Answer:

There are various parameters that affect the electricity production delivered by solar cells. The sun's position relative to the panels, cloud cover, temperature, wind, shading, and dirt are all factors that influence the output obtained from the panels. The nominal power of solar cells is typically specified at a solar irradiance of 1000 W/m2 and a panel temperature of 25°C. This corresponds to approximately the solar irradiance on a clear day with a blue sky during the summer months. However, the panel temperature often exceeds 25°C during summer days.

• Sun's position relative to the solar cells.

The maximum power is obtained from the solar cells when sunlight falls perpendicular to the panels. For a south-facing solar panel installation with a roof inclination of 35°, this occurs at midday during the month of June. It is the sun's position relative to the solar cells that gives the production curve its characteristic clock shape on a cloud-free day.

• Cloud cover:

Cloud cover significantly affects solar irradiance. A clear day with a blue sky yields the highest solar irradiance, while a day with thick cloud cover results in low solar irradiance. Many days may also have varying degrees of haze and thin clouds, which also affect solar irradiance.

• Temperature:

The efficiency of solar cells is temperature-dependent. For common crystalline silicon cells (which are the most common on the market), efficiency decreases by approximately 0.35 - 0.40% per degree Celsius. That is, if the panel temperature rises by 10°C, the power output from the panels decreases by 3.5 - 4%. Note that panel temperature is not normally the same as outdoor temperature. On a sunny day, the panels are heated by the sun, while on a windy day, this helps reduce heating. Although the nominal power of panels is specified at 25°C, the actual output on a day with 25°C outdoor temperature may be significantly lower because the solar cells are heated by the sun. On a sunny day with weak wind, the temperature of the solar cells may be 20-25°C higher than the outdoor temperature, resulting in 8-10% lower output than nominal power.

• Wind:

Wind affects the temperature of the solar cells. Therefore, on a sunny and windy day, the solar cells can deliver more power than on a sunny day with calm winds, even if the outdoor temperature is the same. Windy days cool the panels more effectively.

• Shading:

Shading reduces the output from solar cells because less sunlight reaches the panels. The extent of the power reduction depends on the number of panels in a string affected and how the power optimization in the system operates.

• Dirt:

Dust, pollen, leaves, and algae all contribute to reducing the output from solar cells. Normally, panels are relatively well cleaned by rain in Swedish climate, but in some cases, manual cleaning may be necessary.

• Aging:

All solar cells age and lose efficiency over time. However, this process typically occurs very slowly. For example, many manufacturers provide power warranties of 80-90% of the original output after 25 years. Practical experience shows that this process is usually even slower, although it can vary depending on local conditions. A reasonable assumption may be that panels degrade by 0.5% per year on average.

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