Choosing the power of a photovoltaic system capable of covering the needs of our domestic needs is simpler than one might think.
First we need to get a bill from our electricity supplier and check the data on annual consumption expressed in KWh.
We note that this figure expresses a quantity of energy, not of power, which instead is expressed in KW. In fact, a photovoltaic generator, even if it is sized in peak kW, does not provide power (which would mean an amount of energy per unit of time, forever, 24 hours a day), but it is rather an energy generator, capable of producing on average, in a day or in a year, a certain amount of energy.
This is due to the fact that the panels obviously produce more energy during sunny days than covered ones, during the day rather than at night, more in the summer months than in the winter and so on.
This non-homogeneous distribution of energy is not a problem because we can equip our system with an accumulation system with suitable batteries, in order to have energy available in moments of lower production.
Returning to our annual consumption, let's consider the case of a villa or a large house with 5 or 6 people. Let's suppose that the bill shows an annual consumption of 7400 KWh. We divide this number by 365 and we get approximately 20.27 KWh per day, which is the average daily consumption.
Now, to find the peak kW required for our plant in Italy, we must divide this data by a factor ranging from 3 (Valle d 'Aosta) to 4 (southern Sardinia or Sicily).
These numbers come from a summary of several statistical studies. Obviously where we have a greater radiating power and for a longer time, as in southern Italy, we will need a lower peak power.
Suppose our home is in central Italy, we could take the value 3.5. Dividing 20.27 by 3.5 we obtain approximately 5.8, which will be the peak kW necessary for our system.
By choosing a 6KW peak system we will be able to satisfy our domestic consumption. Clearly there are other factors that affect the energy yield that must be evaluated during the design phase, such as the deviation from the South orientation, the inclination angle of the panels, any obstacles, however with this method we obtain a quick and fairly precise estimate.
Cristoforo Baldoni
