 Do you ever have arguments at home about leaving appliances on and wasting electricity? In this video, we're going to look at measuring energy use in electrical appliances. The unit of energy is a joule. However, when dealing with electricity, we actually use watts. A watt is a measure of power where one watt is equal to one joule per second. So a 100 watt light bulb uses 100 joules of energy every second. An electrical device will have a power rating. This is the amount of electrical energy the device needs to work. We can calculate how much electrical energy a device transfers by multiplying the power rating by the amount of time the device is on in hours. The unit of electrical energy transferred is the watt hour, WH. This is likely to be a very large number, so we usually give this number a kilowatt hours, where one kilowatt hour is equal to a thousand watt hours. Let's look at an example. A computer has a power rating of 250 watts. If it's used for six hours, how much energy is transferred to it? Can you substitute the values into our equation, pause the video and work it out? Did you get 1500 watt hours or 1.5 kilowatt hours? So how much would that cost to use? To work this out, we have to know how much a kilowatt hour costs. This differs between different countries, different companies within a country, and even different tariffs from the same company. Choose one of these countries and work out how much it would cost to use a computer for six hours. Did you get it right? A whole office of computers. The price starts to clock up quite quickly, and that's before we think about the lights and charging our phones. Try this example. Pause the video while you work it out. Did you get 4.4 kilowatt hours and so a cost of 62 pence? So perhaps you can now see while you get nagged to switch off devices, it saves money and also helps us waste less energy.