Understanding kWh and kW in Electric Vehicle Charging and Consumption

• KWh per 100 kilometres or Le/100 km ratings can help consumers understand costs related to EV use

• Understanding onboard charging and fast charging kW capabilities can help compare electric vehicles on the basis of how quickly they charge

Electric vehicles (EVs) have been experiencing a surge in popularity due to their benefits in environmental sustainability and fuel economy. However, a common point of confusion for potential buyers and even some current owners revolves around the measurement of electric vehicle energy consumption. The terms kilowatt-hour (kWh) and kilowatt (kW) are frequently used, but what do they mean? This article aims to clarify these concepts and demonstrate their significance in the realm of electric vehicles.

Kilowatt-Hour (kWh)

To put it simply, a kilowatt-hour (kWh) is a unit of energy. It’s the measure of how much energy is used if a 1 kilowatt (kW) device is operated for one hour. In the context of electric vehicles, a kWh is most commonly used to describe the capacity of the vehicle’s battery. For example, if a vehicle’s battery has a capacity of 75 kWh, this means it can theoretically deliver 75 kilowatts of power for one hour.

Think of kWh as the electric equivalent of the gas tank in a conventional car. The larger the battery capacity (more kWh), the longer the EV can run before needing to be recharged, meaning a greater “fuel tank” or range.

Kilowatt (kW)

While kWh is a unit of energy, a kilowatt (kW) is a unit of power. Power refers to the rate at which energy is used or, in other words, the speed at which work is done. In the context of electric vehicles, kW is often used to describe the power output of the vehicle’s motor, or how fast the vehicle can consume the energy stored in the battery. It also refers to the power capacity of a charging station, indicating how quickly it can transfer energy to an EV’s battery.

A higher kW rating in a vehicle suggests more powerful acceleration, while a higher kW rating at a charging station signifies shorter charging times.

kWh and kW in EV Efficiency

When comparing the energy efficiency of different electric vehicles, you will often come across the term “kWh/100 km” which stands for watt-hours per kilometer or mile. This metric essentially indicates how much energy (in kWh) the vehicle needs to travel a certain distance. A lower number means the vehicle is more energy-efficient.

You can think of this as the equivalent of how many litres a typical gas tank holds. If a gas tank can hold 50 litres of fuel, and the vehicle uses 10 litres per 100 kilometres, you have 500 kilometres of range.

If you have a vehicle with a 50-kWh battery and you average 10 kWh/100 km, you have 500 kilometres of range.

Understanding Le/100 km

Currently, however, electric vehicle consumption is transformed into a litre per 100 kilometre rating just like traditional gas-powered vehicles, which is easier to understand for many consumers. So, when Natural Resources Canada says a particular electric vehicle has an electric consumption of 3.0 Le/100 km, that means that it consumes the equivalent of 3 litres per 100 kilometres of fuel in a traditional vehicle.

This calculation is based on the principle that one litre of gas equals 8.9 kWh of electricity. Therefore, if a vehicle uses 27 kWh per 100 kilometre, it is like using 3 litres of fuel (27/8.9 = 3.03) so that EV will have an Le/100 km of 100 km.

Staying with the same example, imagine that EV has an 81-kWh battery. If it averages 27 kWh per 100 kilometre, it will have 300 kilometres of range (81/27).

Usable Capacity vs. Posted Capacity

Usable capacity refers to the amount of the battery’s total capacity that can actually be used to power the vehicle. Posted capacity, on the other hand, is the total capacity that the battery can theoretically hold.

Electric vehicle manufacturers often limit the usable capacity to extend the life of the battery. Constantly charging a battery to its maximum capacity and depleting it completely can degrade the battery’s performance over time. Therefore, a buffer is maintained to prevent the battery from being fully charged or discharged. For example, a 90 kWh battery may have a usable capacity of around 85 kWh.

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Understanding electric vehicle charging speed, fast charging, and kW ratings for onboard chargers

Charging power, measured in kW, is critical when considering how long it will take to “refill” your electric vehicle. Charging stations can range from slow home chargers that might only deliver 2-7 kW, up to ultra-fast public charging stations that can deliver 350 kW. Keep in mind that your EV’s onboard charger also has a maximum charging rate it can accept.

Let’s look at a specific example by comparing onboard charger and max charging rate for  three models, the 2024 Lexus RZ 450e, the 2023 Kia EV6, and the 2023 Porsche Taycan.

Despite having the largest battery in the group, the Porsche Taycan chargers significantly faster on a home charger than the other two models because of its faster onboard charger. However, the Kia EV6 has de quickest fast charging time because it can charge at a faster rate than the other two. Lastly, despite having the smallest battery in the group, the Lexus RZ 450e is the slowest of the group because of lower max charging rate both on a home charger and a fast charger.

So, what does this all mean?

For one, we see that the size of a vehicle’s onboard charger has a significant impact on charging time. At home, a larger onboard charger can mean cutting down total charging time from 0 to 100% by half. On the road, the higher the max charging rate your EV offers, the quicker it is to charge on a fast charger.

However, this also implies that your home charging station can deliver the available max charging rate. If you install a charging station that can only deliver 7.2 kW, that’s the maximum you will reach in both the EV6 and the Taycan regardless of their differing max capability.

The same can be said about fast charging. Even if you can charge at up to 350 kW in a Kia EV6, if you plug it into a 150-kW charger, that’s the max rate you will get. In such a situation, the EV6 will likely take more time than the Lexus RZ450e because of its larger battery size.

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Final Thoughts

Understanding the units of kWh and kW is key to making an informed decision when purchasing an electric vehicle. Knowing the vehicle’s battery capacity (kWh) can help estimate its range, while knowing its power output (kW) and the power of charging stations can provide insight into its performance and charging time.

As the electric vehicle market continues to evolve, with improvements in battery technology and charging infrastructure, the efficiency and convenience of EVs are only set to increase. By gaining a good grasp of these concepts, potential buyers and users can better navigate the EV landscape and make choices that best suit their needs.