This software takes into account your local condition averages to give you an expected water temp. You can include solar, gas, electric heating as well plus if you have a solar cover. It is about the best method I have found for determining water temp. One more thing, it will do a business case for you to determine if solar is worth it.

http://www.canren.gc.ca/tech_appl/in...aId=5&PgID=484