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How many miles per gallon does your house do?

Most of us know the fuel efficiency of our cars, at least roughly, but how many of us know the fuel efficiency of our homes? We probably know that a car which does 20 miles per gallon(mpg) is pretty poor, whereas 100mpg is excellent. Also a diesel engine gives better fuel economy than a petrol engine, right? Even this basic level of comprehension helps us to make informed decisions when using our cars and is the first step in understanding their environmental impact. This contrasts starkly to the understanding we have about our homes, do you know how much energy your house uses in a day or a year? The commonly held view that heating a house by electricity is expensive and gas is cheaper, is about the limit of our general understanding in this era of payment by direct debits. If you are reading this then maybe you already know a lot more about the subject, but I suspect that many of us outside the technical field of building services could do with a better grasp of the issues.

The UK government hopes that installing smart meters will help us all understand our personal energy use and incentivise changes in our behaviour to reduce consumption. They have also introduced Energy Performance Certificates to help us compare homes in terms of energy efficiency, these have a simple A-G rating just like our fridges and washing machines. Loads of figures from a smart meter are great, if you are that way inclined, but I wonder if they will help us comprehend the scale and impact of our energy use. Similarly an A-G rating is a good way to make simple comparisons, but it won’t help us to understand how energy efficient our homes are and what their environmental impact is. It is rather too easy to speak in comparisons which paint a happier picture of our achievements but hide the truth about our environmental impact. For example it feels good to say ‘my new house/ car/ TV is 25% more efficient than my old one’ or ‘I used 10% less energy at home this year’ but it hides the actual energy use and consequential environmental impact. Inspired by the prospect of understanding my own energy use better and comprehending the environmental impact, I have tried to quantify my own home energy use. So how can we simply quantify the actual energy use of our homes and understand its significance?

5 easy steps to find out the fuel efficiency of your home.

I have set out 5 simple steps to calculate the fuel efficiency of my home. I hope you will be inspired to do the same to see how you measure up. (Apologies if you use coal, wood or oil to power your home. To keep things simple I’ve assumed electricity and gas use. I’ve also ignored renewable sources of energy you might already have on your home, I salute you and assume that you already understand this stuff better than me.)

Step 1

Calculate the floor area of your house in square meters(m2). Don’t worry about measuring each room just calculate the area for each floor and add them together.

Step 2

Look at your gas bill and see how many units you used last year. Now check whether you have a metric or imperial gas meter. For a metric meter (which measures gas in cubic meters), convert the gas use into Kilo Watt hours by multiplying by 11.164. If you have and imperial meter (which measures gas in cubic feet) multiply by 31.594.(See note 1 for a bit more information on this conversion.)

Step 3

Look at your previous electricity bills and see how many Kilowatt hours (KWh) you used last year.

Step 4

Add the gas and electricity used in the last year together, remember to use the Kilowatt hours(KWh) figure for both.

Step 5

Divide the total number of Kilowatt hours used last year including gas and electricity by the floor area (m2) of your house.

That’s it, what did you come up with?

You have calculated the amount of energy consumed in your home per square meter of floor area in the last year. As it is expressed per square meter of floor area it allows you to compare your figure with your friend or colleague, who might have a different size house. Your consumption figure includes all energy required to power your home assuming that you have no other energy sources such as coal, wood or oil heating. It excludes renewable energy sources such as solar panels, wind turbines, heat pumps and your prototype hot water heating system, ingeniously powered by a hamster wheel.

What does the number mean?

‘Energy use in homes is just under a third of total energy use in Britain’ Great Britain Housing Energy Fact file, Department of Energy and Climate Change 2011

So the answer to your calculation is important, how efficient is your house? The UK average is 212KWh/m2/annum (see note 2) so you can see how you compare to Mr and Mrs Average. In order to help me visualise this figure I converted this energy into a petrol equivalent to continue the car analogy. This equates to 1,929 litres of petrol per year(see note 3). How does your consumption compare?

The average house in England uses the equivalent of 1,929 Litres of petrol per year, this would fill 9 water butts.

I managed to beat the average fairly easily but I was still quite shocked at the amount of petrol it equated to. To put it another way you could fill up your average car 35 times and drive over 12,000 miles on the energy the average house in England uses each year (see note 4). If nothing else this should emphasise how profligate the energy we use in our homes is and focus us on reducing the amount we use.

The energy used by an average home in England each year would allow you to drive approx 12,000 miles.

A lack of comprehension into how much energy we consume at home prevents us from understanding our environmental impact and robs us of the motivation to make significant reductions. In the light of the Bruntland Commission definition of sustainability we have to seriously ask ourselves how to control and reduce the amount of energy we use at home. They defined sustainable development for the World Commission on Environment as follows:

“development which meets the needs of current generations without compromising the ability of future generations to meet their own needs”Brundtland Commission 1987

A thorough understanding of our energy use at home seems essential if we want to achieve a sustainable lifestyle, it is not good enough to just say we are reducing energy use or being ‘more sustainable’. I suspect that this is a challenge to most of us. The relatively uncontrolled use of energy in our homes is hugely challenging and is symptomatic of a wider problem as we move towards 2050, how to achieve the required step change in energy use of our entire existing buildings stock. This is needed to achieve the Government emissions reductions targets set out in the 2008 Climate change act. Good luck with your own efforts to grapple with these problems.

Notes

Note 1 – Conversion of Gas units to Kilowatt hours

The single figure conversion factors quoted in the text are taken from the formula provided by British Gas who quote the conversion as follows:-

‘Conversion formula – Used to convert gas units into kilowatt-hours (kWh). The formula is as follows: gas units used x correction factor (1.02264) x imperial to metric conversion factor (2.83) x calorific value (38.9) divided by kilowatt-hour conversion factor (3.6) = kWh.’

1 imperial unit of gas is 100ft3, so the energy in 1 unit of gas measured on an imperial meter is 31.594KWh. To calculation for metric gas meters (measured in cubic meters) simply omit the metric conversion factor, as follows.

Gas units used x Volume correction factor (1.022640) x Calorific value (39.3)/ kWh conversion factor (3.6). 1 metric unit of gas is 1 cubic meter so the energy in 1 unit of gas measured on a metric meter is 11.164KWh

For further information refer to the national measurement office.

http://www.bis.gov.uk/nmo/gas-and-electricity-meters/gas-meters-introduction/Gas-bill-calculation

Note 2 – UK average household energy use

OFGEM quotes figures from Energy watch for median household energy use in 2003. Gas 16,500 KWh/Annum. Electricuty 3,300 KWh/Annum. Median total household energy use is 19,300KWh/year.

http://www.ofgem.gov.uk/Media/FactSheets/Documents1/domestic%20energy%20consump%20fig%20FS.pdf

UK average house size

The Department for Communities and Local government quotes the average house size in England as 91sqm https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/6703/1750754.pdf On this basis the average household energy use per square meter is 212KWh/m2/annum.

Note 3 – Petrol equivalent

The Chief Government Scientific adviser, David McKay, quotes the energy conversion from KWh to a petrol equivalent as 10KWh/ Litre of petrol (http://www.inference.phy.cam.ac.uk/withouthotair/c3/page_29.shtml)

The median total household energy use is 19,300KWh/year (see above). Divide this by 10 to convert it to a petrol equivalent gives 1,930Litres of petrol per year.

Note 4 – Car use

A 2012 ford Focus estate, (fairly average), has a 55 Litre fuel tank. Typical fuel consumption would be about 33mpg, or conveniently 10 Km/Litre. 1,930 Litres of fuel would allow a car to travel 19,300Km or 11,992 miles.

PS

Want to find out more? If you want to estimate your overall carbon footprint, including the flights you took for your summer holiday etc. why not take a look at the World Wildlife Fund calculator which estimates your total carbon emissions based on your lifestyle, it only takes a couple of minutes.

http://footprint.wwf.org.uk/

If you are really keen and want to find out how our national energy might be provided in the future have a look at the Department of Energy and Climate change web calculator. You can create your own grand plan for covering the home counties in solar panels and see it it keeps the lights on.

http://www.decc.gov.uk/en/content/cms/tackling/2050/calculator_on/calculator_on.aspx

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5 thoughts on “Miles per gallon

  1. I think thus would be more meaningful if you used Primary Energy consumed per home as this is what UK plc and The biosphere actually ‘sees’. Not Delivered Energy which is, literally, half the story…

    • This is a really good point, and its something that is so often forgotten! Alex, maybe there is a final factor to multiply by to show how much original fuel was burnt to generate the electricity consumed etc?

      However I think using these exercises to encourage user behavior change (ie: Switching off the light!) will hopefully achieve quicker and easier positive results than trying to encourage the massive infrastructure overhaul required to address the distribution inefficiencies! It is definitely all part of the same story though, as you rightly say…

  2. Looking at the energy rate per square meter is great for comparison and comprehension. The average figure of 212KWh/m2 translates as 21 litres of petrol per annum. That is more than half a tank of petrol per square metre!

  3. Thanks for your comments. I decided to focus on building energy efficiency rather than upstream factors such as primary energy use or carbon emissions which will vary in the future as the energy mix of our national energy infrastructure changes. I take your point that it is only half the story.

  4. This is great way to think about energy use in the home. Just found out that the average for my flat (single-glazed, uninsulated) over a two-year period was 162 kWh/m2, over 90% of which was gas mostly for heating.

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