Efficiency of photovoltaic panels: calculation and optimisation

Choosing photovoltaics for your property is a smart investment into the future. In fact, solar panels make it possible to produce clean energy from an inexhaustible source such as the sun, partly guaranteeing domestic electricity autonomy for the consumer with the prospect of enhancing property values, thanks to an eco-sustainable system that can efficiently produce for up to 25 years. According to the Solar Photovoltaic Report of the National Statistical System by GSE (Gestore Servizi Energetici - Energy Services Manager), as of 31 December 2023, 1,597,447 photovoltaic plants were operating in Italy. This growth continues, with constant and ever more competitive improvements and increasing energy efficiency. Find out how to assess the performance of your photovoltaic system and the factors that influence its energy conversion.

What is the efficiency of photovoltaic panels

The energy efficiency of a photovoltaic system is assessed by its yield, a parameter that expresses the amount of solar energy converted into electricity by the system per surface area unit. It is expressed as a percentage and calculated in the laboratory under STC (Standard Test Conditions), i.e. with a radiance of 1000 W/m², temperature of 25 °C, and spectral distribution = 1.5. The higher the efficiency value, the greater the fraction of solar energy impacting the photovoltaic panel, which is then converted into electrical energy. 

Yield, therefore, is a crucial value for assessing the efficiency and technological optimisation of a photovoltaic system. As a result, we rely on this index when making an informed purchase, which can guarantee maximum panel operability according to the geographical-environmental conditions of the installation site and other factors affecting their efficiency. Before we look at those, however, let's see how to calculate photovoltaic yield. As a result, we rely on this index when making an informed purchase, which can guarantee maximum panel operability according to the geographical-environmental conditions of the installation site and other factors affecting their efficiency. Before we look at those, however, let's see how to calculate photovoltaic yield.

How to calculate the efficiency of photovoltaic panels

On average, modern solar panels have an efficiency of between 15 and 20 per cent. This is an ever-increasing value, which is accompanied by an increase in power that places today's photovoltaic panels between 250W and 370W. To calculate their yield, the formula is as follows:

Yield% = (Power/Surface/1000) x 100

Power is measured in Watts (W) and the receiving area of solar energy radiation is measured in m². Take, for example, the 3kW Black Edition photovoltaic system from Enel Energia, where the value 3kW indicates the power of the system, while the power of each solar panel is 405 W (0.405 kWp), slightly higher than the average for the models on the market. 

Considering a photovoltaic surface area of 1.96 m² (given the measurements of each solar panel of 1.89 x 1.04 m), we can apply the yield calculation formula: Yield% = (405/1.96/1000) x 100. As indicated on the data sheet under 'Module Efficiency', this yields approximately 20.6%, which corresponds to the real or actual yield of our photovoltaic system. 

However, the CEI EN 61724 standard introduced the PR (Performance Ratio) as the ratio between the actual yield and a possible theoretical yield. The Performance Ratio thus makes it possible to evaluate the ratio between the energy produced in kWh by the photovoltaic system in a given period of time, and the amount of solar energy that hit the photovoltaic surface according to the electrical conversion efficiency of the solar panels. On average, the PR is between 75 and 85%.

Factors influencing the yield of photovoltaic panels

While the Performance Ratio makes it possible to determine how efficient the energy production of a photovoltaic system is over time, there are other factors, both internal and external, that influence the performance of solar panels. 

External factors that affect performance: 

• Orientation and latitude: the location of solar panels, together with their orientation, known as the tilt angle, determines the solar exposure gradient. A precise and strategic tilt is decisive for greater solar energy absorption and higher energy efficiency. The best orientation, in the northern hemisphere, is to the south, with the tilt angle of the modules increasing along with latitude and decreasing when approaching the equator. In Italy, the optimal inclination is 30-35°, while in Nordic countries it may be more than 60°. 

• Quality of materials: the technology used for solar panel construction significantly impacts the performance of the system. The most efficient photovoltaic panels are monocrystalline, made of high-purity silicon, which are more expensive, but less space-consuming. Polycrystalline panels, on the other hand, are slightly less efficient and, with a lower price tag, are suitable for structures that have a lot of available installation space. Lastly, we have thin-film panels, which are less efficient but lighter and more portable. 

• Temperature and seasonal variations: as semiconductors in solar panels are sensitive to high temperatures, according to production standards, the highest yield is achieved at a temperature of 25°. Seasonal variations, with longer days in summer and shorter exposure to sunshine in winter, also affect performance, along with constant weather changes. The presence of dust or other volatile materials on the cells can also negatively impact yield.

What’s more, there are also internal and therefore, structural, factors with a role that influences yield:

• Inverter efficiency: this is an electronic device that converts direct current (DC) electricity produced by solar panels into alternating current (AC) electricity used for domestic power. Normally, the energy conversion process of an inverter has an efficiency of around 96-97%. It is therefore necessary to calculate about 3% or more energy losses, which are also due to mismatch, i.e. the factor whereby not all panels of the same model and power output produce in the same way. 

• System ageing: the efficiency of solar panels tends to decay by 0.5% each year; this means that after more than twenty years of operation, the energy production efficiency will no longer be the same. For this reason, manufacturers generally offer guarantees every 10, 20 and 25 years on production and manufacturing defects.

Tips for optimising the yield of photovoltaic panels

Optimising the production of a photovoltaic system is therefore related to maximising the operation of the solar panels based on all factors involved. It is therefore essential to position the panels at the right tilt, in line with the geographical latitude, and to provide the system with proper ventilation to avoid overheating. It makes a difference then to choose quality materials for greater durability, to check the layer of dirt on the panels and the accumulation of dust or other fine materials over time, and to carry out constant maintenance to assess the efficiency of the inverters. 

It is also possible to check the condition of photovoltaic modules using a thermal imaging camera, but this requires the intervention of a specialist technician. It also helps to use photovoltaic optimisers, small electronic devices that, placed on the back of photovoltaic panels, monitor their performance and improve it, ensuring the highest possible yield, even in adverse weather and climate conditions. Operating via an Ethernet or Wi-FI connection, they collect and transmit the production data of each individual module and allow it to be monitored and adjusted independently of the others.

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