How do you calculate the solar load an en electronic enclosure and the subsequent effect on operating temperature or temperature rise?
Try if your weather department has some information about the max. temperatures in your area... in most cases you also get 10,100,... year maximums. Thats what you assume for the outside of the outdoor enclosure - ambient Temperature.
You then can approximate the outdoor enclosure's active surface, your equipments active surface and via physics of radiation heat transfer (for formulas see a physics book) you can estimate how much Energy can be transfered away from your equipment. Then you know if it is ok for your equipment to warm up to 45°C or not.
This of course dismisses other effects (transfer characteristics of the material the outdoor enclosure is made of), but its good enough in many cases. A colleague of me has written a exel calculation for closed housings with passive cooling only. The only difference, your result is not an allowed temperature, but an allowed power dissipation.