What are kWh and Ah?

Electric cars come with lots of new terminology, but it's very easy to understand once you've grasped the jargon...

One of the difficulties with electric vehicles is the amount of new terminology and jargon that surrounds them. Kilowatts and kilowatt-hours are especially important to understand, as they explain charging speeds, range, efficiency and various other factors.

For anyone who's spent their life driving petrol and diesel cars, this is completely new territory that can be a little daunting at first. However, all it takes is a brief familiarisation: the terms and units are very easy to understand, and once you've got the hang of them, you'll never look back.

Anyone with a background in electronics or electrical engineering will soon spot that the terms used are nothing new. The technical terminology revolves around the notion of kilowatts and amperes.

What is an ampere and ampere-hour?

Let's start with amperes: this is 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. Amps aren't often used in day-to-day electric-car terminology, but understanding them will help getting to grips with the more common units of measure.

Electrons are tiny, negatively charged particles. Because they're negatively charged, they repel each other, meaning if one moves closer to another, the latter will move further away from it. Move one electron in a line and the others will move. This is the basic concept behind current or electricity. 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. As an example, a typical laptop computer has a current of about 3A (three amps).

An amp hour (Ah) is a different unit to amps, 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 until it runs out.

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 one 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, so how much energy is consumed in a given period. Electric-vehicle batteries are usually measured in kilowatt-hours: you can think of them as 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 AC 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 electricity 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 0-80% 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.