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kW and kWh Explained

Becky Mckay
By: Becky Mckay
Updated: 25th October 2023

kW vs kWh

When looking for a new boiler you’ll need to consider how powerful it is and this is measured in kilowatts (kW). Likewise, if you’re casting an eye over your energy bill then you’ll come across kWh, which is a measure of how much energy is being used in your home. The more energy you use, the bigger size boiler you need. That is why the best combi boiler for a 4 bed house is different than that for a 3 or 5 bedroom house.

All of these acronyms and technical terms can get a bit confusing so we've put this guide together to help give you a clearer understanding.

If you’re not interested in technical terms and simply just looking for a new boiler, we can help put you in contact with a qualified heating engineer right away.

A heating engineer can help advise you on what kW size you’ll need for your home. This way, you’ll be sure that your boiler is a good fit for your energy needs.

Click the button below to receive up to 3 quotes from qualified heating engineers near you. You can easily compare the quotes and choose the best deal. Our service is completely free and you’re under no obligation to accept any of the offers you receive.

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What is a kilowatt (kW)?

As you might have guessed from the above heading, kW is short for kilowatt, a measure of power made up of 1,000 watts. Watts measure energy in relation to time so a boiler with a maximum output of 24kW will produce 24,000 joules of energy per second when in operation; this energy will heat water for your home. The higher the kW rating of a boiler, the more radiators and taps it can supply at one time.

You'll see this term when looking for a brand new boiler as it shows just how powerful the unit will be and, depending on the heating demands of your home, a different kilowatt may be more suitable.

What is a kilowatt-hour (kWh)?

While a kW is a measurement of power, a kilowatt hour (kWh) measures just how much energy is being used. So, if you have a 24 kW boiler, it will require 24 kWh of energy for each hour that it's working hard to heat your home, if it's operating for 2 hours then it will be 48 kWh and so on.

You'll most probably spot this on your energy bill as it's used by suppliers to calculate the total cost. Should you receive a bill that's higher than you expected, you can work out ways to use your appliances more efficiently.

What's the Difference Between kW and kWh?

Put simply, the difference between a kilowatt and a kilowatt-hour is that a kW shows a unit of power, a kWh is a unit of energy:

  • kW – Power required
  • kWh – Energy needed to keep the power going

Understanding kW and kWh can help you to know how much energy each appliance in your home is using and become more conscious of the energy being used, which could help you to lower your energy bills.

What can 1 kWh power?

Here are a few everyday scenarios, to help give you a better understanding of what 1 kWh will allow you to power in the home:

  • 4 hours on a desktop computer
  • 1 full load in the washing machine
  • Having a broadband router (7-10 watts) on for 5 days
  • A plasma TV (280-450 watts) on for 3 hours

So, it can all add up, which is why knowing the difference between kilowatts and kilowatt-hours is important.

How to convert kW to kWh

kWh can be calculated from kilowatts and the number of hours the appliance has been in operation. With the energy being equal to the power times the period of time or, to put it scientifically: E (kWh) = P(kW) x t (hours).

Average kWh per day

Once you know what kWh represents, the next thing you'll want to know is how that compares to average usage and whether you can use this information to lower your energy bills.

In the UK, when it comes to gas, anything above 18,000 kWh per year is considered high, while below 8,000 kWh per year is low consumption. The average home will tend to use 33-38 kWh each day.

Electricity consumption per year is thought of as high when it passes over 4,600 kWh, with the average household using 8.5 – 10 kWh per day.

What size boiler do I need?

There’s no boiler sizing rule of thumb that shows the exact size of boiler your home will need depending on the number of bedrooms and bathrooms. ‘What size boiler do I need?’ can be difficult to answer, but the table below will help you find the right size.

The output of domestic boilers tends to range from 24kW to 42kW and you'll need to pick the right size as too small and your home won't be well heated while too large could increase your energy bills. You can use the table below to help you find a suitable output for your home depending on the number of bedrooms, bathrooms and radiators you have in your home.

Number of Bedrooms Number of Bathrooms Number of Radiators Boiler Output
2 bed 1 10 24 – 27 kW
3 bed 2-3 15 28 – 34 kW
4 bed 3+ 20 35 – 42 kW

If you're still not sure about which size of boiler is best for your home, then it’s best to ask an installer. You can get free quotes from up to 3 trusted installers in your area by filling in one of our simple online forms today.

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Becky Mckay

About the author

Becky Mckay

Becky has been a writer at Boiler Guide since 2021. Her vast boiler knowledge means she’s ready to help with any home heating query, big or small!

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