Well, so how much energy do I use in a year? And could I use less? I’d always been interested in finding out how much energy I use for my daily life. In 2013 I started monitoring my personal energy consumption more closely. There is a very good book by David MacKay called Sustainable energy without the hot air. In it he explains how our energy consumption is basically divided into three areas: transport, heating and electricity. So, let’s look at those three areas starting with heating and electricity.
The reasons I have done this were an almost lifelong interest in the question if I could sustain my lifestyle from renewable energies and also to see if I could fulfil the 2000W dream. The name of idea bugged me quite a bit until I finally found some evidence that this was meant to be an average of 2000W over 24 hours, i.e. 48kWh per day (per person). Another motivation at the time was working for Passivsystems, which made me realise that I actually had enough instrumentation to answer this question.
Heating and Electricity
I took daily readings of our gas and electricity meters from February 2013 to January 2015, reading the meters in the mornings. In addition I purchased a cheap temperature logger from amazon to measure the indoor temperature. In order to get outdoor measurements I have been monitoring the observations from the MetOffice for the weather station in Benson, which is fairly close by. For each reading I took a picture and noted the reading in a Google spreadsheet.
Analysing the data
I took my four data sources and wrote a short R script to analyse the energy consumption of our house. The reading from MetOffice need some more preparation that isn’t included as the data comes in JSON format.
In Appendix E of David MacKay’s book there is a good description how the energy consumption of a house is expected to increase linearly with the temperature difference inside and outside the house. This is essentially Newton’s law of heat transfer (or Fourier if you prefer), that has been known for quite a while.
So, my R-script goes along and finds the maximum set of observations and then calculates weekly averages for the temperature difference and the gas usage (I neglected the electricity for the time being). The resulting plot of Delta T against the kWh/day looks as follows.
The straight line is fitted for the heating season months only and has an approximate gradient of 10 kWh/(day C). The unit in the denominator of the fraction is the degree day. The house obviously adheres more to Newton’s law during the months of the heating season (October to March). In fact we can plot the data in two different ways. First let’s look at the energy used per day against the outside temperature.
This shows very clearly where the heating season starts and ends. Also, we hardly use any energy for heating when the outside temperature goes above 15C. This would of course change when the thermostat was set to a higher temperature.
So, what does the plot of the energy usage against the inside temperature show?
It shows quite clearly where our thermostat tends to be set: 18-19C. The one outlier corresponds to Christmas 2014, when weren’t at home. Using the data above one could now set out and estimate how much various home improvements would reduce the energy consumption.
Surprisingly, even our electricity usage is very seasonal.
We use significantly less energy in the summer than in the winter. Especially in the summer holidays the consumption hits rock-bottom and is dominated by fridge and freezers and machine on standby. It is interesting to see that the outlier for Christmas 2014 does not go as low as the summer lows. An indication that it is not just the extra lights that cause the winter peak, but probably also the pump of the central heating. This is emphasised by plotting the electricity usage against the temperature difference.
Which shows that the electricity usage goes up with the temperature difference. Though of course the length of the day is related to the temperature difference as well.
Transport
For the past two years my travel to work has been almost exclusively been done by bicycle. Which is about as green as it gets. Though there are of course flights and trips by car with the family. I need to gather some statistics for this aspect still.
Conclusion
Well, comparing the leakage of our house with numbers published elsewhere there is certainly room for improvement. And just heating and electricity cost us on average 52KWh a day in 2014. Just off the 2000W mark, but as with all these statistics there comes the possible fudge factor when you take into account the large number of inhabitants (slightly larger than the national average).
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