How much decrease in efficiency will the PV panels see over their lifetime?

All PV panels degrade over time, somewhere between 0.5% and 1% per year and flattening out over time. Current PV panels come with a 25 year performance warrantee based on 0.5% to 0.7% per year of performance degradation. The warranty is usually for 90% performance after 10 years, and 80% performance after 25 years. The performance is relative to the nameplate performance of the PV module.