Datasheet Values: Rating of a Solar Panel

The power output of a PV panel is affected by the amount of sunshine it gains and the daytime temperature. Manufacturers state that the power output of PV modules is calculated under Standard Test Conditions (STC), at which all solar panels can be more accurately compared and rated against each other.

There are three standard test conditions which are:

• Solar radiation levels (irradiance) = 1,000 W/m², it refers to the amount of light energy falling on a given area at a given time.
• Cell temperature = 25 ℃ (NOT the ambient temperature)
• Air mass = 1.5, this number is minimized when the sun is directly above as the light only need travel a minimum distance straight down; and increases as the sun moves farther from the reference point and has to go at an angle to hit the same spot.

As all PV modules are tested under these conditions, different modules can be compared using their ‘rated’ power outputs – that is, their outputs as calculated under STC. Standard Test Conditions do not necessarily represent ‘real-world’ conditions; the STC solar radiation level is higher than that normally experienced, and the STC cell temperature is very low (‘cell’ temperature represents the operating temperature of the module’s silicon cells). Therefore, solar modules should not be expected to produce their rated power in all instances.

When you look into the datasheets, you’ll probably find maximum power output at NOCT, which is the wattage that more realistically reflects the maximum power outputs of the solar panels. NOCT stands for Nominal Operating Cell Temperature and refers to testing conditions that are much closer to the real world conditions the solar panels will face every day.

• Irradiance (sunlight) of 800W per square meter.
• Panel surface temperature of 45 (+/- 2) °C.
• A wind speed of 1m/s.
• The air temperature of 20°C.

Looking at a panel’s maximum power output at NOCT is a great way to help you pick modules of the same or similar STC power outputs. By choosing the module with the highest NOCT maximum power output, you’ll be selecting the panel that will perform better in most conditions.

P.S.: solar panel = solar module = PV module = PV panel = panel = module = photovoltaic panels

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