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Personal energy footprint

Personal energy footprint

To make the most best use of your efforts in cutting your carbon footprint, it is important for you to get things in perspective. Which parts of your life demand the most energy: is it transport or heating your home? It makes sense to focus on the "easy wins" - those actions that make the most difference for the least amount of effort.

My original Environment Pages (2006) tried to offer an informed set of ideas about how each of us can make a difference. It was a simplistic starting point, and I tried to cut through as much of the "green wash" and spurious information as I could but still fell into traps, for example by getting a bit too caught up noting the hype about turning off mobile phone chargers.

Fortunately, since then Professor David MacKay has published his excellent book on-line...

Sustainable Energy - without the hot air  -  This will give you a great sense of perspective, and is a much better source of information than I can hope to achieve in this simple website. In David's book he converts all aspects of energy use (from cars, to heating, to phone chargers) and energy production (from potential renewable sources). Throughout the book he discusses energy in terms of "kilo Watt hours (kWh) per day", which is useful as it relates to the same unit that we all read from our gas or electric meters [i].

In the following pie chart I have summarised David's findings for the (equivalent) daily energy use of "typical moderately-affluent person". What would your chart look like?

Pie chart capturing the results from David MacKay's book.

Alternative units - thinking in terms of tea

"kilo Watt hours (kWh) per day" is a good general unit but I find it easier to make sense of things with a more down-to-earth unit of comparison. With a colleague at work we hit on the idea of converting energy use into mugs of tea equivalent - i.e. the amount of electricity required to boil enough water to make a mug of tea. You can try this yourself if you have an energy meter or work it out with a bit of GCSE Physics...remembering that the specific heat capacity of water is about 4.2 J/g/ºC ... :o)

Our kettle took 115 seconds to boil 2 mugs [i] worth (as there was a minimum fill line in the kettle) of water from the cold tap. It is a 1.9 kW kettle so the amount of energy required was:

1.9kW × 115sec ÷ 2mugs ÷ 3600sec/hr = 0.03 kWh

Or to convert energy figures in kWh to mugs of tea, you need to multiply by (1 ÷ 0.03) = 33. For example, spending a hour watching a 160 Watt [i] flat screen television = 0.16 KWh or about 5 mugs of tea.

Mugs-of-tea is a useful unit for comparing things around the house. For bigger things you could use miles-travelled-in-a-car...noting that an average car (e.g. Ford Focus) does about 44 mugs of tea to the mile.

Some examples

  • I have longer than average hair so I take more than the apparent average amount of time in the shower, call it 10 rather than 5 minutes. In an electric shower that is equivalent to 50 mugs of tea...or nearly 1 ½ miles driven in an average car...or 6 solid months of a old mobile phone charger.
  • I have an 8-year-old, LED clock radio, which I really like. I was worried that it might be a bit of "energy vampire", as it is warm to the touch. However, I've measured it and uses about 2 Watts, which is equivalent to 1 ½ mugs of tea per day (or 12 seconds in the shower) it is only a tiny fraction of my carbon footprint. I've decided to keep it. It's also worth noting that, whilst a new clock radio might use ½ Watt, the amount of energy involved in building it, and scrapping my old one, would take a long time to pay off.
  • In hunting down the "standby beast" - note that a lot of literature on energy saving tips suggest that we waste approx 8% of our energy through leaving appliances in their standby mode (rather than switching off at the wall), e.g. from The Big Green Switch - I found a few nasty "vampires" (like TV's). However, there were a lot of things registering about 1 Watt (e.g. clocks on microwaves): that is about ¾ of a mug of tea per day per appliance. It is worth knowing which ones are worse so you can make sure you remember to switch them off when not in use. However, it would probably make more difference if you remember turn your engine when you are at the traffic lights!

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