STC and NOCT – Solar Panel Test Conditions Explained
Solar PV panels come in a variety of different technologies and sizes, so it is important to be able to compare them fairly to one another. International standards have been developed to do just that, and the electrical ratings displayed on solar panel datasheets follow these standards.
Standard Test Conditions (STC)
Standard Test Conditions (STC) are the industry standard conditions under which all solar PV panels are tested to determine their rated power and other characteristics. When a panel is advertised as having a capacity of 350Wp for example, this is the power it is expected to produce under STC.
Since all manufacturers follow this same standard, it gives a fair basis to compare them against each other.
The conditions (from IEC 61538):
Cell temperature: 25°C
Irradiance: 1000 W/m²
Air mass: 1.5
Note that the temperature rating is for the cell within the panel. Not the ambient air temperature. Solar panel cells heat up when exposed to sunlight and cell temperature may be 20-30 degrees higher than ambient.
While STC ratings are useful to compare panels, this sort of comparison does have it’s limits. Just because two panels have the same STC rating, does not mean they will produce the same amount of power on site. For example the panels may have different temperature coefficients, or behave differently under low light conditions. STC ratings also do not say anything about the build quality of the panels.
In addition to rated power, solar panel datasheets typically give values for voltage and current at STC. These are also useful, as they are used in standard calculations for string length and equipment sizing. Adjustment factors are typically applied so that the calculations are valid for all likely conditions, not just STC.
Nominal Operating Cell Temperature (NOCT)
The Nominal Operating Cell Temperature (NOCT) (sometimes referred to as Normal operating cell temperature) is defined as the temperature reached by a solar panel under a set of conditions that are more in line with real world conditions than STC:
Air temperature: 20°C
Irradiance: 800 W/m²
Wind speed: 1 m/s
Note that NOCT uses the ambient air temperature, not the cell temperature as in STC.
As with STC, datasheets often (but not always) give values for power, voltage, and current, under NOCT conditions.
NOCT is useful for comparing two panels, with the same STC rating. A panel with a higher rated power at NOCT for example, will generally result in a higher performing panel.
In general you will not need to use NOCT when designing your solar PV system. The NOCT values for voltage, power, and current are typically not used in sizing calculations. Their main benefit is to enable a comparison of how different solar panels will perform under temperature conditions other than STC.
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