My energy efficiency project

Some friends know that I have been devoting an inordinate amount of time to bring the energy efficiency of my house under control and asked me whether I had any results to post, so here they come:

Screenshot 2016-07-08 11.12.09.png

I am conscious this is a bit of an eye test, so I will try to guide the reader through they key figures (all costs include all taxes and tariffs):

  • (line 32) the total energy cost of the house went from nearly €8,400 (2013) down to a little more €4,200 and is projected to further decrease to €3,700 this year. True, 2012 and 2013 were much colder years than these last two, but still the improvement is visible: from around 32 KWh per degree-day (line 35) down to 27.

[NOTE: Degree-days are  a common measurement for external climate, defining 20 °C as the external temperature requiring no heating: a day where the average temperature is 18 °C contributes 2 DD to the year’s total.]

  • It’s always hard to do backwards historical what-ifs, but the 2012-2013 costs were based on energy costs lower than today’s. More importantly, while furious negotiations with suppliers made sure I took advantage of recent energy cost decreases in gas, I essentially eliminated the source of uncertainty linked to electricity costs: while the market price went from €0,16 in 2012 to €0,34 in 2015 to retrace down to €0,26 this year (line 7), thanks to my own photovoltaic production offsetting most of the consumption, my effective price went from said €0,16 down to €0,04 and projected to go to €0,02 this year (line 19). In fact, my net electricity cost decreased over 90% to essentially zero (line 18)
  • My next question mark is whether I should upgrade my heating furnace: the current system is not very efficient (I estimate its yield – line 26 – to no more than 70% while a new one would easily attain 98%). Running the numbers, the new furnace should achieve a further €849 in savings, but would cost around €8,000; even accounting for tax credits, it would take 40 months (line 41) to recoup the investment. Ditto for Hot Water production, which could be replaced by a €6,000 Heat Pump system I would amortize over 42 months. This return period is a little longer than my target (around 24 months) and so I am hesitant to take the plunge: it’s true the last long-term bet I made on energy (the €50,000 photovoltaic system) paid itself back ahead of time (5 years instead of 8), but price volatility is scary so I am thinking hard…
  • as a last curiosity, column K looks at the numbers for a hypothetical wood-fired furnace to replace my gas-fired system: the numbers are very attractive, with a payback period of only 22 months, but the idea of having to refill the furnace twice a day by hand and the fact that wood furnaces apparently pollute more than their gas counterparts made me discard that option.


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