As a general guide you could use the NOCT values from the solar panel specification sheet… Although these are generally for temperate climates, not arid desert regions.
The internal cell temperature can be as high as 80 degC with a very hot 40 degC ambient temperature and little wind, so about a 15-20% loss can be expected in these extreme temperatures. However you should do all estimated generation calculations based on the average peak sun hours (PSH) of your location and average temperatures.
Below is the spec sheet for LG Neon 2 panels showing the STC power rating and the NOCT power rating below.
