Units are something that we use every single day and are a way to help society standardise certain measurements. For instance, one metre is the same in every country because it’s a standardised measurement (though in some cases language may be a barrier). No matter where you are, 10 metres is 10 times one metre. The same can be said for centimetres, celsius, seconds and more. When someone asks us how long it takes to do something, we can reply in minutes or hours and this measurement can be understood by everyone having that conversation. This is why standardised units are so important.
Electricity has its own unit system so that you can understand how much you’ve used and so that your energy company can charge you according to your usage. You may have heard the terms ‘watt’ or ‘joule’, or seen the unit ‘kilowatt hour’ on your electricity bill. Read on to discover exactly what these words mean.
What’s the difference between energy and power?
Before we dive into describing what a watt (W), kilowatt hour (kWh) and joule are, it’s important to understand the difference between energy and power because they aren’t the same thing.
Energy is the capacity to apply a force over a distance or the ability to cause change. So for example, pushing a table across a room takes energy, but simply leaning on it doesn’t, as the table isn’t moving. An electron moving within a lightbulb is another example, as it takes energy to move the electron. The amount of energy needed to move these things can be calculated so long as you know how much force is required and the distance you’re going to move it. There are a few different units of energy, and these include joules, newton-meters and calories, but when it comes to electricity, the most commonly used unit is the joule.
Power is quite different. Instead, power is the amount of energy per unit of time, so while energy describes the work being done, power explains how long it could take, much like miles per hour tells you how fast a car is going. For example, a trailer that is being pulled by a person in a sit-on lawnmower could reach its destination much faster if it was pulled by a car instead. The lawnmower and the car do the same amount of work and burn the same amount of fuel, however the car has more power. Electrical power is usually measured in watts.
Let’s clarify power vs. energy using another example. It takes around one joule (a unit of energy) to lift a can of cola to your mouth. Whether this task takes a few seconds or an hour, the amount of energy required is exactly the same. In contrast, power describes how much energy is being used and how quickly. If you lift the can within one second, you’re using around one watt of power. However, if it takes 60 seconds to lift the can, you’d be exerting a 60th of one watt over the course of a minute.
Now that we understand the basics of energy and power, let’s move on to understanding what the specific units mean.
What is a watt?
A watt is a unit of power and is defined as a current of one ampere that is pushed by a voltage of one volt. So what does this mean in simple terms? Essentially the number of volts multiplied by the number of amps gives you the total number of watts used to power a device.
For instance, in a washing machine, around 10 amps are pushed by a voltage of 230 volts (the standard in the UK), which means the machine uses around 2,200W.
What is a kilowatt and a kilowatt hour?
A kilowatt is simply one thousand watts and is another measurement of power. ‘Kilo’ means a thousand.
However, kilowatt-hours is a very different measurement. If you were to use 1kW for one hour, you have 1kWh. You can find out more about the difference between kilowatts and kilowatt-hours in our handy guide.
What is a joule?
Joules have already been mentioned in this guide – remember we said it takes around one joule of energy for a person to lift a can of cola? A joule describes the amount of energy used when a force of one newton is applied to move an object over a distance of one metre.
How many watts are in a kilowatt hour?
Being able to calculate how many watts or kilowatts are in a kilowatt hour can help you to understand your energy usage. A kWh is simply 1,000W used over 60 minutes. So for example, a 100-watt light bulb that stays on for 10 hours would use 1kWh.
The number of watts in a kWh will obviously depend on how long the appliance or device is on for, so there are some calculations that you may need to do.
Firstly, check the wattage of the device. For instance, the packaging of a lightbulb will provide a wattage, such as 60W. This means it uses 60W for every hour it’s on. Therefore, divide the number of watts by 1,000 to take the number from watts into kilowatts. In our example, 60 divided by 1,000 is 0.06kW. Next, you’ll want to multiply this number by the amount of hours the device will be used, for instance, eight hours. 0.06kW multiplied by eight gives you 0.48kWh.
How many joules of energy are in a kilowatt hour?
Around 3.6 megajoules are in 1kWh and a megajoule is one million joules. This means there are around 3.6 million joules of energy in a kWh.
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