The sizing of roof overhangs to shade windows in the summer but admit light in the winter is a big thing in passive solar design. It's not all that hard to calculate, and there are good resources at Built It Solar
. In the "Window Shading and Overhangs" section near the bottom of that page is a link to some free design and simulation tools at Sustainable By Design
The projection of the overhang depends upon latitude, the height of the window to be shaded, and the height from the bottom of the window to the lowest point of the eave/overhang. In a fixed building design I did several years ago a 24 inch overhang shaded a 36 inch high window which bottomed about 5.5 feet down from the overhang. I worked out the design with those tools at Sustainable By Design, and when I built it, I found that it actually worked. Lots of sun in the winter, window fully shaded in the summer. Some sun still comes in through the very late summer and fall when the heat isn't necessarily wanted; that's because I optimized for full sun in the winter and decided to just deal with what unwanted sun will come in during those months.
Orientation is a big thing in passive solar design too. Glass facing due south is great for heat gain in the winter. Surprisingly, it's not terrible for excess gain in the summer -- IIRC west-facing glass is worst there (at least in the northern hemisphere?) because the sun shines through west-facing windows from just a little after noon until sunset which might be as late as 8 pm.
If the glass is fully shaded then it won't have solar
heat gain. There'll still be some heat transfer just from having hot outdoor air against the glass, but eliminating that direct solar gain will go a long way toward reducing the cooling load.