Understanding your first year output
Why year one rarely matches your full annual figure
Unless a system is commissioned on January 1st, its first calendar year only captures partial months of production. A system turned on in April captures roughly 71% of a full year's output in that first calendar year, not the full annual figure. This is simple calendar math, not a sign of anything unusual about the system. A DOE study of federal PV systems found first-year performance can vary widely from modeled expectations, often due to start-up issues that resolve over time.
What light-induced degradation is and why it's a one-time event
Light-induced degradation is a small, one-time efficiency drop that occurs in a crystalline silicon panel's first weeks of sun exposure, typically 1% to 3%, after which the panel stabilizes at its rated output level. This mostly resolves within the first month or two of operation, so it has only a minor effect on the first year's total and essentially no effect on every year afterward. NLR's PV Lifetime Project tracks this kind of early-life behavior alongside longer-term degradation, and confirms that most of a panel's lifetime performance loss happens gradually over many years rather than in that initial stabilization period.
Why year two often looks better than year one
Often yes, simply because the second calendar year captures a full 12 months of production while the first year likely only captured a partial year from the commissioning date. This isn't a sign of anything wrong, just a full-year comparison versus a partial one. Comparing year one against a properly prorated expectation, not the full annual figure, is the fair comparison.
How commissioning season shapes the total
Yes. A system commissioned in late spring captures its remaining months during typically stronger summer production, while one commissioned in late fall captures mostly weaker winter months, so two systems commissioned the same number of months into the year can still see different first-year totals. The monthly output estimator can help refine this further based on your specific season. If you're still deciding on system size and want to weight that decision around your actual monthly usage pattern rather than a flat annual average, the system sizing by monthly usage tool takes that seasonal variation into account directly.
What to do with this number next
Once your system stabilizes past year one, track long-term performance with the year 10 and year 25 output estimators to see how gradual panel degradation plays out over the system's life. If you want to see what that figure means in dollars and years, the payback period calculator turns the same output figure into a timeline.