Monday, 19 March 2012

Energy consumption down, but efficiency is too

It’s been a warmer than average winter, only 75% as cold as last year according to an independent source, and my calculations bear that out too.

Season Degree Days @17.5C Average Per Day
2008-2009 2134 10.51
2009-2010 2121 10.45
2010-2011 2072 10.21
2011-2012 1567 7.71

This smaller temperature difference  complicates things, as we now have a much bigger effect from our baseline usage, which had been ignored in previous calculations.

  Low Rate
High Rate Electricity
Per Degree Day
08-09 7.20 16.30 71.90 95.40 9.08
09-10 7.09 16.18 62.52 85.79 8.21
10-11 7.14 18.46 60.73 86.33 8.46
11-12 5.49 15.38 50.42 71.29 9.24

As you can see, our total energy use per degree day has actually increased, despite the changes made with better lighting and timed radiator valves.  In addition, we reduced the heating to only 7.5 degrees in a 3m x 4m room at the back of the house with 3 outside walls which uses electrical heating.  I have to admit, I’m disappointed.

Maybe if we look at the numbers differently.  Our baseline gas usage appears to be about 12 kWh/ day, and our electricity usage about 8 kWh for high rate, and 4 kWh for low rate.

Below is the table with units of kWh per degree day.

  Low Rate
High Rate Electricity
10-11 0.307 1.024 4.772 6.104
11-12 0.193 0.958 4.983 6.134

While this is better, you can see that the overall usage is still the same, with no measurable effect from the changes, though High Rate electricity is actually down 7 per cent, and low rate is down 38 per cent, but gas usage is actually up 4 per cent, overall though we used 0.5% more energy per degree day than last year.

I’ll have to put this down the the fact that we have the gas fire on in the lounge in the evenings, and it is really toasty in there.

The other good news though is that we did manage to generate .45 kWh of high rate electricity per degree day over the period, which is included in the consumption figures above.

So in terms of fossil fuels, our consumption has dropped by 7% over the period.

In terms of absolute cost, we used 18%  less energy we did in the previous heating season, but that was unfortunately offset by an 18% rise in prices over the same period, so costs were similar to the previous year.

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