“Pmax”, “Vmp”, “Voc”, “Isc”… So many mysterious terms that can be found on the technical sheets of photovoltaic panels. We are going to explain this lexicon to help you understand all that this implies and better understand the design of your photovoltaic installation.
The data sheet of a solar panel contains a wealth of technical information. Some are obvious to understand such as weight or dimensions. But very quickly, we notice that several levels of power, voltage and intensity coexist and that other information seems almost esoteric. We will therefore clearly detail the elements that you can find on the technical sheet of a solar panel and above all explain their impact on your energy production.
The maximum power that can be delivered from a solar panel (Pmax)
This is the value most highlighted by manufacturers. It corresponds to the maximum power that a solar panel can generate under conditions called optimal by the manufacturer (solar power of 1000 W/m², a temperature of 25°C, ideal inclination, etc.). Do not hesitate to read our tutorial explaining how to estimate the energy yield of your solar panels.
The optimal voltage at the terminals of a photovoltaic panel (Vmp)
The voltage supplied by a solar panel is zero at night and reaches its maximum when the sun is at its highest and most generous in energy. It can therefore vary greatly during the day and this also influences the energy efficiency of your installation. The Vmp, or Maximum Power Voltage, corresponds to the optimum operating voltage, the one that makes it possible to get as close as possible to the Pmax and therefore to draw the most possible energy from the sun. The Vmp is automatically deducted and applied to the panels by the MPPT (Maximum Power Point Tracker) modules connected to your solar installation. This can be an inverter, a solar charge controller or a battery charger. This is a value that therefore makes it possible to optimize its production.
The power tolerance of a solar panel
Even in the laboratory, the Pmax varies slightly over time. The tolerance of a solar panel therefore corresponds to the margin of error in the delivery of Pmax under conditions said to be optimal by the manufacturer. Thus, if you have panels with a tolerance of ±3% and a Pmax of 1000 Wp, the actual maximum power may vary from 970 watts to 1030 watts.
The maximum voltage or Max Voltage
Please note that this is not the maximum voltage that a solar panel can generate. This information refers to the voltage not to be exceeded. But this figure should not be understood for a single panel, it applies to your entire photovoltaic installation. If your panels indicate a Max Voltage of 1000 V, then the voltage generated by all of your panels connected in series must not exceed 1000 V of voltage. For example, if you have panels of 1000 V in Max Voltage and 50 V of individual voltage, you must not exceed the 10 panels mounted in series. One more panel and you will reach the cumulative 1050 V, which risks damaging your panels and associated devices.
The no-load voltage, or Voc of a photovoltaic panel
As with any electrical device, there is a voltage that must not be exceeded for proper operation. The Voc or off-load voltage therefore corresponds to the maximum voltage tolerated when the panel is not connected to anything. When you connect it to an inverter or any other device, you must be certain that the Voc resulting from the sum of the Voc of each of your solar panels is lower than the maximum voltage admissible by these devices. Otherwise, in the best case, you will damage the equipment, in the worst, it can start a fire. This information is therefore important for sizing your electrical installation as well as possible. Please note that this Voc has been measured under so-called optimal conditions for the manufacturer, with sunshine of 1,000 W/m² and a temperature of 25°. If you have a strong sun and a temperature below 25°, the voltage produced may be higher than the Voc. However, these are elements that will be taken into account by your installer.
Ideal intensity or Imp
The intensity of the current produced by a photovoltaic panel is highly variable depending on the level of sunshine during the day. The stronger the latter, the higher the intensity delivered and vice versa. The ideal intensity is the equivalent of the Vmp, so it corresponds to the optimal value to obtain the best energy yield. Again, MPPT devices can determine this optimal intensity automatically.
The maximum current in the event of a short circuit, or Isc or Isc
The stronger the sun shines, the more intense the current generated by photovoltaic panels will be. The Isc or Icc (Short Circuit or Court-Circuit) is very important information which is expressed in Ampere. The size of your entire installation will depend on this value. Thus, the choice of cables, connectors and all devices connected to the solar panels must be made according to this value. By making the right choices, you will avoid overheating which can lead to energy loss or worse, fires.
The applicable temperature coefficients
As we have already seen, the temperature influences the performance of a solar panel. This includes Maximum Power (Pmax), Open Circuit Voltage (Voc) and Maximum Current (Isc). Manufacturers can indicate how these parameters evolve by giving the temperature coefficient of Voc, Isc or Maximum power. It will be noted in percentage of evolution per additional degree as shown in the example below:
- Temperature coefficient of Voc [%] -0.34/°C
- Temperature coefficient of Isc [%] +0.045/°C
The operating temperature in °C
The photovoltaic cells that make up a solar panel are usually fixed under glass. This produces a kind of “greenhouse effect”, which will raise the temperature of the entire panel. Each is designed to operate within a temperature range, if it is exceeded or too low, operation is no longer ensured optimally and the device may suffer damage. However, the panels are designed for optimal use at a temperature of 25°. Thus, in summer, solar radiation can raise the temperature of the panels well beyond 40°, even if the ambient temperature is only 25°. This has the effect of greatly reducing the energy efficiency. Hence the importance of good ventilation behind the solar panels to avoid the accumulation of hot air.
The weight of the solar panel
This may seem futile to you and yet unless you install your panels on the ground, this data is essential. Indeed, the frame of a roof can only support a limited weight. If you overload it, then it risks damaging the roof and there is even a risk of collapse.
The maximum surface load
This information is more accurate than just the weight of a panel. It allows you to know the pressure that each panel will exert on the roof. It is expressed in kilograms per square meter (kg/m²).
The maximum surface load
It is simply the ratio between the production of electricity and the production capacities of the solar panels under optimal conditions which are, among other things, sunshine of 1,000 W/m², an inclination in line with your longitude and a temperature of 25 °. The yield is different depending on the technologies used, the two main ones being:
- monocrystalline silicon panels, dark in color: they offer on average an efficiency of 18% to 19%;
- those in polycrystalline silicon, blue in color: their average efficiency is less efficient (12 to 15%), but they are less expensive to purchase.
The photovoltaic panels are designed to withstand bad weather, which includes hail. This information can be given in two ways:
- The resistance of the glass in Kilopascal (kPa), the most technical information, but which will perhaps not be very meaningful for the general public.
- The second way to see it displayed is by indicating the diameter and the maximum speed of the hailstones that the panels can support. Thus, in France they must meet the standard of the International Electrotechnical Commission CEI 61 215, which withstands hailstones up to 1.25 cm in diameter at a speed of 140 km/h. There is a variant where the diameter is replaced by the weight of the hailstone.
The type of photovoltaic cells
There are several types of solar panels, the two main ones being monocrystalline. [couleur bleue] and polycrystalline [noir]. These are the least effective, but also the least expensive.
All these data are important and allow you to properly size and above all optimize the performance of a photovoltaic solar installation. However, unless you are of a fairly advanced level, it is better to leave your calculations in the hands of professional installers.
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