Performance of solar pool heating system depends on numerous factors, including panel orientation, whether a controller is used, shading on both the panels and pool, in the pool indoor or outdoor, wind exposure on both the pool and the panels, sizing, whether a cover is used, and of course the weather. That said, here are some general rules of thumb:
-The system typically take 3-5 days to reach their maximum pool temperature and maintain that temperature.
-Solar pool heating systems typically heat pools 8°C degree warmer than non-solar pool temperatures. By adding a pool cover, the pool can be heated 10°C-14°C higher that normal non-solar pool temperatures.
-The system should maintain that temperature increase based on average local weather conditions.
-A rough rule of thumb for comfort is to expect the solar pool heating system to allow you to enjoy swimming in the pool as long as the weather conditions don’t require wearing a sweater or coat. After which you are going to need a removable pool blanket and backup heater.
Of course, if the system is undersized (for example, a large pool with limited roof space, or budget constraints) the system will underperform. It will still efficiently collect energy and be cost effective, it will just underperform giving the sense that it’s not working well. The common rule of thumb for sizing is you will get a 1.5°C temperature increase compared to an unheated pool, from each 20% of the pool surface area that you put up in solar collectors.