The easiest and fastest way to calculate PV string size and voltage drop is to use the Mayfield Design Tool. Our web-based calculator has data for hundreds of PV modules, inverters, and locations so you don't have to look up datasheets nor do manual calculations.
This can vary depending on your north-south location, but it generally ranges from 30-45 degrees for homeowners in the continental United States. Factors such as roof pitch and time of year can affect the overall efficiency of your arrays.