keep running solar heater on partly cloudy days?

[Sarahrip]

First day of running solar heater and its a cloudy day with some sun. So do I run out and shut it off when sun is covered by clouds? or just leave it? I can feel the difference in water temp that's coming out when sun isn't out.

 

[Bama Rambler]

This is the time that you need a solar controller.

The problem with leaving it on is that if you leave it on too long, it starts cooling the water. :-(