How Much Electricity (kWh) Does It Take To Charge An Electric Car?

Electric cars are, on the face of it, much simpler than petrol or diesel cars. But that doesn’t mean they won’t take some getting used to.  

It’s not just a matter of ‘plugging in’ vs’ ‘filling up the tank’. Electric cars also come with an abundance of acronyms that may leave you scratching your head - either because you haven’t heard them before, or (more likely) you think you haven’t heard before (remember those high school physics lessons when you should have been paying more attention?).

In this article, we break down some of that EV charging jargon. By the end, you’ll know your ‘kW’ from your ‘kWh’, your ‘A’ from your ‘Ah’, and your ‘rapid’ from your ‘ultra-rapid’.

What does A and Ah mean?

‘A’ and ‘Ah’ stand for ampere (or amp) and ampere-hours. An amp is a unit of electrical current. 

Amp-hours describe the amount of current multiplied by a period of time. It can express the current produced or consumed, or the capacity of something to produce or consume current. It is often used to express the amount of current a battery can supply in an hour, or the ‘battery life’. 

Amp hours divided by amps tell us the battery life in hours. A 4Ah battery could draw 4 amps for an hour before it runs out, or 8 amps for half an hour.

Although amp-hours are frequently used to measure the battery capacity of smaller electricals, they are rarely used in relation to electric cars. The terms kilowatt and kilowatt-hours are much more common.

What does kW and kWh mean?

‘kW’ and ‘kWh’ refer to kilowatts and kilowatt-hours. 

‘Watts’ are a unit of power. It’s a measure of amps and voltage combined, or the potential difference in charge between two points in an electrical field (Voltage) and the volume of electrons (Amperage). 

Watts refer to how much power runs through a given power supply. A kilowatt (kW) is a thousand watts. A kilowatt-hour (kWh) is the amount of energy consumed in a given period. 

Electric car battery capacity is usually measured in kilowatt-hours. It’s the electric car equivalent to the size of the fuel tank in a petrol or diesel car.

How to use battery capacity (kWh) and charging speed (kW) to calculate time to charge

Electric car chargers are rated by power, measured in kilowatts (kW). This allows you to easily calculate how long it takes to charge an electric car. 

A 7kW wallbox would take one hour to deliver 7kWh of energy to your car. If your car has rapid charging capabilities, a 50kW DC charger would be able to deliver 50kWh of energy to your car in one hour. 

As a general rule of thumb: divide a car’s battery capacity (kWh) by the power of the charger (kW) to work out the amount of time it would take to charge your car. So, it would look like:

Car Battery Capacity (kWh) / Power of the Charger (kW) = Time to Charge.

Let’s look at an example:

Hyundai Ioniq 5 

• Battery Size = 73kWh
• Power of Wallbox Charge: 7kW
• Time to Fully Charge = 73 / 7 = 10 hours 25 mins

But the Ioniq 5 also has 350kW DC ‘ultra-rapid’ charging capabilities. This means if you plugged into an Ionity chargepoint (one of the best electric car charging companies if you’re looking for a speedy charge), you could cut that time down to just 20 minutes, according to our formula. 

Unfortunately, this is where our general formula becomes less accurate. All batteries have a ‘charging curve’. Although DC rapid and ultra-rapid chargers are able to charge electric car batteries at a faster rate than the 7kW AC wallboxes in our driveway, they are not able to deliver full power to the battery all the way from 0-100%. 

The first 10% and the final 20% (80-100%) is usually much slower to protect the battery. 

This isn’t a major issue, however, as the Ioniq 5 can still charge from 10-80% in 18 minutes according to the manufacturer’s website. That’s 208 miles of range recovered in the time it takes to get a coffee at the service station!

How much does it cost per kWh to charge an electric car?

Electric cars may be cheaper to refuel than petrol or diesel cars, but the actual cost to charge can vary substantially between charging stations. 

As a general rule: the higher the charging speed in kW, the more you pay per kWh.

For example, charging at home using a 7kW AC wallbox would cost around 18p per kWh, the average rate for electricity. 

A BP Pulse public AC charger starts at around 18p per kWh. 

However, 50kW DC and 150kW DC chargers cost much more, starting at 32p and 38p per kWh respectively.

At the top end of the price range is Ionity’s ultra-rapid 350kW charger. If you have a compatible car, you can expect to charge from 10-80% in less than 20 minutes. However, the added convenience comes at a price: 69p per kWh!

To calculate how much it would cost for a full charge, simply multiply the cost per kWh by your EV’s battery capacity in kWh. 

You can learn more about how much it costs to charge an electric car here.

What is the difference between nominal battery capacity and usable battery capacity in kWh?

In EV reviews and advertisements, we often see two different kWh figures used to describe the capacity of a battery.  For instance, the Fiat 500e has a 24kWh nominal battery capacity but only 21.3kWh of usable capacity. 

Don’t worry, you’re not being conned! The usable capacity of an electric car battery is limited by a battery management system to protect the battery. By preventing an overcharge or over-discharge, you can extend the life of the battery significantly. 

How does the battery capacity in kWh differ between car models?

Electric car batteries have a much greater capacity than they did a decade ago. This means electric cars have a much longer range than they used to. 

However, there is a considerable difference between electric cars when it comes to battery capacity. 

The Mercedes EQS has one of the largest batteries on the market, with a nominal battery capacity of 120kWh. This also gives it one of the longest range figures: 453 miles on a single charge. 

At the bottom end of the scale is the Smart EQ ForTwo, which has a 17.6kWh battery and a much more modest range of 81 miles on a single charge.  

But more kWh doesn’t always translate to more miles. Manufacturers like Tesla are putting less emphasis on the size of batteries, focusing instead on efficiency. 

Tesla’s ‘structural battery’ is a great example. It removes 370 superfluous parts and relies instead on the battery as a key structural component in the car. This results in a 10% reduction in mass and up to 14% more range! 

Your choice of model will definitely affect how often you have to charge your electric car.

Conclusion

Charging your car might seem like an alien concept, but it’s really quite simple. Once you understand terms like kilowatt and kilowatt-hour, you can easily calculate how much electricity, time and money it will take to charge your car. 

If you still can’t decide whether you should buy an electric car now or wait, take a look at our electric car pros and cons. 

If you decide to take the plunge, having a home charging point is a must-have. You can compare electric car charging points with Rightcharge so you can find the most powerful charger for the best price with ease. This is an affiliate link where Lease Fetcher earns money if you choose to go with one of Rightcharge's providers via Rightcharge. You will not be charged for using this link.