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Solar panel efficiency: useful evaluation factors

Solar panel efficiency strongly depends on different variables. Discover the factors that can influence the performance of the entire system

The installation of a photovoltaic system can have numerous advantages from both an environmental and economic point of view.

However, the efficiency of solar panels is influenced by various aspects that significantly affect annual energy production. Inadequate evaluation of these elements could compromise the efficiency of the entire plant.

To obtain the desired results and correctly size a photovoltaic system, I recommend you to try immediately the trial version of a photovoltaic software that can assist you throughout the design.

What affects solar panel efficiency?

During the design phase, it’s always useful to consider the characteristics that influence the amount of energy captured and effectively produced and therefore the relating system efficiency.

But before defining the efficiency of the photovoltaic system, you’ll also need to analyze the efficiency of each individual photovoltaic panel.

The power of the photovoltaic panels is calculated under the so-called STC (Standard Test Condition) conditions, i.e. with a solar radiation of 1000 W/m², temperature of 25 °C, spectral distribution = 1.5.

By definition, the efficiency of a panel is the amount of solar energy that it can convert into electricity per unit area and is always the maximum efficiency under STC conditions.

The efficiency of a photovoltaic panel is calculated by knowing the peak power and its dimensions as shown below:

PV efficiency calculation formula

PV efficiency calculation formula


  • “Power” is the peak power expressed in W;
  • “Surface” is the surface of the panel in m²;
  • 1000 is the radiation of 1000 W/m²;
  • 100 is the percentage factor.

Let’s take a look at a practical example: in the case of a 250 Wp panel, measuring 1.65 m x 1 m (surface area equal to 1.65 m²), we get:


Efficiency = (250 / 1.65 / 1000) * 100 = 15.15%


The dimensions and peak power can be acquired from the Panel technical data sheets or from the labels placed near the connector box.

At this point, we can introduce the concept of efficiency of the photovoltaic system. It expresses the relationship between the quantity produced and the maximum amount that can be produced.

Plant performance is influenced by several factors (and not just module performance).

Factors Affecting Solar Panels Efficiency

The main factors that affect plant performance are:

  • the intrinsic characteristics of the panels;
  • the PV panel inclination;
  • panel exposure ;
  • effects of shading;
  • the distance between each row;
  • energy loss.
Performance of photovoltaic panels: influencing factors

Performance of photovoltaic panels: influencing factors

Orientation and inclination of solar panels

Before installing the photovoltaic system, it is advisable to verify that the panel has the correct exposure to the sun and the appropriate inclination.

To capture the maximum solar radiation, the solar panels must be oriented towards the South; however, the modules exposed in the South-East and South-West direction can also be high-performance.

The inclination (tilt) is variable and depends on the latitude. Guidance values at our latitudes are 20-40°.

To determine exactly the best value depending on the installation area, I recommend you to rely on a photovoltaic software, which allows you to estimate the photovoltaic solar production from concrete solar radiation data taken from the main reference climate databases:

  • PVGIS for Europe, Africa, Mediterranean Basin and South-West Asia.




Distance among the panels

In the case of horizontal surface installation, in addition to establishing orientation and inclination, we’ll also need to calculate the distance among the module rows in order to find the right compromise between the maximization of installable power on the surface and the minimizing due to shading between rows. This is a fundamental aspect to maximize the PV system efficiency.

To establish the optimal distance between the rows, consider the surface on which the system will be installed:

  • if the surface is inclined, the panels are generally installed coplanarly with the roof slab and doesn’t require any distance calculation (inclination is the same as the slab);
  • if the system is installed on the ground or on a flat roof, this particular scenario requires a calculation in order to determine the correct inclination and distance between rows avoiding any shading due to adjacent modules, also in consideration of the different time and periods of the year.

This basically means that we need to calculate the height (H) and the distance (D) between one row of modules and the other. In the event that the distance is not calculated correctly, the phenomenon of load-bearing shadows can occur, that is, the modules placed in front create shadows on those behind.

Distance between solar panel rows

Distance between solar panel rows

To manage the phenomenon of load-bearing shadows without wasting time and to make sure you have reliable results, I suggest you rely on photovoltaic software that can automatically calculate the minimum installation distance of the rows of photovoltaic panels on any surface (horizontal, vertical or inclined).

Shading coefficient

Another key factor in ensuring optimal panel performance is the shading coefficient, which is calculated using the so-called shading analysis.

The Shading analysis phase invloves the study of the architectural or natural elements surrounding the PV system, which allows the designer to verify how they may affect incident solar radiation and whether it can cause shadows on the solar capturing surfaces.

A correct shading analysis involves an inspection, during which an inclinometricanalysis is carried out, that is, the presence of any objects that could cause shadows is verified and their azimuth is calculated.

Azimuth detection is addressed as follows:

  1. position yourself in the actual site location where the system is to be installed and using a compass simply measure which cardinal point the shading element is located with respect to our reference position (in this case the tree is located to the East);
  2. its elevation value is then read (for example 10°);
  3. the inclination value is calculated: looking towards the tip of the tree with binoculars or a camera, which represents the boundary between the celestial vault and the element that can cause shading, the inclination level is observed and is represented by the angle α. To measure α, we can use the inclinometer (a goniometer with a lead wire attached), which allows you to understand your altitude compared to a horizontal view.
Azimuth measurement of a tree

Azimuth measurement of a tree

This factor is calculated for each object that may cause shading on your PV panels. Subsequently, these values will be integrated on a solar map of Cartesian coordinates (solar diagram) and analyzed with a specific photovoltaic software that allows to verify if during some periods of the year, those elements will bring shadows on the plant.

Solar diagram for shading checks

Solar diagram for shading checks

For example, the image shows a solar diagram in which we can see:

  • the cardinal points on the X-axis;
  • elevation values on the Y-axis;
  • the solar curves represented by the yellow lines;
  • the geometric description of elements that can cause shading (e.g. green circles to represent trees).

The precise calculation of the elevation and orientation values of the elements surrounding the modules can become a very difficult process if you do not rely on specific reference software. With the Photovoltaic calculation software Solarius PV, you can evaluate the shadows coming from all the obstacles far or close to the plant (heights, buildings, trees, etc.) starting from a simple photographic survey and create the solar diagram of the installation site.

PV system Energy Losses

To calculate the actual efficiency of the solar plant, the various energy losses that can occur during the conversion of solar energy into electricity must be considered.

In this regard, we can consider the balance of system (BOS) which expresses as a percentage the energy losses in the system due to various factors, such as the coupling between the various PV modules, the connections with the inverter, the losses in the panels, the losses in the conductors, etc.

These can be summarised as:

  • losses due to reflection;
  • losses due to shading;
  • mismatching losses;
  • temperature losses;
  • leaks in continuous circuits;
  • inverter losses ;
  • losses in alternating circuits.

The losses in the DC panel depend on the length of the cables: longer wiring will cause greater losses.

The mismatching phenomenon is due to the small differences in electrical characteristics that exist between the various modules; depending on their configuration within the strings, this can result in a decrease in produced energy.

To to be able to calculate the actual efficiency of photovoltaic panels, it is advisable to consider all the factors mentioned above. This strongly underlines the fact that it can turn out to be very useful to use a photovoltaic software that you can download for free in order to evaluate shadows, calculate solar irradiation and consider the various energy losses.