What is a kilowatt-hour?
A kilowatt-hour tells you how much energy you're using. But what does it mean and how does your kWh usage compare to other homes? Find out here.
A kilowatt-hour (or kWh) is the unit of energy utility companies use to measure how much gas and electricity you’re using. Put simply, it refers to the use of power over a period of time – for example a 1kW drill used for an hour will use 1kWh. The amount you pay per kWh varies between suppliers but you can see how much each of our tariffs charge per kWh using our tariff information labels. If you already know what you’re looking for, you can jump straight to that section using the links below:
Why is it useful to know what a kWh is?
The kWh is the most important bit of information on your bill. It lets you know how much energy you use in your home - and how much you’re paying for it.
If you’ve had a bill that was higher than you expected for example, knowing your kWh usage makes it easier to work out which appliances use more gas or electricity than others. Armed with this information, you can work out what uses the most energy in your home, and think of ways you can use them more efficiently.
Seeing exactly how many kWhs you’re using can also be great motivation when it comes to saving energy around the home, helping you understand why you should turn appliances off at the wall and not just leave them on standby.
What’s the difference between a kW and a kWh?
Knowing the difference between a kilowatt (kW) and a kilowatt-hour (kWh) is the key to understanding what’s really driving your home’s energy use – and your bills.
An easy way to understand the difference between the two is to think of a kW as the power needed to make a device work and a kWh as the amount of energy that has actually been used over a period of time. So, if the label on your drill tells you it has a 1,000 watt power rating, it needs 1kW of power to make it work, and would use 1 kWh of energy if it was switched on for an hour.
Another way to look at it is to think of your electrical device as a car. The kilowatts would refer to the speed you’re driving (for example, 30mph), whereas the kWh refers to the distance you’ve covered (for example, 25 miles). Everyone knows that the more miles you cover, the more you pay in fuel costs – and the same is true for your energy bill. The more kilowatt-hours you use, the more you pay in energy charges.
How can I convert kW to kWh?
Every appliance has a power rating which tells you how much electricity it needs to make it work. You’ll find this on a label (for example, on the bottom of your kettle), on the packaging it came in or by looking at the manufacturer’s website online.
This is usually given in watts, so you’ll first need to know how many watts are in a kilowatt. There are 1,000 watts in a kilowatt, so divide the wattage by 1,000 to get the amount of kilowatts it uses. Then multiply the kW power rating by the amount of time it’s on to find out how many kWh it uses.
For example, imagine you have a dehumidifier with a 500W power rating that’s on all day – and you suspect it may be costing you a lot to run. To find out exactly how many kWh it consumes, first convert watts to kilowatts to give your dehumidifier a kW rating of 0.5. Then multiply this by the time the appliance is switched on – in this case, 24 hours. So your 0.5kW dehumidifier uses 12kWh per day.
If you know from looking at your bill that your electricity price per kWh is 12p, your appliance costs £1.44 per day to run – which can clearly add quite a lot to your bills over time.
It’s worth remembering though that sometimes higher-wattage appliances use less power overall than lower-wattage ones. That’s because they’ve been designed to do their job more efficiently. An example of this would be an energy efficient washing machine – it may have a higher-wattage because it takes less time to complete its cycle overall, so ends up costing less.
How can I use my home’s kWh figure to reduce my bills?
It’s much easier to work out how to reduce your energy bills once you know your home’s average kWh per day, month, week or year. You can use the information to see how your household compares to the national average. If your energy usage is much higher, you can use that information to monitor your gas and electricity use more closely, to find out exactly what’s going on.
The fact that energy bills are measured in kWh means that ALL usage counts, even if it’s something as small as leaving a mobile plugged in or a TV on standby. These devices are still using power that add up to higher kWh amounts – and a larger energy bill.
Where will I find kWh on my energy bill?
You’ll find this vital bit of information on your bill or annual statement, normally under usage. It will probably use the abbreviation kWh. Your usage breakdown will tell you how many kWh you used, how much you pay per kWh and your latest meter readings.
Electricity prices per kWh
Energy suppliers set their own price per kWh and these depend on many factors, like the kind of meter you have (credit or prepayment, for example) and what tariff you’re on.
Your location can also have an impact on your kWh cost, depending on how much demand there is in your area and how easy it is for companies to supply energy to your home from the National Grid.
For some people, an Economy 7 meter can save them a lot of money. Economy 7 tariffs have one kWh price for energy used in the day time and then a significantly cheaper tariff for overnight use – perfect if you use electricity to heat a large tank of water.
What kWh formula is used on my gas bill?
While your electricity usage is stated in kWhs on both your meter and bills, gas can either be measured in units by your meter, or in kWhs on your bill - which can lead to confusion.
Luckily, there are many kWh calculators available online, which can convert gas units into kWhs for you. You’ll need to read your meter from left to right, ignoring any numbers in red.
Converting kWhs to Joules
It’s pretty easy to convert kWhs to Joules. Each kWh is worth 3,600,000 Joules so you just need to multiply each kWh by 3,600,000 to work out the number of Joules. However, while joules are used to measure small units of energy, kWhs are preferred when it comes to measuring the energy used in your home as the numbers expressed in Joules would quickly become unwieldy – it would be like buying potatoes by the milligram.