As temperature decreases, voltage increases, increasing the overall wattage. Its quite dramatic, a system at 150F will operate at 240V and at -20F will rise to 600V. As far as predicting airflow in various outdoor conditions? Sounds like the Science guys were running tests on the mushrooms again. You could probably net better results in manufacturing ground mounted panels to better vent the air they ate in contact with. The Building Codes make it difficult to engineer practical passive cooling on roof mounted systems.