Applying Temperature Correction Factor

The testing conditions to measure photovoltaic cells or modules nominal output power.  Standard Test Conditions are used with performance figures so that modules can be compared effectively. Test conditions have been developed to more represent performance of PV devices in real-world conditions.

Standard Testing Conditions [STC]

=plane of array irradiance of 1000W/m2

=1.5 air mass

=25° C

Irradiance level is 1 000 W/m², with the reference air mass 1,5 solar spectral irradiance distribution and cell or module junction temperature of 25°C.

Project Testing Conditions [PTC]

=plane of array irradiance of 1000W/m2

=1 meter/sec wind speed

=20° C

=1.5 air mass

Nominal Output Ratings

When shopping for a solar electric system for your home or business, make sure that the dealer that you are speaking with is quoting you in actual PTC rated kilowatt hours per day.

We have noticed several dealers that are listing only STC rated kilowatt hours per day which is not the rating that the CEC uses to determine rebate amounts. Make sure when comparison shopping that you are comparing apples to apples. Insist upon receiving an actual system PTC rating which includes the efficiency rating of both the modules and inverter.

PTC refers to PVUSA Test Conditions, which were developed to test and compare PV systems as part of the PVUSA (Photovoltaics for Utility Scale Applications) project. PTC are 1,000 Watts per square meter solar irradiance, 20 degrees C air temperature, and wind speed of 1 meter per second at 10 meters above ground level. PV manufacturers use Standard Test Conditions, or STC, to rate their PV products.

The PTC rating, which is lower than the STC rating, is generally recognized as a more realistic measure of PV output because the test conditions better reflect “real-world” solar and climatic conditions, compared to the STC rating. All ratings in the list are DC (direct current) watts.

Neither PTC nor STC account for all “real-world” losses. Actual solar systems will produce lower outputs
due solar cell voltage drops about 0.08 volts per
°C. In environments which exceed 25°C.
Thus, an STC rating of 17 volts can actually become a PTC rating of 15 or 16 volts. Using
Ohm’s Law, volts times amps is equal to watts which equals power, so a reduced voltage, means
reduced watts.

Temperature Correction Factor

To take in account a drop of cell voltage due to temperature= [(operating cell temperature C° – 25°C) x (-0.5%/C°)], and to correct to the testing conditions. Use correction factors when solving problems with STC, PTC applied in the problem.

NEC table 690.7–Ambient Temperature Correction Factors

Ambient Temperature (°C)

Correction Factors

Ambient Temperature (°F)

25 to 10


77 to 50

9 to 0


49 to 32

-1 to -10


31 to 14

-11 to -20


13 to -4

-21 to -40


-5 to -40

When encountering testing conditions (e.g PTC, STC) in exam questions or real life design situations you’ll want to use the Voc and Isc for voltage and amperage integers.

When using  the Temperature Correction Factor:

use Voc (open circuit voltage)
use I
sc      (short circuit current)

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