What is the temperature coefficient worth in the sun?
The page opens on a worked example: two modules with the same rated power, one heterojunction and one PERC, on a hot afternoon. Pick your own two and set the weather; the chart shows what each one gives as its cells heat up.
In the heat
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Output by the module's temperature coefficient alone, at the sunlight chosen. Cell temperature from each datasheet's NOCT on an open rack; a module whose datasheet gives none is assumed at 45 °C, the median of the 12 877 catalogue modules that do.
Pick two modules
Search for a module in each slot. Only modules whose datasheet gives a temperature coefficient can be compared.
Questions
It is how much power a module loses for every degree its cells run above 25 °C. A module at -0.34 %/°C loses 0.34 % of its rated power per degree; one at -0.24 %/°C loses 0.24 %.
On a sunny roof the cells run 30 to 40 degrees above 25 °C, so the gap between two coefficients becomes several percent of the output during exactly the hours the module produces most.
Sunlight the cell does not turn into electricity turns into heat. The datasheet's NOCT is the cell temperature at 800 W/m² of sun, 20 °C air and a 1 m/s wind on an open rack, and the page scales that rise with the sunlight you choose.
A module flush on a roof with little air behind it runs hotter than the test rack, so treat these figures as the better case.
No. It compares two modules under the same steady conditions: one air temperature, one level of sunlight, no passing clouds, no inverter.
A year's energy depends on every hour of weather at your address, which PVGIS gives. This page shows the part of the difference between two modules that comes from heat alone.