How is driving range measured for electric cars?

Fuel consumption figures play an important role for buyers when choosing a new car.

Obviously, a lower consumption figure means the car is cheaper to run and produces fewer toxic emissions. But it doesn’t clearly explain how far you can travel between refills.

It’s the opposite for those looking to buy an electric car, which are defined by their driving range – the distance it can cover on a full battery charge.

So, how do they come up with those numbers?

All light passenger vehicles (classified as those that weigh less than 3.5 tonnes) sold in Australia must currently comply to the Australian Design Rule 81/02.

This requires them to meet the UN ECE 101 regulation that uses the outdated New European Driving Cycle (NEDC) and display both fuel consumption (measured in L/100km) and CO2 output (measured in g/km) on a windscreen sticker and all marketing communications.

However, you may also notice car-makers are using what is called WLTP consumption figures in brochures and on their websites. And in our reviews on carsales…

The Worldwide Harmonised Light Vehicle Test Procedure (WLTP) replaced the NEDC test when it was introduced in 2018 and is designed to better replicate real-world driving conditions.

For electric vehicles, this calculates the vehicle’s driving range (displayed in km) and energy consumption (measured in kilowatt hours per 100km) with a unique variation of the testing program compared to vehicles with an internal combustion engine.

In all cases – no matter what type of engine the vehicle uses – the majority of the WLTP test program is conducted on a dynamometer in a laboratory, and car-makers are required to supply a fleet of vehicles (including examples of the lightest and heaviest specifications) to ensure consistent results.

The lab environment is heated to 23 degrees Celsius – which is considered the optimum temperature for batteries to work at – and each car is subjected to a pre-programmed driving test on the rolling road without using any auxiliary functions such as air-conditioning and headlights.

Each car is subjected to two dynamic runs through the Worldwide Harmonised Light Vehicle Test Cycle (WLTC) which is divided into four sub-sections that are designed to replicate both urban and highway driving conditions.

The WLTC parameters vary depending on the power to weight ratio, with vehicles divided into three categories: less than 22W/Kg, 22-34W/kg and those with greater than 34W/kg.

All electric cars currently sold in Australia fall into the highest Class 3 category, which means they are subjected to the most rigorous version of the test cycle.

Each WLTC lasts 30 minutes and covers a total of 23.25km at an average speed of 46.5km/h.

The first Low Speed phase lasts for 3095m and 9:53min, with the car travelling at a maximum speed of 56.5km/h and average speed of 18.9km/h, and sitting at a standstill for 26.5 per cent of the time. This is designed to simulate heavy urban conditions with numerous intersections and traffic lights.

The second Middle Phase lasts for 4756m and 7:13min, increases the maximum speed to 76.6km/h and average speed to 39.4km/h, and lowers the stationary time to 11.1 per cent. The combined results from the first two segments are used to calculate the vehicle’s City Cycle consumption.

The third High Phase takes 7065m and 7:35min, includes a portion of driving at 97.4km/h with an average of 56.5km/h, and the car is only stationary at the beginning and end of the phase for a total of 6.8 per cent duration.

In the final Extreme High phase, which takes just 5:23min, the car travels 8254m and spends the majority of time beyond 100km/h (up to a maximum of 131km/h at an average of 91.7km/h) with only 2.2 per cent of time spent at a standstill. The combination of the third and fourth phases are used to determine the Extra Urban Cycle energy consumption figures.

Uniquely for electric vehicles, each dynamic segment includes one full WLTC plus the first Low Speed phase (for a total of 31.113km) followed by a constant speed section driving at 100km/h. This is because an electric car starts the first run with a full battery, which limits the benefits of regenerative braking at the beginning of the cycle.

The second dynamic segment is completed when the vehicle can no longer maintain a constant speed run in order to finish with a fully depleted battery.

The vehicle is then left idle for two hours before the battery is fully recharged using an AC charger. Because there is energy lost during the conversion of alternating current from the mains to direct current energy storage, there is a variation in the energy used by the vehicle over the two WLTCs and the electricity required to fully replenish the battery.

All of this data is then analysed to determine the vehicle’s official WLTP driving range and energy consumption.

While the WLTP program provides more accurate figures to compare one vehicle to another, these are still only a guideline as there are many other variables that will determine how far you can drive an electric vehicle before needing to recharge.

For starters, your own driving behaviour will be a major contributing factor.

EVs are more efficient around town than when cruising on the highway because you can take advantage of the regenerative braking more often in heavy traffic.

And tapping into an EV’s impressive acceleration will obviously consume more energy than moving away gently.

Also, driving with the windows down and fitting roof racks will create more aerodynamic drag that will reduce driving range, as will low tyre pressures.

Furthermore, using auxiliary systems such as the air-conditioning and stereo system will also consume extra electricity.

Finally, the topography of the landscape and road conditions will have an impact on driving range too, as does the environment with changes in wind, rain and temperature all effecting the efficiency.