Previously, we went through the ins and outs of charging an EV. In case you missed it, you can read it here. Now that you’re clear on AC vs DC charging and the various charging ports on EVs, it’s time to move on to the EV itself.

Let’s say you’re considering the Nissan Leaf and you have its brochure on your hands, you’d see terms like 110 kW AC motor, 40 kWh lithium-ion battery, 6.6 kW charger, etc. And if you’re already feeling a wave of sleepiness, you’re not alone. So, let’s us try to break things down as simple as possible.

kW vs kWh

First up, the difference between kilowatt (kW) and kilowatt-hour (kWh). kW is a measure of power while kWh is a measure of energy. To put it in an oversimplified analogy, kW is how fast you can go, kWh is how long you can go.

In EV terms, the electric motor’s power output is usually denoted in kW, which can then be converted to PS (metric horsepower) for an easier reference. The battery’s capacity (or size, think fuel tank size for combustion-engine cars) on the other hand, is expressed in kWh.

Using the Nissan Leaf as example, its electric motor puts out 110 kW (150 PS) and the battery is rated at 40 kWh. Running the electric motor at its peak power for one hour will consume 110 kWh, which is not possible since the battery is rated at 40 kWh. Theoretically, it’s only possible for the electric motor to run at its peak for 23 minutes.

The same method of calculation applies to charging as well. Charging from a wall box that is rated at 6.6 kW, the Nissan Leaf will require 6 hours and 40 minutes to be fully charged from 0%, assuming a linear rate of charge. The brochure states 7 hours which is a reasonable estimate when factoring variables such as ambient temperature and charging rate.

The brochure also states that the Nissan Leaf has a maximum DC fast charging rate of 50 kW. Charging from 0-100% would take about 52 minutes (brochure states 1 hour).

400 V vs 800 V architecture

Currently, most EVs for sale run on a 400-volt (400 V) system. Some manufacturers, like Porsche and Hyundai, have made the shift to an 800-volt (800 V) system with the aim of increasing power without bumping up current that leads to more heat. The first production car that has an 800 V system is the Porsche Taycan.

Porsche claims that an 800 V system enables consistent high performance, reduces the charging time, and decreases the weight and installation space of the cabling.

“If the charger provides 800V and a minimum of 300A, the Taycan can charge from five to 80 per cent in 22.5 minutes. 400V chargers typically provide 50 kW only. The same charging capacity would need 90 minutes,” said Otmar Bitsche, director of e-mobility at Porsche. 

Think of the 400 V vs 800 V system as a wide versus narrow river. In this analogy, width of the river is the voltage, water velocity is the electrical current, and power is the volume of water. With the same water velocity, the volume of water in the wide river would be higher, which in this case translates to higher power.

Why not just increase the current to have more power?

In one word – heat. With all things equal, heat generated in a conductor is related to its thickness and level of current. To reduce heat, the cable must be made thicker, hence DC fast charging cables are rather bulky.

To get more power without making the cables even bulkier, the obvious solution is to increase voltage. There are, of course, challenges of having a high-voltage system including more insulation for conductors and high-voltage grade components. But the purpose of going through the effort is to get more power without increasing heat.

Conclusion

In a nutshell, kW denotes power – either the electric motor’s output or charging output – while kWh indicates the battery capacity (think fuel tank size in a combustion-engine car). To reduce charging time without increasing heat, manufacturers are moving on from 400 V to 800 V system.

If you’re confused by the charging modes and ports in EVs (3.7 kW vs 6.6 kW charging for example, and ports like Type 1, Type 2, CHAdeMO, CCS, etc.), read all about it here.

Also read:

What is CHAdeMO and CCS? Everything you need to know about charging EVs

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