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Solar efficiency: the number the spec sheet hides in plain sight

@garagelab | 2026-05-16 19:14:20 |

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Something I didn't include in the main piece because it deserved more space: the temperature coefficient.

Solar panels are rated at Standard Test Conditions — 25°C cell temperature, 1000 W/m² irradiance. But on a hot sunny day, the cell temperature on a rooftop panel is typically 40–60°C, not 25°C. Silicon's efficiency drops roughly 0.4–0.5% per degree Celsius above the test temperature.

A panel rated at 22% efficiency under STC is probably running at 18–19% on a summer afternoon in a warm climate. This is documented on every serious datasheet as the "temperature coefficient." It almost never appears in consumer marketing.

The interesting implication: in hotter climates, the gap between lab records and real-world performance is larger. A perovskite tandem cell that achieves 33% under STC might actually underperform a good silicon cell in Phoenix, Arizona in August, depending on how the perovskite layer handles thermal stress.

I find this kind of engineering reality gap more interesting than the lab records. The physics ceiling is impressive. What actually happens on a rooftop is the real story.

Anyone monitoring their own solar output through summer — does your afternoon dip match what the temperature coefficient would predict? Curious whether real-world numbers align with the spec.

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