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What are kWh and Ah?

You might be wondering what some of the new electric-car jargon means – and we're here to help with that

Renault Twizy

Moving from a petrol or diesel car to an electric car can feel a little daunting at first, mainly because there are so many new terms to learn. Words like kilowatts and kilowatt-hours are everywhere, and you need to know what they refer to, as they give you lots of information about things like charging speeds, battery capacity, range, efficiency and more.

It’s not too difficult to get the hang of, though. After all, you probably know what mpg means, probably have heard of NOx emissions and can piece together what’s meant by internal combustion. All those terms can be put to bed now, though, and it’s time to learn some others.

All the terms to do with electric cars are nothing new to anyone who works with electricity, and you may even already know them – but the abbreviations like kWh and Ah might add more confusion. Read on to find out what each one means and why you need to know it.

What is an ampere and ampere-hour?

The short version is that the term ampere or amp measures electric current over time. You don’t often hear it mentioned in regard to electric cars, but it’s worth knowing what it is. The long version is that it's a unit for measuring the rate of electron flow (think electricity) or the flow of current in a conductor (something that conducts or controls electricity) in the space of a second. 

Here’s how electricity flows. Electrons are all put in a line, and because they're negatively charged, they repel each other. This means when you push one at the end of the line, all the rest of them move as well. Imagine a line of snooker balls – when you strike the one at the end, the one at the other end moves away.

The measure of this current is known as ampere. One ampere (A) has a standard definition of 6.24 x 10 to the power of 18 electrons flowing in a second. The more amps you have, the higher the current. It’s not necessary to understand this, but just know that a typical laptop computer draws a current of about 3A (three amps).

An amp hour (Ah) is a different unit to amps; it's used to estimate the amount of energy a battery can hold. In simple terms, it’s used to define the amount of current a battery can supply in an hour. Amp hours are therefore used to determine battery life. Amp hours divided by amps tell us the battery life in hours. So a 2Ah battery can draw two amps for an hour before it runs out, or four amps for half an hour.

What are kilowatts and kilowatt-hours?

Watts, on the other hand, are a unit of power. A watt is a measure of amps and voltage combined. Voltage can be thought of as the amount of electrical pressure a conductor or circuit has; the force that pushes the electrons along the circuit. Amps are the speed at which electrons move past a given point. Power, then, is a relation of both force and speed. The formula runs as follows: Power (Watts) = Amps x Volts

Watts are used to define the amount of power that runs through a given power supply. A kilowatt (kW) is simply a thousand watts. A kilowatt-hour (kWh) – much like an amp hour – is different from a watt. A kilowatt-hour is a measure of energy – how much energy is consumed in a given period. Electric-car batteries are usually measured in kilowatt-hours: you can think of this as being equivalent to the fuel-tank size of a petrol or diesel car. The bigger the battery, the more energy you have and the longer your electric car's range will be.

This is useful for working out charging times, as chargers are always rated in terms of their power, measured in kW. So if you have a 7kW wallbox charger at home, it'll take one hour to deliver 7kWh of electrical energy. Therefore, as a rule of thumb, you can divide a car's battery capacity by the power of a charger to work out the charging time. So a Nissan Leaf with a 40kWh battery that's plugged into a 7kW charger will take around five-and-three-quarter hours to charge (40kWh ÷ 7kW = 5.71 hours).

However, it's important to note that this isn't always true, especially in the case of fast or rapid chargers, which typically use a DC supply. In these cases, charging the final 20% of a nearly full battery will take longer than the first 20% of an empty one, as it becomes increasingly difficult to cram the energy into the cells at such a high rate. This is why you'll see many manufacturers quoting '10-80%' rapid-charge times. And because that final 20% is harder to fill using a rapid charger, many drivers choose to leave public charging stations once they've reached 80% rather than wait longer for the battery to fill up completely.

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