Money
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6 min read
What half a percent a year does over twenty-five
Degradation is the clause every datasheet carries and no quote models. Compounded across a warranty period it removes about a ninth of the output, and it removes it from the years that matter most.
Written by Tobias Ekwueme
Every panel datasheet carries a performance warranty, and every performance warranty concedes that the panel gets worse. The usual figure is half a percent a year after the first.
Compounding, not subtracting
Half a percent a year is not twelve and a half percent over twenty-five years. It compounds, so year twenty-five produces about eleven point three percent less than year one.
That is a smaller number than most people fear and a larger one than any quote models, because most quotes model year one and multiply.
Where it actually bites
In the second half of the payback window, which is also where the inverter replacement lands. Two modest effects arriving in the same decade is what turns a thirteen year payback into a twenty-two year one.
Next step
Run the numbers this argues about
The calculator carries every effect described here. Change one input at a time and see which of them your payback is actually sensitive to.