Shootnpar, our system has two settings, "on" and "auto", with a dial for low, medium, high. If you select "on" - the system is on regardless of the weather. If you select "auto" it is on up to the temperature you have selected, and only if it's sunny. There's a sun sensor on the roof near the panels. Also, if it doesn't seem to be getting warmer, it will loose any heat it gains during the day, overnight, unless it's covered... Hope this helps!

I have four options for heat: (1) Solar only, (2) Heater (gas), (3) Solar Preferred, (4) Off.

I set my desired maximum temperature (84 for pool, 103 for spa) and let the system do the rest. With Solar Only, the Gas Heater doesn't go on at all. Once the water temp reaches 84 degrees, it won't go to solar. With Solar Preferred, it will go to solar if it's warm and sunny enough outside, otherwise the gas kicks in. The other 2 options are fairly self-explanatory :-)