kW, kWh and EV Charging Speed Explained

kW, kWh and charging speed explained simply

When charging an electric car, you will often come across the abbreviations kW and kWh. They look similar but describe different quantities. Knowing the difference helps you estimate charging times, compare vehicles sensibly and understand the information displayed at charging stations.

What kW means for an electric car

The abbreviation kW stands for kilowatt and describes power. When charging, this figure indicates how quickly electrical energy is delivered to the vehicle. A 150 kW fast charger can generally supply more energy per unit of time than a 50 kW charging point. Under ideal conditions, it could theoretically deliver three times as much energy in the same period.

However, the actual charging power also depends on the electric car. If a vehicle supports a maximum of 100 kW, it will charge at no more than that power even at a 300 kW charging station. The higher capacity of the charging point cannot override the vehicle's technical limit.

When charging with alternating current (AC), the onboard charger also plays a role. Many electric cars charge at up to 11 kW from a wallbox or public AC charging station. With DC fast charging, direct current flows straight into the battery, allowing much higher power levels.

What kWh tells you about batteries and consumption

Kilowatt-hours, abbreviated kWh, measure an amount of energy. In electric cars, the unit describes battery capacity, among other things. An 80 kWh battery can store more energy than a 50 kWh battery.

An electric car's energy consumption is also expressed in kWh. If a vehicle uses 18 kWh per 100 kilometres, for example, it consumes approximately that amount of energy over that distance under the conditions in question.

How far you can travel on a single charge depends on more than just battery capacity. Speed, outside temperature, driving style, heating, air conditioning and the route profile all affect the actual range.

The difference between kW and kWh

kW and kWh answer two different questions. The figure in kW shows the power at which the electric car is charging right now. The figure in kWh tells you how much energy has flowed into the battery in total.

If, for example, a vehicle charges at an average of 100 kW for 15 minutes, it theoretically receives about 25 kWh of energy. To calculate this, first convert the 15 minutes into hours. Since 15 minutes is a quarter of an hour, multiply the charging power of 100 kW by 0.25 hours: 100 kW × 0.25 h = 25 kWh. The kW figure describes the charging speed; the kWh figure shows the amount of energy delivered over that period.

You can also see the difference on a charging station's display in everyday use. The current charging power is usually shown in kW. The amount of energy charged and the billing, by contrast, are generally expressed in kWh.

How charging power and charging time are related

You can estimate the charging time by dividing the amount of energy needed by the average charging power. The average power is more meaningful than the maximum peak value.

To charge a 60 kWh battery from 20 to 80%, you need to add 60% of its total capacity. The calculation is: 60 kWh × 0.60 = 36 kWh. Next, divide the required 36 kWh by the average charging power of 120 kW: 36 kWh ÷ 120 kW = 0.3 hours. Multiplied by 60, this gives a theoretical charging time of 18 minutes. In practice, charging losses and fluctuations in power can make it take a little longer.

A larger battery therefore does not automatically mean longer charging stops. What matters is how much energy is needed and how consistently the vehicle can accept a high charging power.

Why charging speed varies

Charging speed rarely remains constant. When the charge level is low, the battery can generally accept particularly high power. As the state of charge (SoC) rises, the battery management system reduces power. That's why many EVs charge especially quickly between about 20% and 80%.

Besides the state of charge, battery temperature also affects charging power. If the battery is too cold or too warm, the vehicle throttles the charging process. Some models can precondition the battery before fast charging, bringing it to a more favorable temperature.

Even if the vehicle meets the necessary requirements, the power available at the charging point also matters. JOLT brings battery-buffered fast-charging stations to urban areas, providing high charging power there. That lets you use short stops to add exactly the range you need.