That 1% finger in the air estimate is what installers or sales people tend to use to give people a vague idea what to expect. Your panels will not make as much power in 25 years as they do today. But because weather happens the hypothetical input power - sunlight falling on the panels, can vary maybe 5% from one year to the next, larger than the degradation in any single year no question. So it's very possible a solar PV roof has its "best" year in year two or three even though that's not intuitive.