July 27, 2023
kW vs kWh
The terms kW and kWh can be a little confusing at first, but they're pretty straightforward once you get to know them.
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You will see these numbers quoted throughout adverts and spec sheets for EVs, so see below for a simple explainer:
This is a unit of power. In the context of EV charging, it refers to the charging rate, or how much energy is being transferred to the vehicle from the charger per unit of time. Think of it as the speed at which your car is charging. A charger rated at 7 kW will charge your car twice as quickly than a charger rated at 3.5 kW, for example.
This is a unit of energy, and it's used to measure the amount of energy stored in a battery. When talking about EVs, this number often refers to the battery's capacity. A battery's capacity tells you how much electric energy the battery can hold when it's fully charged. So, if you have a car with a 60 kWh battery, it can store 60 kilowatt-hours of electricity when it's fully charged. For context, the average UK home uses between 8.5 and 10 kWh of electricity each day.
Here's an analogy: Think of electricity like water flowing through a pipe. The kW (power) would be the rate at which water is flowing through the pipe - this is like the speed of charging. The kWh (energy) is the total amount of water that flows out of the pipe over a specific period of time - this is like the total amount of energy that your battery can store.
So, if you plug your car into a 7 kW charger, it's like you're filling up your "water tank" (battery) at a rate of 7 "buckets" (kilowatts) every hour. If your "water tank" can hold 60 "buckets" (60 kWh), it will take you a little over 8.5 hours to fill it up from completely empty to completely full. All EVs have a maximum charging power, so sticking to the same analogy as before, if too many buckets are poured into the water tank at once, the force would cause the water tank to crack. This is analogous to charging a battery too quickly which can cause it to catch fire. This never happens in reality because the vehicle controls the charging power.
To approximate the time required to fully charge a vehicle from empty, it’s very simple. Divide the battery capacity (kWh) by the charging power (kW).
Charging time (hours) = Battery Capacity (kWh) / Charging Power (kW)
Remember, this is a simplification, and actual charging times can be influenced by a number of factors including the car’s onboard charger, the remaining battery life when you start charging, the type of battery, and the battery's temperature.