https://phys.org/news/2018-10-solar-power-largest-date-loss.html

After quantifying the data, Dr. Dhimish discovered that the panels found to have hot spots generated a power output notably less than those that didn't. He also discovered that location was a primary contributor in the distribution of hot spots.

Photovoltaics hot spots are areas of elevated temperature which can affect only part of the solar panel. They are a result of a localised decrease in efficiency and the main cause of accelerated PV ageing, often causing permanent damage to the solar panel's lifetime performance.