Solar Photo Voltaic (PV) modules and systems convert sunlight to electric energy. When sunlight strikes a PV module, it produces a voltage at its output terminals. If a load is connected to these leads, current flows. Electric power is measured by the product of voltage and current. The duration and intensity of sunlight incident on a PV module determines how much electrical energy it produces. The energy also depends on the electrical, thermal, and optical properties of the module. Several models and algorithms exist to predict the electrical output of PV modules. These performance models are used to size, design, and predict performance of PV plants.
While these performance models are detailed and empirical, a rough first order relationship between the weather input and electrical output can be described. The incident solar radiation primarily determines how much current a module produces. The higher the incident radiation, the higher the current. Incident irradiation is reduced by the cosine of the angle-of-incidence on the module surface. Hence, solar trackers produce more energy compared to fixed tilt systems. The module temperature primarily affects the module voltage. As the module temperature rises, the module voltage decreases. So PV systems typically have degraded performance in warm locations. Module manufacturers typically provide temperature co-efficient of voltage ratings of modules. In order to classify and rate PV modules, a standard test condition (STC) of 1000 W/m2 radiation perpendicular to the module and a module cell temperature of 25° C is defined. The electrical output of the PV module at STC is typically given by the short circuit current, open circuit voltage, and peak output power. To roughly estimate the performance of PV modules in a typical field deployment, the Nominal Operating Cell Temperature (NOCT) is defined as the cell temperature at 800 W/m2 and 20° C ambient temperature. In conclusion, PV energy production is optimal in cold climates with plenty of sunshine.