Electrical Characteristics of PV Modules

STC

The industry standard against which all PV modules are rated and can be compared is called Standard Test Conditions (STC). STC is a defined set of laboratory test conditions which approximate conditions under which PV modules might be used. Although there are other standards that offer better real-world approximations, STC offers the most universal standard. The same standard is also used to evaluate potential installation locations, since it is the basis for Insolation values. STC includes three factors:

1. Irradiance (sunlight intensity or power), in Watts per square meter falling on a flat surface. The measurement standard is 1 kW per sq. m. (1,000 Watts/m2)
2. Air Mass refers to “thickness” and clarity of the air through which the sunlight passes to reach the modules (sun angle affects this value). The standard is 1.5.
3. Cell temperature, which will differ from ambient air temperature. STC defines cell testing temperature as 25 degrees C.

Maximum Power Point — Go For The Knees!

Every model of solar module has unique performance characteristics which can be graphically represented in a chart. The graph is called an “I-V curve”, and it refers to the module’s output relationship between current (I) and voltage (V) under prevailing conditions of sunlight and temperature. The curve looks like a seated person’s leg:

Theoretically, every solar module has multiple I-V curves (several of which are shown above for one particular module)— one each for all the different combinations of conditions that would affect the STC rating parameters above: temperature, air mass, irradiance… that’s a lot of possible graphs! You can see from the illustration above that this module loses voltage as the cell temperature increases; that effect is common to all crystalline modules. Because of Ohm’s Law (and the equation Power = Voltage x Current), the result of reduced voltage is reduced power output. The ideal position on any I-V curve—the sweet spot where we can collect the most power from the module—is at the “knee”. That’s the maximum power point (MPP), and you can see that its position changes with temperature and irradiance.

In battery-based PV systems, a maximum power point tracking (MPPT) charge controller monitors the array constantly to find the ever-changing MPP and thus capture the most power from the array. In straight grid-tied systems, MPPT technology is built into all the inverters, so these systems tend to have very high efficiency.