Performance and output
Enter two systems' year-one output and degradation rates. Compare their cumulative output over 25 years and see if one ever overtakes the other.
How it works
Step 1
Use your own numbers, or the annual output estimator if you don't have them handy yet.
Step 2
Check your panel manufacturer's warranty documentation, since degradation rates vary by panel tier and brand.
Step 3
The tool sums each year's output for both systems and flags the year, if any, where one overtakes the other in cumulative total. Pair this with the output vs usage gap analyzer to see how either system tracks against your usage. That decline is not the same for every brand, as the degradation rate comparison by brand tool shows.
A system with slightly lower year-one output but a slower degradation rate can produce more total energy over 25 years than a system that starts higher but fades faster, so a single year's number can be misleading on its own. That's the whole reason this tool projects both systems across their full lifetime instead of stopping at year one, unlike the quick output estimator, which is built for a fast single-year number. A DOE assessment of federal PV systems notes that degradation due to age is typically modeled at around 0.5% per year, a figure worth checking against any long-term projection.
Most modern panels degrade somewhere between 0.3% and 0.8% per year, with premium panels often at the lower end and budget panels at the higher end of that range. Even a small difference compounds meaningfully over 25 years, which is exactly what this projection is built to show. NLR's PV Fleet data, drawn from thousands of real installed systems, has documented this same tier-based spread in practice, with panel quality and local climate both contributing to where an individual system lands within that range.
It means the gap in starting output was too large for the degradation difference to close over 25 years, which is common when the two systems differ mainly in size rather than panel quality. In that case, size matters more than degradation rate for total output, and it's worth double-checking your inputs with the real-world vs nameplate gap calculator.
Not necessarily does a lower degradation rate always mean a better long-term investment. A lower degradation rate helps most when the starting outputs are close, but if one system is significantly larger to begin with, that size advantage often matters more than a small difference in degradation rate. Weigh both factors together rather than picking a system on degradation rate alone, and check the household output calculator if you're still deciding how much output you actually need. Once you have a projected output curve you trust, the 25-year lifetime savings calculator can translate it into an actual dollar figure over the same time horizon.
Questions
A system with slightly lower year-one output but a slower degradation rate can produce more total energy over 25 years than a system that starts higher but fades faster, so a single year's number can be misleading on its own.
Most modern panels degrade somewhere between 0.3% and 0.8% per year, with premium panels often at the lower end and budget panels at the higher end of that range.
It means the gap in starting output was too large for the degradation difference to close over 25 years, which is common when the two systems differ mainly in size rather than panel quality.
Not necessarily. A lower degradation rate helps most when the starting outputs are close, but if one system is significantly larger to begin with, that size advantage often matters more than a small difference in degradation rate.