Peak power of your panels and practical power of a solar installation
Are you going to buy or have you recently bought a solar installation? Congratulations, it's the path to energy autonomy, a great contribution for your wallet and for the planet!
However, you have questions about the performance of your installation: you have purchased a panel power of 4500 Wc, but your monitoring never shows you more than 3600W of instantaneous power in production. This is completely normal and we explain why: what is the difference between peak power and nominal operating power of the panels , why do we size the inverters at 70 or 80% of the peak power, we answer all these questions?
Power of solar panels: Wc purchased and W produced
The generator of your solar system is the photovoltaic panel . It is made up of unit cells that generate a flow of electrons whose power varies according to the quantity of light received (irradiation) and the temperature of the cells . In short, at a given moment:
- The more light it receives, the more it produces: shade and cloudy weather are unfavorable to your instantaneous production, as is an orientation that is not perpendicular to the sun.
- The hotter it is, the less it produces: heat reduces the production of a panel.
Temperature coefficient of solar panels
The order of magnitude of the performance degradation of a monocrystalline silicon solar panel is 0.35% per degree Celsius above the reference temperature of 25°C. That is to say that for any temperature rise of 10°C at the cell level, a loss of 3.5% of the panel's performance is observed. This temperature coefficient is a technical characteristic of the panel given by the manufacturer. It systematically appears on the technical sheet.
Purchased toilets: measurements under standardized test conditions
To allow a comparison of solar panels with each other, and their performance in particular, the European standard IEC 60904 defines very precisely the laboratory measurement conditions (and more precisely its section 9 for pulse panel performance tests also called flash test). These standard test measurement conditions (STC for standard test conditions) are draconian. The measurement is controlled by comparison with a calibrated reference device (reference cell) that only a few laboratories in the world are able to deliver. The measurement conditions include:
- The temperature measured directly on the cells to within 1°C, this temperature is generally 25°C.
- The angle of incidence of the luminous flux
- the power (generally 1000W/m²) and the spectral nature (AM1.5) of the luminous flux as well as its homogeneous distribution on the measurement surface
Under these very controlled conditions, the voltage and current of the panel are recorded during a light pulse. This measurement results in the peak power value of the panel that will be sold (with a tolerance usually positive of a few Wc or a few % depending on the manufacturer). The flash measurement is very fast, so the cells of the panel do not have time to heat up during this measurement.
W products: nominal operating conditions
However, the operation of the panels in real conditions is very different from STC conditions . Indeed, as soon as a solar cell is exposed to a luminous flux, part of the energy of the flux is transformed into current (this is the photovoltaic effect that we use in our solar system, frequently of the order of 20 to 22%) and the other part is dissipated as heat (i.e. a little less than 80%) which very quickly leads to a rise in the temperature of the cells . The heat dissipation of the cells is then a function of the nature of the material of the solar panel, but also of the wind speed on the panel.
In order to have a more realistic picture of the optimal performance of solar panels in real conditions, manufacturers provide, on the technical data sheets of the panels, values measured in more representative conditions. The nominal operating conditions (NOCT or NMOT for nominal operating temperature conditions) differ significantly from the STC conditions but are more representative of the optimal operation of the panel in real conditions.
Nominal operating conditions are not standardized like STC conditions but manufacturers have agreed on a certain number of values:
- the outside temperature of 20°C
- the luminous flux of 800W/m²
- wind speed of 1 m/s
For example :
- On a Tongwei TH420PMB7 monocrystalline silicon solar panel, when the ambient temperature is 20°C, the wind speed is 1m/s and the luminous flux is 800W/m², the actual cell temperature is 42.3°C. As a first approximation, with a temperature coefficient of 0.34%, we then have a power degradation compared to STC conditions of 0.34*(42.3-20)=7.58%.
- If we extrapolate this calculation to an ambient temperature of 35°, assuming that the cells heat up as much at 35°C as at 20°C, we then have a power loss of 12.7%.
- In practice, on roofs where ventilation can be moderate and heat reverberation from the tiles significant, it is not uncommon to see cell temperatures above 70°C.
- Similarly, a wind speed of 1m/s
Comparison of performance values between standard conditions and nominal conditions for a solar panel
For the same solar panel, the peak power value between standard conditions (lab test) and nominal conditions (optimum in real conditions) can therefore vary significantly. This information is generally present on the technical data sheet of solar panels. For example, these different values are highlighted on the data sheet of a 425Wc Trina Vertex panel.
To go further, the power values measured in the 2 types of conditions are represented in the following table for different solar panel references:
|
Brand
|
Sign
|
Wc STC
|
W NOCT
|
% variation
|
Temperature coefficient Pmpp%
|
|
Trina
|
TSM-DE09R.08
|
425
|
321
|
-24.47
|
0.34
|
|
Trina
|
TSM-DE09.05
|
390
|
294
|
-24.62
|
0.34
|
|
Tongwei
|
TW420PMB7 44SCS
|
420
|
316
|
-24.76
|
0.34
|
|
Tongwei
|
TW410PMB7 44SCF
|
410
|
309
|
-24.63
|
0.34
|
|
Longi
|
LR5-54HIH
|
410
|
306.56
|
-25.23
|
0.34
|
|
Bisol
|
Duplex BDO
|
375
|
NC
|
0.35
|
|
|
DMEGC
|
DM410M10-B54HST/HBT
|
410
|
311
|
-24.15
|
0.335
|
These measurements show that, in the best case, solar panels produce 25% less in real representative conditions than the peak power measured in laboratories .
It may happen, on particular days, that these nominal performance values are exceeded if:
- The outside temperature is low, but the sun is intense (10° and below)
- The luminous flux is concentrated: certain cloud edge phenomena or reverberation on the snow can induce a light concentration effect. On the surface of the panel, we will then have more than 1000W/m² of irradiation, which would correspond to more than a sun. This is rare, but it happens on certain spring days with large scattered white clouds.
The illustrations below show the productions on the same installation with Trina 405Wc panels on March 15, 2023 and July 20, 2023 during solar noon.
We note that it is on spring days, with scattered sunshine, that the highest production powers per panel are reached (385 to 390W per panel in March compared to 334 to 349W per panel in July).
What impact on my solar system: choice of inverter?
The fact that solar panels, in real operation, very rarely produce more than 75% of their peak power has an impact on the sizing of another major component of the solar system: the inverter .
The inverter transforms the direct current from the panels into alternating current that can be directly used in the home's distribution network. An inverter will have a better conversion efficiency if it is operated in a regime close to its nominal power.
Therefore, in order to maximize the conversion efficiency and minimize investment costs , it is technically relevant to choose an inverter whose nominal power is between 70 and 90% of the peak power of the solar field . It is indeed counterproductive to oversize the inverter in relation to the power of the solar field.
For more information on inverter sizing in a solar system, you can find our article on the subject .
Instant power and energy harvesting
The fact that the instantaneous performance of solar panels degrades with increasing temperature can lead to misinterpretations. For example, there are rumors in the media that solar panels produce less in summer.
It is important to separate two quantities: the instantaneous power produced and the total energy . Solar panels produce very well in summer, even in very hot weather. They produce a little less than they could because of the loss of efficiency linked to the rise in temperature, but summer days are longer, and with very few clouds, so daily energy harvests are excellent.
So, you have understood: the instantaneous power records, expressed in Watts W or kilowatts kW, of a solar system are not obtained in the middle of summer, but during cool days of strong irradiation, generally in spring. At the beginning of summer, you will see records of daily energy production expressed in kWh/day (the days are longer and very sunny, especially in July, shortly after the solstice).
