What do kWh and KW really mean?
The top electric car models on the market have a long list of tech specs, but two of the most important are measured in kW and kWh.
Knowing what these units mean and how they affect the vehicle’s performance is worthwhile if you’re looking at leasing an electric car for work or personal use.
Let’s look at kWh and kW to remove any confusion about what they mean and how they can help you compare top EVs on the market.
Terminology: What’s Watts?
First, we need to know what kWh stands for — and what that extra ‘h’ means.
• k is the symbol for ‘kilo’, meaning thousands
• W is the scientific unit Watt, a measure of power
• h is the symbol for ‘hours’ (more on that in a moment)
Watts are quite versatile for scientific purposes, with a couple of useful definitions:
• The power needed to keep an object travelling at one metre per second against an opposing force of one Newton.
• The electrical work done when a current of one Amp flows across a potential difference of one Volt.
You don’t need to understand the science, but both definitions are important when it comes to electric vehicles.
What Is A kWh?
One Watt is a unit of power or the ‘rate’ of energy transfer — aka, the amount of energy transferred over time.
By taking kW (one thousand Watts) and multiplying by hours, the result is a quantity of energy equivalent to one kilowatt-hour (3.6 million Joules in standard scientific units).
Again, you don’t need to know the science to understand what this means for battery electric vehicles — we’ll clear up more of that mystery below.
KW And KWh In Electric Cars
To summarise what we’ve discussed above:
• kW (thousands of Watts) is a unit of power
• kWh (kilowatt-hours) is a unit of energy
In an electric car, you can think of this as the electric motor’s power and the total energy the battery can store.
For example, the BMW IX3 SUV has a 210 kW motor and an 80 kWh battery, while the Tesla Model Y SUV offers a 255 kW motor and 85 kWh battery.
Again, these specs will vary between the top electric car models on the market, and can even change depending on the trim of the vehicle, so check the tech specs to make sure of what you’re getting.
What Do KWh And KW Mean For Drivers?
We’re moving from the science theory into the real everyday world now, and the battery size and electric motor power of an EV have a bearing on several parts of the vehicle’s performance.
Maximum Range
A larger battery (in general) means you can drive further between charges. A bigger battery generally adds to that distance if you want an EV that can cover the most miles.
EVs are tested to determine their maximum range, so you don’t need to work this out based on the battery size in kWh.
If you accelerate harshly, you use more energy and will eat into your maximum miles more quickly.
Acceleration
A bigger motor will usually mean your vehicle can gain speed faster.
Note that power (kW) is the ‘rate’ at which energy is transferred, and acceleration is the ‘rate’ at which your speed increases.
Faster energy transfer means faster acceleration. The weight and other characteristics of the vehicle will affect this, but as a rule of thumb, more power means it takes less time to pick up speed.
Remember the first definition of a Watt from earlier? That’s directly relevant here — more kW in your motor will keep your car travelling at speed uphill or in a strong headwind, even when external forces are trying to slow it down.
Fuel Efficiency
The energy stored in your vehicle’s battery is the EV equivalent of having petrol in your tank, and the amount of energy used per mile (or km) tells you the fuel efficiency and economy of the car.
Fuel efficiency is calculated using the Worldwide Harmonised Light Vehicle Test Procedure (WLTP) and may be given in kWh per 100 km, or miles per kWh.
Either way, it’s about how much energy the battery takes to cover a specific range, affecting things like maximum range and recharge costs.
What Does Kw Mean For Charging?
Another place you may see kW used in relation to electric cars is on charging posts. Some of the most common EV chargers (at home and on the road) include:
• Standard 3-pin wall socket: 3 kW
• Domestic wall chargers: 7 kW
• Fast public chargepoint: 22 kW
• Rapid public chargepoint: 50 kW
• Ultra-rapid chargepoint: 100 kW and above
Again, these are just examples. Different charging stations offer different kW ratings, and some of the fastest ultra-rapid chargepoints now reach 200-350 kW.
It’s pretty clear that more kW means faster charging, and again this is down to the ‘rate’ of energy transfer — except that, in this case, the energy is going into your battery rather than being taken out of it by your vehicle’s motor.
Maximum Kw For EV Charging
Each model of EV has a maximum charging rate. The car’s own charger will usually limit this to prevent damage to the sensitive electronics in the vehicle.
For example, the Tesla Model Y SUV has a maximum charging rate of 250 kW, while the BMW IX3 SUV mentioned earlier is capped at 150 kW.
Both the vehicle and the charging point include controllers to prevent overcharging the battery. However, it’s still a good idea to only use chargepoints with output in the same ballpark as your vehicle’s maximum charging rate.
Key Points
So what have we learned – and what do we need to remember?
• kW is power, the rate of energy transfer
• kWh is a measure of the total energy stored or used
In electric vehicles, this is important for:
• Rates of energy transfer: Acceleration and battery charging speeds
• Energy used: Maximum range, battery capacity and fuel efficiency
When comparing electric cars, look for electric motor power in kW, battery capacity in kWh, maximum charging rate in kW and fuel economy in kWh per 100 km (or miles per kWh), and you’ll have a clearer idea of what they all mean.
