Energy project update

Intro – the problem

This is a follow up post to something I posted on Facebook – silly me – and, predictably, can’t find anymore now. Serves me well for entrusting FB with my stuff.

Anyway, here is the gist:

  • my household consumes about 8,600 cubic meters of gas and 8,400 KWh of electricity per annum
  • this translates to an energy expense of about €10,000/year. This bill increased more than 10% in 2013, and projecting it over a few years clearly shows it will quickly spiral out what is sustainable – barring a major lottery or huge inheritance
  • unfortunately, incentives for installing renewable energy solutions have run out, but a 50% tax credit over 10 years is still in place until the end of the year.

Calculating the solution

In itself, the calculation of exactly what consumes how much is not at all easy – invoices from utility companies are incredibly detailed these days, very generous in information of no use what-so-ever, so I had to resort to literature and to some basic measurements to figure out where all this energy goes. My conclusions (all to be confirmed) are as follows:

  1. heating: 7500 SMC = 80,000 KWh @80% yield of the (old) furnace = 64,000 KWh of thermal energy
  2. hot water: 1100 SMC = 11,600 KWh @94% yield of (brand new) boiler = 11,000 KWh of thermal energy
  3. appliances: 8,400 KWh of electricity

Replacing #1 and #2 with heat pump technology, which is much more efficient (COP of 3.5) , decreases energy requirements from the current total of 75,500 KWh to 21,600 KWh in ideal conditions: however, these conditions are not attainable, as external temperature does get at times (I assumed 10%) below the minimum working temperature of heat pumps; additionally, there is a portion of the consumption (assumed at 30% of total) which happens at night.

There is also some additional consumption, as the inverter systems I will be installing double up as climatisers, offering us the additional benefit of cooling down the summer months.

Under the current “on place exchange” system, the maximum benefit is achieved if you generate the energy you consume but of course the curves of consumption (red) and production (dark blue) do not match; a much better fit could be achieved through the use of accumulators, but there are still at early stages and therefore very expensive. However, extra generated energy can be credited back (purple) at a discount (about 30%) against future purchases within the year..

This means my energy requirement addressable by a photovoltaic system (green) is in the region of 22,400 KWh; divided by the about 1,100 hours of sun we get at our latitude, I would need a 19KW system, which by chance is the maximum eligible for the private individuals tax credit and the maximum my south-oriented roof will carry.

curveI am therefore currently trying to decide whether a 15KW system would be enough for now, knowing they are completely modular, and should I find out I need more, another 4KW can be “clipped” on the existing system without changing anything else.


Of the five companies I called, three I have discarded already and looking into the final details to pick the winner – the order of magnitude for the investment  is 45k, which I can finance at 7.5% paying off the investment in 10 years off the energy bill savings and tax credit, with another 15 years of useful life in the system left after that.

If the assumptions in my calculations are right, I should achieve about 77% energy independence with this system; the residual gas bill should be <2,50 SMC and electrical bill should be negligible, as it should more or less balance out. Payback of the investment is achieved in 5.5 years.


One obvious development would be the installation of accumulators; in a couple of years hopefully the data I have on my consumption will be much more granular and precise, allowing me a much more precise dimensioning of accumulators. Roughly speaking, however, with accumulators I should be able to achieve around 85% energy independence, a yearly benefit of about €4k, which therefore means a 5,000KWh pack should cost no more than €12k to be really attractive.

The other form of energy independence is the lowering of our family gasoline costs – our two cars consume around 5,000 liters of fuel between them, which translates to about €8,500; replacing one of them with an EV (e.g. Nissan Leaf) which for 30,000km consumes about 6,500 KWh is however not really attractive, yet: the car itself is at least €10k more expensive than its equivalent diesel and to generate said amount of energy I would have to forego the additional electrical saving I described above, trading €4k saved on electricity against €4k saved on gasoline.


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