Blog · 2026-02-05 · 6 min
Panel degradation and what a 25-year warranty actually guarantees
Linear degradation assumptions of ~0.4–0.5%/yr are planning tools — warranties guarantee a floor, not your weather.
What 0.5%/yr means
If year-1 output is 100%, a 0.5%/yr linear model implies roughly 88% by year 25 (compounded slightly differently depending on formula). WattPayback applies a simple compound factor each year.
Warranty vs. prediction
A performance warranty usually promises that output will not fall below a curve (for example 80–85% at year 25) under defined test conditions. It is a floor, not a forecast of your roof.
Inverters are the other mid-life cost
Many string inverters are replaced once around year 10–15. Include that cash outflow explicitly — the payback calculator does.