Solar pool heating panels collect heat from direct solar radiation, but also from the ambient air when it is warmer than the pool water. If the roof is warmer than the pool water, heat can also be gained via conduction from the back side of the panel. 

Shade will decrease the efficiency of your system depending on the percentage of area and length of time they are in the shade. Tree shade and shade from narrow structures like chimneys and towers are not nearly as bad as they are for solar electricity. As the sun moves across the sky each day, only a portion of the solar panel is affected. The rest of the panel continues to heat the pool.

When selecting the best location for your solar pool panels use Google Maps to look around for potential obstacles that may cause your solar panels to receive shade. Be sure to choose a suitable location on your roof or on the ground that you believe to have the least amount of shade during the daily solar window period [9:00 am to 5:00 pm]. Little to no shade is optimal but not necessary. 

If in the event the location you selected does have some shade during the solar window period, to compensate for the decreased efficiency you may want to add an additional solar pool panel to your system to increase your solar panel square footage. The more panels you have installed the more solar radiant energy you will collect during the solar window period.