For better or for worse, the nameplate power rating of PV modules is based on standard test conditions. Efficiency is also essentially an STC rating, as it is a ratio to 1,000 W/m2.
Unfortunately, the 25ºC cell temperature used for testing is not a typical operating condition. In full sun, PV modules typically operate between 15°C and 30ºC (approximately 60°F to 90ºF) above the ambient temperature depending on how they are mounted (flush on a roof, on top of a pole, etc.). Thus, on a 25ºC (77ºF) day, cell temperature may be closer to 50ºC (122ºF). On a hot summer day, cell temperature can routinely approach or exceed 65ºC (149ºF). As cell temperature increases, voltage decreases. A decrease in voltage, at the same level of irradiance, means a proportional decrease in power.
A module’s nominal operating cell temperature (NOCT) is measured with the module exposed to 800 W per m2 irradiance, close to the weighted annual average irradiance a PV array is subject to; and at an ambient temperature of 20°C. This provides a better way to determine the effect of putting the modules in the sun—the lower the number (meaning the lower the cell temperature), the better.
About 330 modules on the list have a NOCT of 45ºC or lower; the lowest NOCT is 43ºC, and only 33 modules have this rating, mostly polycrystalline modules made by Day4 Energy, Astroenergy, DelSolar, and Schüco. On the other end of the spectrum, about 30 modules have an NOCT of 49 or 50º C. The cooler a cell can remain under high irradiance, the more power it can produce. Module current increases with temperature, but by a very small amount. As a result, the overall effect of increased cell temperature is an inversely proportional reduction in power output. Side by side, and all else being equal, a module with a 43ºC NOCT should produce about 3% more power than a module with a 50ºC NOCT.
Module spec sheets include data on temperature coefficients of voltage, current, and maximum power (Pmp), specifying how much those characteristics are impacted by changes in temperature. Temperature coefficients (Tk) of voltage are negative—for each degree Celsius that cell temperature exceeds 25ºC, the open-circuit voltage (Voc) and maximum power voltage (Vmp) drop by a certain amount, either given as a millivolt value or as a percentage. (The average temperature coefficient of open-circuit voltage [TkVoc] for all the modules on the list is -0.33%/ºC). Temperature coefficients of current, while positive, are negligible.
Also a negative value, the average temperature coefficient of maximum power (TkPmp) for all the modules on the list is -0.45%/ºC. The closer to zero the coefficient is, the less power the module will lose when operating at real-world cell temperatures. Nearly 100 modules on the list have a temperature coefficient of power of -0.40% or better; only 19 have a TkPmp of -0.37% or better, including modules from Sanyo, Schüco, Grape Solar, and MAGE Solar.
The NOCT cell temperature is used with the maximum power temperature coefficient to calculate a module’s PVUSA test conditions (PTC) rating. All modules on the CEC list are required to have independent testing done to confirm their NOCT and TkPmp. PTC module power ratings will always be lower than their STC ratings; some states and utilities use the PTC rating to determine up-front rebate incentive payments, and PTC ratings are also used as a more accurate portrayal of real-world performance.
Because modules are typically purchased based on dollars per watt, the closer the PTC rating is to the STC rating, the more power the module should produce, all other conditions being equal. More than 230 modules on the list have a PTC-to-STC ratio of 0.90 or better. For example, that means that a 200 W STC module has a PTC rating of at least 180 W. Just 20 modules have a PTC/STC ratio of 0.92 or better, including modules from Sanyo, Sun Earth Solar Power (aka Nbsolar), and SunPower.