PV Solar Panel Generation Deterioration Over Time?

Solar panels are expected to deteriorate over their lifetime and according to the manufacturers the generation of solar power can drop by around 0.5% per year.

What's interesting is that after 8 years this deterioration isn't visible and that the natural variation of the solar electricity generated is a far more obvious factor than the performance of the panels themselves.

http://solar-panels-review.321web.co.uk/index.php/yearly-comparison-of-solar-pv

Annual generation has varied between 2794 and 3343 kWh per year but this has not been in a straight line and generation can vary by 10% which easily masks a 0.5% variation in panels.