What is the difference between kW and kWh?

If you’ve been researching the pros and cons of electric cars, the chances are you’ve seen values written using kW and kWh. But what do these measurements mean and why are they so important to you?

First, you need to know that kW stands for kilowatts, and kWh is short for kilowatt-hours. Crucially, the two units are not compatible, so cannot be compared. Confused? You won’t be.

Imagine the car battery is a swimming pool – kWh is the amount of water the pool can hold, while kW is the rate at which water will flow into the pool. Sure, water and electricity don’t mix, but this analogy helps set the scene.

Next, take a look at this video:

What is kW?

So, as Lexi explains, kW is a measure of power – one kilowatt is simply 1,000 watts. Kilowatts (kW) is the measurement of energy used for electric car chargers, i.e. 3kW, 22kW and 50kw, etc.

In other words, it’s the rate at which power is transferred from a charging station to your electric car. In theory, then, it would make sense to use a rapid charger, because a 50kW or 100kW input is going to replenish a battery more quickly than a fast or slow charger.

That might be so, but not all electric cars can handle a rapid charge, while batteries do not respond well to regular rapid charging.

It’s worth noting that kW can also be used as a reference to the output of the electric motor. In a standard car, this would be measured in hp or bhp.

We admit, it’s confusing to have two definitions for kW, but put it this way: when kW is used in the context of a charger, it’s the rate at which you can charge your battery. When it’s used in the context of a car, it’s the size of the electric motor.

What is kWh?

Meanwhile, kWh is the total amount of energy stored in a battery. For example, there are two types of Nissan Leaf electric cars available: a 40kWh version and a 62kWh model. In general, the larger the battery, the further you can go on a single charge.

Here are various battery sizes with their associated range:

  • Seat Mii Electric 36.8kWh: 125 miles
  • Nissan Leaf 40kWh: 168 miles
  • Vauxhall Corsa-e 50kWh: 211 miles
  • Nissan Leaf 62kWh: 239 miles
  • Kia e-Niro 64kWh: 282 miles

To put it another way, it’s the amount of energy expended in one hour by a kilowatt of power. You’ll be familiar with kWh, because it’s the measurement used by your energy provider when billing you for powering your domestic appliances.

Still not getting it? Well, imagine an electric car with a 50kW electric motor and a 50kWh battery. If you ran the motor at maximum speed, you would drain the battery in one hour.

In summary, kWh is the size of the battery, while kW is either the size of the electric motor or the rate at which your battery will recharge. Got that?

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