You can buy a General Electrical book from any bookstore, or a book that at least has a chapter or 2 about this in it. Or, you can go to your local inspector and see what the BOCA code is. Or, you can walk around the house and measure the height of other outlets that are under windows. BUT, beings that you have an enclosed porch, I don't know if my last suggestion is one that should be followed. Does anyone else you know have an enclosed porch with outlets under their windows?! Be sure you have the proper outlet and wiring, depending on whatever.