The nameplate power rating of a PV module is determined under standard test conditions (STC): insolation of 1,000 watts per square meter and a 25°C (77°F) cell temperature. Unfortunately, these conditions are not typical of real-world situations. For instance, on sunny days the actual temperature of a cell in a module can be 25°C to 40°C above the ambient temperature, raising the operating temperature well above that at which the module was tested for rating purposes.
As the temperature of a PV cell increases, voltage decreases—this is known as the temperature coefficient. Module current (Isc) actually increases as the temperature rises, but not as fast as the voltage decreases, which means that a module’s output will normally be less than its rated power. The overall effect of temperature may be published as the coefficient of maximum power (Pmax or Pmp) of the module. For example:
- Module rating, STC = 180 W
- Temperature coefficient, Voc = -0.346%/°C
- Temperature coefficient, Isc = +0.057%/°C
- Temperature coefficient, Pmax = -0.478%/°C
In this case, for each degree Celsius that the cell temperature rises above 25°C, the module output (Pmax) will decrease by almost half a percent. A cell temperature of 45°C (113°F—not an uncommon operating condition) would mean a 9.6% loss of power:
(Cell temperature – STC temperature) x temperature coefficient Pmax = Adjusted power at operating temperature
(45°C - 25°C) x -0.478%/°C = -9.56%
180 W x -0.0956 = -17.2 W
180 W – 17.2 W = 162.8 W at 45°C cell temperature
In cold, sunny conditions with cell temperatures below 25°C, power increases proportionally. However, “less than labeled” output is normally the case with PV modules. For more realistic ratings, the www.gosolarcalifornia.org Web site lists module PTC ratings. Based on different test conditions—a 20°C ambient (rather than cell) temperature—PTC ratings typically run 10% to 20% less than the STC ratings for the same module.
Leaving an air space between the roof surface and mounted modules allows for airflow to keep operating temperatures as low as possible. Four inches of space is the minimum, though sometimes aesthetics trumps performance. Adding a space between rows of modules can have a similar effect (and also allow easier access).
Pole- and ground-mounted arrays typically operate at cooler temperatures compared to roof-mounted systems, and usually offer the ability to match the ideal angle and orientation. However, even though a few more kWh can be wrung from a ground- or pole-mounted system, increased rack and labor costs mean that the cost per kWh is likely to be higher than a roof-mounted system.