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Despite all the changes made by the Flemish government, solar panels are still a lucrative investment. But how many panels need to be self-sufficient and what is the annual yield? 


We explain how you calculate this yourself and what you should pay attention to when calculating income.

CALCULATION OF SOLAR PANELSHow many solar panels do I need?

The amount of solar panels required depends on: your energy consumption per year, the efficiency of the panels and the capacity of the installation.

 Calculate the number of solar panels by dividing the total energy consumption per year by the conversion factor (0.85).

Suppose you use 3500 kWh of electricity per year, then you need an installation that can deliver 3500 / 0.85 = 4117 Wp (watt peak). That equals 16 solar panels of 260 Wp (16 x 260 = 4117).

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Why a conversion factor of 0.85?

A 260 Wp solar panel theoretically supplies 260 kWh of electricity per year. In practice, this is not feasible, because the sun does not always shine brightly and the circumstances are not always ideal (ideal = south orientation, roof slope of 30 degrees, no shadows).

That is why when calculating the performance of the panels a loss of performance of 15% is taken into account. The conversion factor 0.85 is derived from this. 

If the roof is perfectly oriented to the south (without shadows) and the slope of the roof is 30 degrees, then use the conversion factor 0.9 when calculating the yield.


Calculate the performance of solar panels

You calculate the performance of a solar panel installation by multiplying the total number of kilowatts per hour by the current price of energy (per kilowatt hour). 

In the calculation you must take into account the conversion factor (0.85 or 0.90). This is due to the return loss of 10-15%. Suppose you have 16 panels of 260 Wp. That’s a total of 4160 Wp. X 0.85 = 3536 kWh. With an energy price of € 0.25 per kWh, that equates to an income of € 885 per year.


Number of people Annual energy consumption Power required (kWh / 85%) Number of solar panels Required ceiling area (in m²) Target price (VAT included, installation, investor) Savings / year (energy price € 0.25 / kWh)
one 2500 kWh 3000 Wp 12 18 € 6,200 € 630
two 3000 kWh 3500 Wp 14 twenty-one € 7,400 € 750
3 3500 kWh 4100 Wp 16 24 € 8,900 € 870
4 4000 kWh 4700 Wp 18 27 € 10,000 € 1000
5 4500 kWh 5,300 Wp twenty-one 32 € 10,800 € 1120
6 5000 kWh 5900 Wp 2. 3 37 € 11,900 € 1250


How can I calculate the required area of ​​the roof?

A solar panel occupies an average of 1.5 m². Calculate the total area you need to install the panels by multiplying the number of solar panels by 1.5. 

For example, if you want to install 16 panels (average home), you need 16 x 1.5 = 24 m² of ceiling space. If you do not have enough space on the ceiling, choose a smaller installation. You still save a lot on your energy bill with this.

Is my house suitable for solar panels?

In principle, all houses are suitable for solar panels, provided that the roof is large enough and strong enough to support the panels. To achieve maximum efficiency, the panels should be oriented between the southeast and the southwest at an angle of 20 to 60 °.

What return can I expect?

The efficiency of solar panels has increased considerably in recent years. This is because the panels are of higher quality and provide much more energy than, say, ten years ago. The price level has also dropped considerably. 

The return will depend on the geographical area where it resides, the climatic conditions of the area and the efficiency of the solar panels.



Factors that influence the return.

As solar panels work with solar energy, it is obvious that there are several external factors that have a great impact on efficiency. These are certainly not important to consider:


1. Type of solar panel

In general, it can generate the greatest amount of energy with monocrystalline or polycrystalline solar panels. Monocrystalline panels are more expensive to buy, but offer higher performance per square meter than polycrystalline solar panels. 

With a small roof surface, it may be attractive to opt for a high efficiency solution. Read more about the different types of solar panels here .


2. Location

The number of hours of sunlight and the degree of sunlight vary by location. For example, the sun’s rays are higher in the west than in the east, which means that it will generate relatively more energy on the coast than in the center or east of the country.


3. temperature

At higher temperatures, a solar panel operates less than at low temperatures. That has to do with the conductivity of the solar cells. The ideal temperature is 25 ° C. At this temperature, a solar panel can generate electricity more efficiently.

Therefore, it is possible to generate more energy on a sunny day in spring than on a hot summer day. It also depends on the method of panel placement. The panels mounted on the roof are cooled naturally by the wind. This is not the case of the panels that are integrated into the ceiling.


4. Shadows

The efficiency of a solar panel can be negatively affected by the shadows of nearby objects, such as lampposts, trees, other houses, a chimney or the presence of an attic window. When installing solar panels, you should take into account the presence of such objects that alter the light.


5. Solar panels of slope angle.

You achieve a high efficiency of solar panels at an angle of 30 to 40 degrees. Many inclined roofs have an angle of 35 °, which makes them suitable for installing solar panels. 

If the roof slope is less than 30 degrees or greater than 40 degrees, you must take into account an energy efficiency lower than 10% on average.

 With solar panels on flat roofs, the angle of inclination is never an objection because the solar panels are placed in a frame whose angle of inclination can be adjusted optimally.


6. Orientation of a solar panel.

You achieve the highest efficiency with a solar panel facing south. Solar panels facing southwest or southeast can yield 10% less. That does not mean that this orientation is not profitable. 

The amortization time is only greater and you may need more panels. The least good orientation is an eastern or western orientation. 

The yield is then on average 20% lower than with an ideal orientation to the south.

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