How to calculate your own demand for your PV system

How many square metres of PV system do you need? Depending on whether you want to cover part or all of your own needs, the amount of modules you use differs.

In the following, you will learn how to calculate your own demand per year and thus determine the number of modules for your PV system.

Calculate photovoltaic own consumption

1. If available, take your electricity bills for the last five years. Add up the total consumption figures and divide the result by five. Now you have calculated your average electricity consumption for the period. Let's assume this was 6,000 kWh/a.
2. In order to cover your electricity consumption, the PV system should ideally produce 125 % of your calculated average electricity consumption. In our example, this would be 125 % x 6,000 kWh/a = 7,500 kWh/a.
3. Now read off the regional energy yield from the map of the German Weather Service (see Figure 3). If you live in Cologne, for example, this would be about 1,000 kWh/m².
4. Probably your roof is not oriented south under an inclination of 30°. It is helpful if you now take a deviation amount from a corresponding table (see Figure 4). Let us assume that your roof is oriented 40° to the southwest with a roof pitch of 20°. Then a deviation value of 95 % results from the table.
5. Now we calculate the personal energy yield as: Regional energy yield x deviation value = 1,000 kWh/m² x 95 % = 950 kWh/m².
6. Next, we calculate the required size of the PV system: (7,500) /(950) = 7.9 Kwp.
7. A typical solar module is 1.67 m x 1.00 m and delivers 300 Wp = 0.3 kWp power. To achieve 7.9 kWp, 7.9 kWp / 0.3 kWp = 27 modules are needed. These require an area of 27 X 1.67 m² = 45 m².