Solar Cell, Module and Panel Construction and Rating
Construction
Solar Cell, Module and Panel Construction and Rating covers the materials inside a module and the meaning of its label. A crystalline module stacks tempered glass, EVA encapsulant, solar cells, another EVA layer, a backsheet, a junction box and an aluminium frame. Cells are connected by busbars; bypass diodes in the junction box protect shaded cells. Monocrystalline cells are cut from a single crystal and usually look black; polycrystalline cells show visible grain. Common configurations are 60-cell and 72-cell modules.
Ratings and standards
The label gives Voc, Isc, Vmp, Imp and Pmax at STC. NOCT or NMOT is a more realistic operating condition, about 800 W/m², 20 °C ambient and 1 m/s wind. Temperature coefficients matter: power falls about 0.35–0.45 %/°C, Voc falls as temperature rises, and Isc rises slightly. IEC 61215 covers design qualification and IEC 61730 covers safety; neither is a performance guarantee for every site.
Worked example
A 72-cell module has Voc 46.0 V at STC and a Voc temperature coefficient of −0.30 %/°C. At 5 °C, the cell is 20 °C colder than STC. Voltage change = 46.0 × 0.0030 × 20 = 2.76 V. Cold Voc ≈ 48.8 V. Check that the charge controller's maximum PV input voltage is above this, not just above 46 V.
Traps
Do not use Isc as the normal operating current for cable sizing; use Imp and then apply derating. Do not cover a module with a cloth and leave it connected without isolating it. A failed bypass diode can cause overheating, not just a small yield loss.