If you are the computer inclined, here is an application that can help you determine what your average temp is with and without solar. You can specify not only what direction the panels are but also, what type of material they are over (i.e. roof, ground etc.). It takes a bit of time to set up but it is better than a swag.

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