat the full-pack number as what is available on a trip day rather than every day.

Temperature also changes the usable energy in the short term. A cold battery delivers less than a warm one, so the figure available on a winter morning can be several percent below the same pack in mild weather. Pre-conditioning while connected to a charger helps recover it.

Planning a trip around the number

1. 1Choose consumption for the speed and weather you expect, not the label figure.
2. 2Multiply usable energy by the state of charge you will really start with.
3. 3Subtract a reserve so you arrive with a margin, commonly 10% or more.
4. 4Divide by consumption to get kilometres, then compare with the distance between chargers.
5. 5Allow extra on hilly or very cold legs.

Keep a record of arrival percentages on your regular routes. After a few trips you will know your own safe range for each, which is worth more than any single calculation.

Range and the next charging stop

Charge speed tails off as the battery fills, so the most time-efficient plan is usually to stop at 10–20% and charge to around 80% rather than to 100%. With 50 kWh usable and 0.16 kWh per km, the 10% to 80% window is 35 kWh, which is about 219 km of driving between stops.

The plan changes with the charger network. On a corridor with chargers every 100 km, a short range is a minor annoyance. In a region where the next charger is 250 km away, an extra 30 km of range is the difference between reaching it and not. Check the chargers on the route and their status before you rely on the figure.