Performance of photovoltaic panels – How is it calculated?
In this article we will talk about the performance of photovoltaic solar panels, which is the way to calculate it. The formula for the calculation (simplified in symbols for non-experts) is as follows: The performances of photovoltaic systems are susceptible to variations: the variables and calculations to make the correct considerations on performance and duration.
Performance of photovoltaic panels – How is it calculated?
Photovoltaic panels are capable of converting the sun’s energy into electricity in different percentages depending on different factors. The degree of efficiency of a photovoltaic system indicates the percentage of energy captured and transformed into electricity with respect to the total amount reached on the surface of the module itself and, therefore, can be considered a correlation index between the watts supplied and the surface occupied .
What does the performance of a photovoltaic panel depend on?
The performance of the solar panels of those declared by the manufacturer is always calculated under standard conditions, with an irradiation of 1000W per m2 of surface, a temperature of 25 ° C and equal to 1.5 spectral distribution. But the yields of photovoltaic panels are not constant, it also varies substantially depending on several factors, such as:
- characteristics and performance of the different materials (monocrystalline, polycrystalline, amorphous), of the inverters and of the other components of the system
- correct exposure of the modules to solar radiation (the optimal conditions for Italy are: exposure to the south, but also to the southeast or southwest currently good results)
- tilt of modules (optimal 30-35 degrees) and any temporary shading
- operating temperature of materials, as request to reduce performance in excessively hot environments
- composition of the light spectrum.
In aerospace panels, yields have reached 50 percent, but costs do not allow for large-scale deployment. In silicon-based panels currently on the market, the values are instead around:
- 21-25 percent in heterojunction moduli
- 19-21 percent on monocrystalline silicon modules
- 16-18 percent in polycrystalline silicon modules
- 10 percent on microspherical silicon modules
- 8.5 percent in amorphous silicon modules.
It is estimated that the efficiency of the panels will tend to grow thanks to technological innovation that has currently reached good levels , making the panels increasingly convenient from an economic point of view.
It follows that for the same required electricity production, the surface occupied by an amorphous photovoltaic system will be more than double compared to an equivalent crystalline photovoltaic field. However, to increase the producibility of the systems by 20-30%, it is possible to mount the panels on supports with variable orientation (mono or biaxial), capable of constantly following the movement of the sun (tracking systems).
The calculation of the Performance of Photovoltaic Panels i is quite simple because it correlates the power with the surface of the implant. The mathematical formula to calculate the performance of photovoltaic panels is as follows:
% Efficiency = (power / area / 1000) * 100
The power is the peak power expressed in W, the surface is the surface of the panel in square meters including the frame, 1000 is the irradiance of 1000W / m2, 100 is used to obtain the efficiency in percentage.
Let’s take a practical example, using a 230W peak panel, with dimensions in meters of 1,650 × 0.98 and therefore 1,617m2 of surface, and apply the formula:
Return% = (230 / 1,617 / 1000) * 100 = 14.22%.
The maximum efficiency of the panel is 14.22%, this means that at a time of day with a soil irradiation of 1000W / m2 and a temperature of 25 ° C, our panel will convert 14.22% of solar radiation into electricity. The dimensions and peak power can be found in the technical sheets of the panels or on their labels.
It should be noted that we are only talking about maximum performance under STC conditions (standard test condition) and how to calculate it, we are not talking about the productivity of photovoltaic systems, which depends on much more complex factors.
We also add that if there are no space problems, the efficiency of the panel is a relatively important data, while it must be taken into account more important when it is necessary to create systems with the maximum possible power on the available surface. In the latter case, the maximum power per unit area cannot be neglected.
Operating temperatures
Better outdoors! It is often overlooked that photovoltaic systems work best at a temperature of 25 ° C. If we think that temperatures close to 40 ° C can already be reached in summer in our latitudes (and on the roofs of certain buildings even 50- 60 ° C) it is easy to understand how the efficiency
of the system may decrease slightly due to heating of the forms. In common silicon cells, the drop in efficiency is about 0.4 percent for every degree ° C. The colder temperatures in northern European countries actually compensate for the lower level of irradiation in those areas.
Duration of photovoltaic solar panels
Another factor that affects the performance of a module is the deterioration of its performance due to the natural fatigue of the materials, with values that can reach an average of around 1 percent per year. To ensure the quality of the materials used, it is common practice for manufacturers to cover the deterioration of panel performance over time with an appropriate warranty. The most common warranty today is 90 percent of the nominal for 10 years and 80 percent of the nominal for 20 years. It is good to always ask for product warranties at the time of purchase. Current photovoltaic modules have an estimated life of about 40-50 years, although it is plausible to assume that they will be decommissioned after a life cycle of 20-25 years, due to the obsolescence of the technology of the entire system.
Correct power
Photovoltaic systems are characterized by a “maximum power” indicated with kWp . To get an idea of the peak power that will be installed to allow full coverage of the annual electrical needs of a specific user (for example, a single family or a company), one must calculate:
peak power (kWp) = average kWh production per year of the panel for the area where the panel is to be installed / annual user consumption
In fact, the annual energy consumption of users is divided by 1200, a value relative to the average annual kWh production of a panel.
Taking into account that the average electricity consumption in Italy of a single-family home is 3000-4000 kWh / year, a photovoltaic system for domestic use in a city in northern Italy should have a power of between 2.5 and 3.3 kWp (slightly less in central and southern Italy). due to its greater insolation).
To have a nominal installed power of 1 kWp, generally 8m2 (for the most efficient modules) to 15m2 (for the least efficient modules) are needed. The most common crystalline silicon PV modules range in size from 0.5 m2 to 1.5 m2 (sometimes even 2.5 m2 in samples for large plants). For a 3 kWp system, therefore, at least 20-24 m2 of available surface is required.
The correct dimensioning of a photovoltaic system is in any case the result of a more complex calculation that takes into account multiple and variable factors: the solar radiation of the place in question; altitude and climatic characteristics; exposure of the building and / or land; surface available for the plant; inclination and orientation of the modules; conversion factor of the photovoltaic module used; electrical assembly of the system; period of use of the system (always, only weekends, holiday periods, etc.); users to be used and their power; system supply voltage (12, 24, 220 volts).
