Contrary to popular belief, solar panels work better at lower temperatures. This means that panels installed in warm climates, where there is a lot of direct sunlight, won't be operating at maximum efficiency due to the excess heat.
Higher temperatures will make the electrons in the panel more excited, which will reduce the voltage of each cell. Furthermore, heat will increase the resistance of all the circuits in your solar array, making them less able to carry current.
When installing solar systems in warmer climates, always leave enough space between the panel and the roof to facilitate airflow and cooling.