Dimensioning a PV system

Your energy consumption is the sum of several components:

  1. heating (typically gas powered)
  2. hot water production (may or may not be the same furnace as for #1)
  3. everyday electrical appliances
  4. air conditioning

The template (download it here) assumes the installation of a PV system and the supplementing of your existing heating system with a heat pump system which also works as an inverter; it also allows for a separate heat pump boiler to be installed for the production of hot water. The cost items in my case did not include any masonry work as I had already pre-installed the necessary connectors when I did the renovation, but please make sure you estimate yours correctly.

Using the template

The simplest way to use the template is to focus on section 1:

  • fill in your parameters (cells with yellow background)
  • run simulations with the solutions you are evaluating
  • pick the one that maximises the net present value of your savings over the life of the system (cell I5).

Other information in the template

Part of your benefit is in the form of a tax credit, whose parameters are recorded in the TAX panel in Section 1. Any tax credit depends on your yearly tax bill being HIGHER than the amounts in cells O2..O10. If that’s not the case in one of more year, that year tax credit is lost; moreover, if you are a business, the tax benefit comes from the deductability of the asset, and therefore on your marginal tax rate (cell D21)

In assessing the size of the system maximising your benefit, please remember that the italian system works on the basis of in-situ exchange; in other words, the energy you consume is priced at whatever price you are paying (cell E5) but energy produced ON TOP of your consumption is bought back by the grid at a much lower rate (set in cell E10). Therefore the best dimensioning is the one that keeps the value in the Electrical Balance control cell J4 negative, but as close as possible to zero.

Most financing options have a 6-months grace period (i.e. you start paying back 6 months after the installation is completed); moreover, gas prices are going to increase: the template conservatively estimates this increase at 7% (cell D10). All this means that your yearly balance will change – you may want to make sure that such balance is always positive (in your favor) by looking at the Saving (column P). In an ideal world, the Saving is higher than the Tax Credit, which you only get at end of the year, to avoid straining your monthly cash flow, but that is not easy to achieve.

The template allows for a certain percentage of time in which the weather is too cold for heat pump to perform efficiently; this is currently set at 20% (cell D6)

Refining the template

In Section 2 of the template you find some cells with a light blue background.
These are a sun hours distribution (row 29), a heating consumption distribution (row 32) and a hot water distribution (row 39); these are obviously based on my latitude for a roof that’s perfectly exposed to the south and has no obstacles or trees shading it.

It is moreover based on the history of my family consumption and the (poor) isolation performance of my house – for a more precise modeling of your situation, you may want to look at these additional parameters.

Finally there are three cells which bear the consumption of the Air Conditioning system you will be installing in your home: the theoretical models are very imprecise, as this heavily depends on behavioural factors (how many people in the house? how many go on holiday and for how much? do they like it VERY cold or just a little? how hot is it outside? how windy? is it shaded? how humid?). At the end of the day, a model seems futile, so I simply allowed for manual input, to be confirmed by actual data. Doing a little what if with these figures however shows a very limited variation on the overall picture, so I thought this was an acceptable approximation.

One thought on “Dimensioning a PV system

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