What Is a kWh? EV Charging Units Made Simple

A kWh, or kilowatt-hour, is a unit of energy: it is how much electricity your EV stores and uses, and how you are billed for charging. A kW (kilowatt) is a unit of power, meaning the rate at which energy flows, so charging at 7kW for one hour delivers 7kWh of energy. Your battery size is quoted in kWh, your charging speed in kW, and your electricity in pence per kWh. Once you grasp this, working out running costs and charging times becomes simple, and choosing the right Type 2 charging cable makes more sense too. Think of kW as the tap and kWh as the water in the bucket.

At a glance: kWh vs kW

What exactly is a kWh?

A kWh is the amount of energy used by a one-kilowatt appliance running for one hour. For EVs, it is the unit that describes both how much energy your battery holds and how much you use to drive. A 60kWh battery, for example, can store 60 kilowatt-hours of energy when full. Your electricity supplier also bills you per kWh, so understanding this single unit lets you connect battery size, charging cost and how far you can travel on a charge.

What is the difference between kWh and kW?

The difference is energy versus power. A kWh measures energy, the total amount stored or used, while a kW measures power, the rate at which that energy flows. Charging at 7kW for one hour delivers 7kWh. Your battery capacity is quoted in kWh; your charging speed is quoted in kW. Mixing them up is common, but the tap-and-bucket analogy helps: kW is how fast the tap runs, and kWh is how much water ends up in the bucket.

How does kWh relate to battery size?

Battery size is measured in kWh, so a bigger number means more stored energy and, broadly, more range. A 60kWh battery holds more energy than a 40kWh one, so it can usually travel further on a full charge. How far depends on efficiency, measured in miles per kWh, which varies with the car, speed, weather and driving style. So two cars with the same battery size can have different ranges, but the kWh figure is your starting point for estimating range.

How do I use kWh to work out charging cost?

To estimate charging cost, multiply the energy you add in kWh by your price per kWh. Filling a 60kWh battery from around 10 to 100% adds roughly 54kWh. On a cheap overnight rate of around 7 to 10p per kWh, that costs roughly £4 to £5, while on a standard rate of around 25 to 28p it costs roughly £15 to £17. Public rapid charging is dearer, often around 45 to 80p per kWh. Always treat these as approximate and check current rates.

Quick cost method

• Find the energy added in kWh (roughly battery size times the percentage you add)
• Multiply by your price per kWh
• Compare off-peak, standard and public rates before you charge

How does kW affect charging time?

Charging power in kW sets how long a charge takes. At 7.4kW, a car adds roughly 25 to 30 miles of range per hour, while a 3-pin portable charger at around 2.3kW adds roughly 8 to 10 miles per hour. To add 54kWh at 7.4kW takes several hours, which is why overnight home charging works so well. Rapid DC chargers of 50kW to 350kW push far more power, so they add the same energy in minutes rather than hours.

Frequently asked questions

What is a kWh in simple terms?

A kWh, or kilowatt-hour, is a unit of energy equal to a one-kilowatt appliance running for one hour. For EVs, it describes how much energy your battery stores and uses, and it is the unit your electricity supplier bills you by. A 60kWh battery holds 60 kilowatt-hours of energy when fully charged.

What is the difference between kW and kWh?

A kW measures power, the rate at which energy flows, while a kWh measures energy, the total amount stored or used. Charging at 7kW for one hour delivers 7kWh. Battery size is quoted in kWh and charging speed in kW. Think of kW as how fast the tap runs and kWh as how much ends up in the bucket.

How many kWh does my EV battery hold?

It depends on the model, but common EV batteries range from around 40kWh to well over 70kWh. The figure is quoted in your car's specification. A larger kWh capacity generally means more range, though actual range also depends on efficiency, measured in miles per kWh, and factors like speed, weather and driving style.

How do I calculate my charging cost?

Multiply the energy you add in kWh by your price per kWh. For example, adding around 54kWh to a 60kWh battery at 7 to 10p per kWh costs roughly £4 to £5, while at 25 to 28p it costs roughly £15 to £17. Public rapid charging is dearer. Treat these as approximate and check current rates.

Does a higher kW charge mean a bigger battery?

No. kW describes charging speed, not battery size. A higher kW rating means the battery fills faster, but it says nothing about how much energy the battery holds, which is measured in kWh. A small battery can charge quickly, and a large battery can charge slowly, depending on the charger and the car.

What is miles per kWh?

Miles per kWh is a measure of efficiency: how far your car travels using one kilowatt-hour of energy. A higher figure means the car goes further on the same energy. It varies with speed, weather, load and driving style. Combined with your battery size in kWh, it helps you estimate real-world range between charges.

Once kWh and kW make sense, the rest of EV ownership is easier to budget. For real-world figures, see our guides on which EV charging cable you need and 7kW vs 22kW charging, then match the right lead to your car on the cable finder.