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Energy Explainers: What is a Watt-hour?

Blog : 21 December 2016

Welcome to our series of blogs explaining the science that supports the goals of Exeter City Futures. We will break down the different aspects of energy independence and zero congestion into their simplest terms, in preparation for Exeter’s energy report which we will be releasing in early 2017.

In our last blog, What is a Watt?, we looked at what a watt means and how it measures the speed (rate) at which energy is used rather than the total quantity. In this blog we’ll explain how watt-hours are used to actually measure large amounts of energy.

Energy by the Hour

Previously we used the example of a 60W light bulb. Its 60W rating means that for each second that it’s on it uses 60 joules of energy. Over the course of 60 minutes, or 3,600 seconds, this means it would use 216,600 joules of energy.


What is a watt-hour


Because we use so much energy and for long periods of time it’s easier to talk about the energy we use in small numbers of watt-hours, rather than large numbers of joules.


Working out Watt-hours

To work out watt-hours we need to multiply the power rating in watts on our device or appliance (which tells us how many joules of energy it uses each second) by the number of hours it has been on.

So if we leave our 60W light on for one hour, we will have used 60 watt-hours of electricity. This is a simpler way of saying the sum for the amount of energy used in joules:


What is a Watt-hour


If we leave our 60W light on for ten hours, we will have used 600 watt-hours of electricity. This is an easier way of saying 2,160,000 joules.


Converting to Kilowatt-hours

The average UK home uses around 11,000,000,000 or 11 billion, joules of electricity each year. This is equal to 3,000,000 or 3 million watt-hours.

When we talk about domestic energy use we use kilowatt-hours, or thousands of watt-hours, to keep the numbers on our bill as small as possible.

To convert to kilowatt-hours we simply take our figure in watt-hours, and divide by one thousand.

So a 60W light left on for one hundred hours uses 6,000 watt-hours, or 6 kilowatt-hours of electricity.


So what is a watt-hour? It’s an easier way of measuring the amount of energy we use than joules. It tells us how many watts, or joules per second, are used over a period of time measured in hours. To work out the number of joules we multiply the number of watts by the number of seconds that amount of energy has been used for.