The Heat Penalty
Solar panels are rated under Standard Test Conditions at 25°C, but real-world cell temperatures routinely reach 50-70°C on sunny days. Every degree above 25°C reduces output by the temperature coefficient — typically -0.4%/°C for crystalline silicon. A panel rated at 400 W under STC might produce only 340 W when its cells hit 60°C. This discrepancy between nameplate and real-world output is one of the most important factors in solar system design.
Physics of Thermal Derating
The fundamental cause is the temperature dependence of the semiconductor's open-circuit voltage. Higher temperature increases the intrinsic carrier concentration exponentially, raising the diode's dark current I₀. Since Voc depends on ln(Iph/I₀), more dark current means less voltage. The short-circuit current Isc increases slightly with temperature (the bandgap narrows, absorbing more photons), but this small gain is overwhelmed by the Voc loss. The net effect is a nearly linear power decrease with temperature.
Technology Matters
Different cell technologies have different temperature coefficients. Standard PERC silicon cells have β around -0.37%/°C. Heterojunction (HJT) cells achieve -0.26%/°C thanks to superior passivation. Cadmium telluride (CdTe) thin-film modules show only -0.25%/°C. Perovskite cells vary widely but some formulations show even lower thermal sensitivity. In hot climates, technology choice based on temperature coefficient can make a meaningful difference in annual yield.
Cooling and Mitigation
This simulation plots efficiency versus temperature for your chosen cell technology. Watch the efficiency curve slope as you adjust the temperature coefficient. Compare a standard silicon panel to an HJT panel on a hot summer day. The thermal loss readout quantifies exactly how many watts you lose to heat, making the engineering and economic case for ventilation, raised mounting, and advanced cell technologies.