Your questions answered

What are kWh and Ah?

These two units often crop up when electric cars are discussed – but what do they actually mean?

Renault Twizy

For many buyers, one of the most intimidating things about the move from petrol to electric power is getting to grips with all electric-car terminology. Where you might be used to all the usual car-related jargon like ‘mpg’ and ‘bhp’, words like ‘kilowatts’ (kW), ‘kilowatt-hours’ (kWh) and ampere-hours (Ah), which are used when talking about electric cars, may be less familiar. 

It’s important to understand units like kWh and Ah and what they refer to when looking for a new electric vehicle, as they help to explain things like charging speeds, battery capacities, range and efficiency.

So, with the UK heading to an all-electric future fairly quickly, now’s a good time to brush up on some of the electric vehicle acronyms you should expect to come across increasingly frequently within the next decade.

For anyone already familiar with the workings of electricity, many electric-car terms should already make sense, although abbreviations such as kWh and Ah can still cause confusion. We’ve put together a guide to all the electric-car terminology you need to know, what each term means and why you need to know it.

What is an ampere and ampere-hour?

An ampere, also known as an amp, is a unit that measures electric current over time. Think of an electrical circuit as a tap with running water. To get water to come out, you need two things: water, and also pressure to force it out of the tap. Electrons flow through a circuit much like water flows through a tap – if you push water through a pipe from one end, it moves forward at the other end, too, just as electrons do in a line. The ‘electrical pressure’ used to push these electrons along is known as ‘voltage’.

An amp is a measure of how many electrons (water) flow past a certain point in the circuit (the end of the tap) in one second. 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.

An amp hour (Ah) is a different unit to amps; it's used to estimate the amount of energy a battery can hold, or, using the previous analogy, the amount of water in a bucket. 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?

A watt is a unit of power calculated by multiplying how much electrical current (amps) is flowing by how hard it is flowing (volts). Using the water analogy, let’s say you wanted to spin a water wheel using two different types of hose. One hose is narrow, and the total volume of water (electrons) flowing per second is low (low amperage), but the high pressure (high voltage) means that the total power (wattage) exerted on the water wheel causes it to spin. 

A wider hose that lets out a higher volume of water per second (high amperage) but at a lower pressure (low voltage) could make the water wheel spin at the same speed if the total power (wattage) from the combined pressure and current are equal. The total power produced from an electrical circuit (wattage) is determined by the following formula: Power (Watts) = Amps x Volts – so amperage and voltage can vary, but can also result in the same wattage.

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.

Recommended

Pop-up solar-powered charging hub launches in UK
Papilio3 charging hub
Charging

Pop-up solar-powered charging hub launches in UK

27 May 2022
Rapid charging prices rise by 21%, but EVs are still cheaper to run
Gridserve Swansea
Charging

Rapid charging prices rise by 21%, but EVs are still cheaper to run

26 May 2022
Tesla Supercharger network: complete guide to Tesla charging stations
Tesla Superchargers
Tesla

Tesla Supercharger network: complete guide to Tesla charging stations

23 May 2022
Complete guide to the BP Pulse (formerly Polar Plus) charging network
BP Pulse
Your questions answered

Complete guide to the BP Pulse (formerly Polar Plus) charging network

19 May 2022

Most Popular

Where can I buy hydrogen and where is my nearest hydrogen filling station?
Mercedes fuel cell
Hydrogen

Where can I buy hydrogen and where is my nearest hydrogen filling station?

A guide to where you can find UK hydrogen fuel stations for filling up a hydrogen fuel-cell car
23 May 2022
Tesla Supercharger network: complete guide to Tesla charging stations
Tesla Superchargers
Tesla

Tesla Supercharger network: complete guide to Tesla charging stations

The Tesla Supercharger network is expanding fast, providing reliable rapid charging throughout the UK to Tesla and non-Tesla owners alike
23 May 2022
Top 10 best electric SUVs 2022
Top 10 best electric SUVs
Best cars

Top 10 best electric SUVs 2022

Electric SUVs combine style and practicality with low running costs, making them a very popular choice in the UK. Here are our top 10 favourites to bu…
17 May 2022