Solar Photovoltaic Fundamentals: Radiation, the PV Effect, I-V Curves, STC and Efficiency
This section covers solar photovoltaic fundamentals: the solar resource, the photovoltaic effect, cell/module/array terminology, I-V characteristics, standard test conditions and efficiency. The exam tests whether you can read a datasheet curve and convert it into a safe, realistic design.
Solar resource and the PV effect
Irradiance is instantaneous power per area (W/m²); irradiation is energy per area per day (kWh/m²/day). STC means 1000 W/m², 25 °C cell temperature and AM 1.5 spectrum. A cell is a large p-n junction: photons above the bandgap create electron-hole pairs, and the junction field separates them to drive current. Cell → module → string → array is the physical hierarchy.
I-V curve, fill factor and efficiency
Isc is the current at zero volts; Voc is the voltage at zero current; Pmax is the knee product Vmp × Imp. Fill factor FF = Pmax ÷ (Voc × Isc). Efficiency = Pmax ÷ (module area × 1000 W/m²). Voc falls as cell temperature rises, while Isc rises slightly with irradiance. Datasheet ratings are at STC, not at noon in Punjab summer.
Worked example
A module measures 1.7 m × 1.0 m and is rated 400 W. Area = 1.7 m², so efficiency = 400 ÷ (1.7 × 1000) = 400 ÷ 1700 = 23.5%. With Voc 49.5 V and Isc 10.2 A, FF = 400 ÷ (49.5 × 10.2) = 400 ÷ 504.9 = 0.79. A 10-module string has 495 V at STC; at −5 °C with a −0.29%/°C Voc coefficient, the rise is about 8.7%, giving 538 V. Check the inverter maximum input before approving the string.
Traps
Confusing Isc with Imp; multiplying Voc by Isc and calling it power; ignoring cold-temperature voltage rise; mixing W/m² with kWh/m²/day; assuming STC output equals field output.